Liabilities or Assets? Some Australian Perspectives on Weeds

Nimal R. Chandrasena Ecosystem Restoration 1, Kawana Court, Bella Vista NSW 2153, Australia nimal.chandrasena@gmail.com

Abstract: Australians generally have a strong negative attitude towards weeds1, and a tendency to label a large number of potentially useful plant resources as invasive species2, which should be controlled at any cost, while ignoring considerable evidence of the use by both Indigenous Australians and by European settlers of plants, regarded as weeds. This attitude may stem from the adjustments that early settlers had to make, to farm in an environmentally harsh continent. More recently, the application of ‘species-focused’ weed risk assessments- a field well developed in Australia, may have also contributed to the maligning of many plant taxa as ‘invaders’ in the public mind. A large number of recent publications, including government-sponsored reports, have highlighted the losses of agricultural production caused by weeds, and the threats posed by such species to biodiversity. The plants in question are mostly ‘colonisers’, which have the capacity to rapidly occupy human-modified environments. The focus in Australia has been so much on waging a protracted war on weeds that land managers have tended to overlook the potential of using these plants as resources.

The literature in a number of fields provides evidence that many colonising taxa may form worthy resources in diverse areas of human interest. These include their traditional uses as food, for both humans and animals, and continued use as therapeutic plants. There is considerable interest at present in obtaining pharmaceuticals from many taxa occupying disturbed habitats. Other major areas of significant interest include the use of the colonising strengths of several species in the remediation of water and terrestrial environments that have been damaged by human activities. Among some outstanding prospects are the potential to use aquatic species, such as Water Hyacinth [Eichhornia crassipes (Mart.) Solms] in pollution removal, and the use of some strong colonisers in wastewater treatment systems, or in the rehabilitation of riparian zones of watercourses and rivers. The Common Reed [Phragmites australis (Cav.) Trin. ex Steud.] and Cattails (Typha spp.) are examples of such taxa. Globally, there is considerable interest in using the large biomass produced by these species in a variety of beneficial ways, including as raw materials for a range of products and as bio-fuels of the future.

1 The general use of the term weed in Australia encompasses its broadest contemporary meaning, i.e. a ‘weed’ is any plant growing where it is not wanted. A more technical commonly used definition is: ‘a weed is a species that adversely affects biodiversity, the economy or society’ (Groves et al., 2005). 2 Invasive plants in Australia are defined as naturalized species that are spreading; ‘Naturalized’ species are those from distant ecosystems, outside a given region that can maintain populations in the wild.

In addition, there are many opportunities for using colonising plants in phyto-remediation, to scavenge soil pollutants. Furthermore, the awareness of the role of weeds as part of biological diversity is increasing, and there is continuing interest in creating more sustainable farming systems, in which colonising species are appreciated. There are also significant opportunities to further exploit chemical warfare between plants (allelopathic phenomena) in beneficial ways. These include the discovery of new bioactive chemicals and the use of allelopathic plant residues within low input agricultural systems. Many colonising plants are useful in providing such benefits.

The conflict between humans and weeds will continue, so long as humans modify ecosystems. However, a fresh look at the potential of ‘co-existing’ with weeds and using them as resources is overdue, given the many possibilities demonstrated. In many cases, the focus is on managing problematic species in specific situations, rather than on their utilization. However, if land managers can be led to appreciate the extraordinary strengths of the colonising taxa, this will allow a better integration of these species into our economies. Improved understanding of the causes of biological invasions should help to reduce the current confusion and negative attitudes towards invasive species. This essay discusses the above viewpoints, and provides examples to illustrate that not all weedy taxa are bad all the time, just because they may interfere, under certain circumstances, with human interests.

Introduction

We tend to dislike some plant species, because they interfere with our endeavours, such as agriculture or animal farming, recreational pursuits, including gardening, bush walking, transport, and water sports. Collectively, we call them ‘weeds’ or ‘invasive species’ and they include many herbaceous species- grasses, sedges, and small broad-leaved plants, as well as woody shrubs, medium-sized trees and scrambling vines.

There is no doubt that weeds compete with crop plants, reducing yields and crop quality, and take the space of native bushlands or garden plants. Some can also taint milk, and others could be poisonous to humans and domestic animals. Still others have attributes like thorns and spines, which cause physical injury. They may act as host plants for parasitic insects or diseases, while yet others can be parasitic on other plants. Through these direct or indirect effects, weeds often increase the cost of farming and decrease the value of land. Under some circumstances, they may even threaten the biodiversity of landscapes. Our dislike for weeds is reflected in the global figures from agrochemical sales. In 2005 alone, globally we spent approximately $ 33,600 million on agrochemicals, of which 45.8% was spent on herbicides (AGROW, 2006).

Species can become weeds because they are competitive, adaptable, highly fecund, and are capable of exploiting man-made habitats. They are not ecologically ‘plants out of place’, as some older definitions have suggested. In fact, in an ecological sense, the opposite is true. Weeds are opportunistic species or ‘pioneers of secondary succession’ (Bunting, 1960), that are well adapted to grow in locations where disturbances, caused either by humans or by natural causes, have opened up space (Grime, 1979). A set of common biological characteristics (Baker, 1965) appears to allow them to colonise such habitats, to form extensive populations and, sometimes, to dominate landscapes. These include high fecundity, the ability to grow rapidly, and a wide range of tolerance to environmental conditions. However, a species may become an invader of landscapes only if a chance combination of circumstances makes its attributes particularly advantageous to its growth and survival (Naylor and Lutman, 2002). In many cases, this opportunity arises because the lack of specific parasites or herbivores (i.e. ‘natural enemies’) gives them an advantage over

the crops and the native flora. However, in the right place, many of these extraordinary plants can provide benefits that can be sustainably exploited for human welfare.

In terms of evolutionary success, generally accepted to mean the continuation of a genetic line over time, most weeds are highly successful because of the numbers of individuals they produce, their reproductive capacity, and the area and range of habitat they can occupy (Baker, 1965). Therefore, in terms of the Darwinian concept of the struggle for existence, weeds, as a class are amongst the most successful plants that have evolved on our planet (Auld, 2004). However, instead of a balanced view, a somewhat negative attitude towards weeds is prevalent in Australia, which has led to a large number of plants being branded as ‘thugs’. This negative attitude towards weeds could arise, at least in part, due to the harsh farming life that the early European settlers and their descendants experienced over the past two centuries. It may also be a reflection of the uncompromising way in which colonisation of the continent occurred3. Most people tend to agree that past activities, such as the large-scale clear felling of native forests to make room for pastoral agriculture, have left the Australian environment in a perilous state.

Many books, review papers, and government-sponsored reports (e.g. Humphries et al., 1991; Low, 1999; Brinkley and Bomford, 2002; Virtue et al. 2004; Coutts-Smith and Downey, 2006) have highlighted the adverse impacts weeds have on biodiversity. These, and other publications (Lonsdale, 1994; Groves, 1999; Groves and Willis, 1999; Richardson et al., 2000; Williams and West, 2000; Randall, 2001; Grice and Ainsworth, 2003; Glaznig et al., 2004a; 2004b; Randall and Kessal, 2004; Spafford-Jacob et al., 2004; Glaznig, 2005; Groves et al., 2005), have focused attention on the dangers posed by environmental weeds4 and sleeper weeds5. The primary objective of these publications has been to highlight that some species are already serious problems for Australia, and others can become so in the future. However, their message can be misconstrued. Weeds are a favourite topic of the popular media, including magazines, newsletters, and Gardening TV programmes. Most people are quite happy to use a variety of costly means, in terms of both energy and resources, to undertake control of various species, regarded as undesirables. There is little discussion in the same articles that ‘weediness’ is an ill-defined man-made concept; if man thought about the world differently, there would be no weeds. The absence of discussion on beneficial aspects of ‘colonising’ species in the afore-mentioned publications may have also contributed to the hardened negative attitude towards a large number of taxa, presumed to cause problems all the time. Such attitudes prevent people from considering that colonising species are useful resources.

3 A controversial concept- terra nullius (Latin for ‘land of no one’) has been used to justify the colonisation of the continent (Horton, 2000). Essentially, the rationale was, if you did not farm in the European sense - and Indigenous Australians did not - then, you did not deserve to keep the land. Anthropologists like Horton (2000) believe that much of Australia’s environmental problems are the result of the destruction wrought by the application of British farming practices to a ‘land that deserved better’ 4 Environmental weeds are defined in Australia as naturalized plant species that are deleterious to native vegetation (Humphries et al., 1991). The species are presumed to impact negatively on native species diversity or ecosystem function. Environmental weeds are usually non-native species, although native plant species that are invasive beyond their indigenous range are also included (Groves et al., 2005). 5 ‘Sleeper Weeds’ have been defined as a sub-group of invasive plants that arrive at a region, naturalize (i.e. establish and self-reproduce), and remain localized for a period of greater than 50 years between naturalization and a marked increase in population growth and then commence to spread and become seriously invasive (Groves, 2006).

The inclusion of some plants of world importance, (e.g. Taro [Colocasia esculenta (L.) Schott], Neem (Azadirachta indica A. Juss), Guava (Psidium guajava L.) and ‘Ipil Ipil’ [Leucaena leucocephala (Lam.) De Witt]), as ‘invasives’ within an Australian context, tends to confuse the consideration of which taxa should pose serious problems. Often landholders and public officials do not take into account the full-published details or the rationale for listing such species. Instead, the information is often misread and misquoted, leading to the summary condemnation of useful plants. Listing of such species is the result of a narrow, species-focused approach based on ‘weed risk assessments’, which is largely a product of the past twenty years. Assessing the potential risk of a species becoming an invader by its invasiveness elsewhere is widely supported in Australia and New Zealand (Williams and West, 2000; Arcioni, 2003; Gooden et al. 2006). However, this approach could lead to errors in judgement, because plant invasions are not taxonomic phenomena, but are often related to complex bio-geographical factors (Colautti and MacIsaac, 2004). The tendency to categorise individual taxa, or groups of taxa, as ‘invasives’, because of weed risk assessments, raises doubts in the public’s mind whether all non-native species are bad for the environment. It also leads to a negative perception of plant resources of potentially significant value, and this ‘branding’ tends to be perpetuated.

The proliferation of descriptions like ‘invasives’, ‘aliens’, ‘noxious’ and ‘thugs’ in Australia has led to divergent interpretations, and subconscious associations of some plant taxa as undesirable6. The more serious issue is that, by attributing blame to imprecise plant characteristics, the significance of human-mediated factors in the spread of plants that may later invade human-modified environments is downplayed. Human activities are, directly or indirectly, the major factors implicated in plant or animal invasions in different regions or continents. Whilst some invasions are related to large-scale disturbances, such as land clearing or to the discharge of ships’ ballast, vehicles, and machinery, others result from deliberate or accidental introductions of organisms into new areas. One survey in Australia (Lonsdale, 1994) showed that 463 grasses and legumes were deliberately introduced between 1947 and 1985, and 60% of those are weeds from an agricultural or conservation perspective. Other activities, like increased farming in marginal lands, changing farming from sheep to cattle, and provision of artificial water holes in grasslands, where none existed before, are also implicated in the spread of invasive species. The latter, presumably to increase productivity from the plains, has led to farm animals grazing over vast extents of land, and causing a range of significant negative impacts on the environment (James et al., 1999).

An indicator of a ‘trend’ in thinking within a scientific discipline is often the number of papers presented at successive conferences of that discipline. Based on this indication, the ‘trend’ in Australian Weeds Conferences over the past 20 years has been almost exclusively to highlight weed problems, of one kind or another, and their management. The number of papers espousing any virtues of colonising plants is very low, and is restricted to occasional papers with different anthropocentric angles; e.g. those which trace the origins of Australian weeds to overseas floras, or the changes in weed floras occurring in Australia due to human influences (Kloot, 1987). Perhaps, attitudes have so hardened that consideration of weeds as a potential resource is seen as outside the realm of Australian Weed Science!

Globally, the utilization of weeds has been patchy over the past five decades. The most significant uses have been largely restricted to aquatic weeds. These have most often been used for nutrient removal, with the plant residues largely utilised as mulches. Some species have been used as sources of food, fibre and pharmaceuticals, and a variety of minor

6 The use of emotive terms and phrases, like ‘Garden Thugs’ (Randall, 2001; Glaznig et al., 2004b), ‘Damned Weeds!’ (Arcioni, 2003), and ‘Jumping the Garden Fence’ (Groves et al., 2005) is common in Australia.

uses, mostly in developing countries. Nevertheless, there is a renewed interest in focusing on utilization of weeds in productive ways, so that people may benefit from an aspect that has been largely ignored. The need to have a fresh look at weeds was highlighted by several contributors (Ishizuka, 2001; Harada, 2001; Kim, 2001) to the recently launched Japanese journal Weed Biology and Management.

Given the magnitude of the negative publicity attached to weeds, it seems paradoxical to consider that these ‘maligned’ plants may be beneficial as resources. Nevertheless, if we are concerned about man’s relentless impact on the environment, perhaps we should question the validity of the summary condemnation of plant taxa. The main objective of this paper, therefore, is to examine some examples of human uses of colonising plants, from both Australia and elsewhere, and to discuss the prospects for a better appreciation, and utilization of such species as biological resources. Part of the essay provides an overview of prevailing attitudes and opinions towards weeds that have thus far delayed this exploration. If an attitude change can be stimulated, it will be for the benefit of both the environment and humans (both present and future generations) in Australia and elsewhere.

Weeds, invasive species and attitudes in Australia Australia’s hardened attitude towards weeds appears to be entrenched in its land use practices. In spite of the continent's harsh environment7, agriculture is the most extensive form of land use in the continent. In 2005, the estimated total area of establishments with agricultural activity was 445 million hectares, representing about 58% of the total land area (Australian Bureau of Statistics, ABS 2007). Rangeland pastoralism occurs on 53% of the total land area of Australia (70% of the continent’s arid and semi-arid zones). Pastoral agriculture accounts for the largest area of land use in agriculture (≈382 million hectares), which is 85% of all agricultural land. The ABS Report (2007) states that in the higher rainfall and irrigated areas, livestock grazing has led to the replacement of large areas of native vegetation with ‘more productive’ introduced pastures. However, creating artificial waterholes in vast grass plains in the arid and semi-arid zone has led to dramatic negative impacts. These include areas of extreme degradation around the water holes; spread of unpalatable perennial shrubs in the areas disturbed, and decreased abundance of palatable native perennial grasses, due to selective grazing (James et al., 1999).

Australian National Weed Strategy (ARMCANZ, 1999) states that weeds exist all through Australia. Although some are native species, which have spread beyond their natural range, most are introduced. The introduction of weed species commenced with the white

7 With the exception of Antarctica, Australia is the world's driest continent. More than a third of the continent is effectively desert; over two-thirds is arid or semi-arid. The wet summer conditions of northern Australia are suited for cattle grazing in inland areas, and the growing of sugar and tropical fruits in coastal areas. The drier summer conditions of southern Australia favour wheat and other dryland cereal farming, sheep grazing, and dairy and beef cattle. Within regions, there is a high degree of rainfall variability from year-to-year, which is most pronounced in the arid and semi-arid regions. The seasonality and variability of rainfall in Australia is very high by global standards and often results in lengthy periods without rain. This requires that water be stored. The development of large-scale irrigation schemes has opened up areas of inland Australia to agricultural activities, which otherwise would not have been possible, and 70% of water consumption is accounted for by the agricultural sector (ABS, 2007).

colonisation of Australia in 17888, with species imported for productive and ornamental purposes. From that beginning, through to the mid-1980s, Australian governments actively encouraged the use and introduction of species, which are now recognised as weeds (ARMCANZ, 1999)9. There is no evidence of Aborigines introducing species into Australia. However, their use of fire may have had some minor impact on the distribution of existing species within the continent (Horton, 2000).

In Australia, weeds are disliked, and are not seen as of much use except for some recognition that they are part of nature or ‘wilderness’. The main reason for the dislike is that weeds were estimated to have cost the Australian economy $4 billion in 2001-02 in lost agricultural production and control costs (Sinden et al., 2004). This figure does not include flow-on costs to the environment, and would have been significantly higher had those been estimated. Thee figure of $ 4 billion per annum losses to agriculture is often quoted, and there is significant concern that Australia’s primary production, unique environment, and biodiversity are under threat from invasive weeds. The campaigns and government-sponsored attempts to influence farmer attitudes and modify poor land practices are decidedly less vigorous compared to the assault on weeds.

The relationship of many Australian weeds to Settlers’ origins, patterns of settlement, and ‘lines of communications’ between their countries of origin and Australia, as well as to pastoralism and land management practices is well established (Kloot, 1987, and references therein). Many problematic species were unintentionally introduced by Europeans throughout the world they went and ‘colonised’. Some of Australia’s weeds, like Prickly Pear, resulted from the first contact, but others came later, being associated with continuing lines of communication (i.e. Europeans maintaining contact with their original lands of descent). Still others are associated with collections of plants en route to Australia, contaminated ballast, fodder, and packing. Spread of species, which were later labelled weeds, occurred through contaminated machinery, footwear, and implements, as well as through propagules attached to animal coats and fleeces during the earliest periods of settlement. The establishment of some species can be traced to the way settlement of the continent occurred. These include the initial sub division of land as grazing properties (Kloot, 1987), and later sub divisions of such properties for cereal cropping. The spread of weeds was facilitated further by the abandonment of old gardens and their amalgamation with adjoining properties. Later additions of species were largely intentional - as fodder plants. Kloot’s analysis shows that other critical factors implicated in the spread and successful establishment of these so-called ‘undesirable’ plants are biological (i.e. lack of natural enemies) and environmental (i.e. climate similarities, water availability, and suitability of soil). The introduction of species to Australia reads as a human folly of large proportions, which is likely to be repeated many times over again.

Since the First Fleet arrived in late 18th century with their crops and ornamental plants, approximately 30,000 plants have been imported into Australia (Groves, 2006). Now, slightly less than 10% of these are established in the wild (‘naturalised’), and this number increases by about 10 species per year (Groves et al., 2005). Some naturalised plants have become environmental weeds, and a large number of others remain as ‘sleepers’ until the right conditions to ‘awaken’ arrive, which could be a disturbance of some kind. The heightened concern is that some of these may pose significant threats to biodiversity in

8The manifest of the First Fleet of 1788 indicates that plants and seeds of Cocoa, Cotton, Coffee, Eugenia, Guava, Lemon, Oranges, Tamarind were brought, along with Spanish Reed and Prickly Pear (Opuntia spp.). [http://home.vicnet.net.au/~firstff/story.htm] 9 Promotion of Pampas grass [Cortaderia selloana (Schult. & Schult.f.) Asch. & Graebn.] as a windbreak, ARMCANZ, 1999. National Weeds Strategy, pg 23.

important wilderness areas, largely through habitat modification, competition, and alteration of disturbance regimes (Mack and D’Antonio, 1998; Coutts-Smith and Downey, 2006).

There is little doubt that some colonising species have the ability to displace native species, and the effects may flow on to animals that rely on native plants for food and shelter. Some of these may also be either more flammable or more fire retardant than the Australian native species that they allegedly displace. This can alter the fire patterns of the communities they invade, which may have effects on native animals living in those communities. However, the same species provide food and shelter for native fauna, birds, and pollinating insects, as well as to some introduced animals, which are also often maligned as ‘invasive’ from agricultural or conservation perspectives.

Many land managers and most farmers recognise that weeds are an integral part of the landscape, and often seek information in order manage their adverse effects. Scientists agree that the factors, which cause biological invasions to occur, or ‘sleeping weeds to ‘awaken’ are poorly understood (see discussion in Groves, 2006). Among the conservationists, there is strong appreciation of strengths and attributes of all plant species, and an interest in utilizing plants to improve not just the environment, but also the general well being of humans. However, the hardened attitude towards weeds is likely to continue due to the past agricultural history, despite the fact that combined pastroalism and cropping agriculture has contributed less than 3% to the Nation’s Gross National Product (GDP) in recent years (ABS 2007)10. Yet, Australian farmers are very protective of their land-holdings and are usually suspicious of any discourse that might lead to a reduction of profitability from their farms.

The Australian National Weed Strategy11 (ARMCANZ, 1999) defines a weed as a plant, which has, or has the potential to have, a detrimental effect on economic, conservation, or social values in Australia. The Strategy evolved within a wider political and social climate that was supportive of change towards greater environmental accountability. The Strategy states that weeds cause major economic, environmental, and social impacts in Australia, causing damage to natural landscapes, agricultural lands, waterways, and coastal areas. It also provides guidelines under which more effective weed management can be achieved strategically, with community participation, integrating the efforts of all stakeholders - government, industry, landholders, community groups and the public.

Thorp and Lynch (1999) explained that the emphasis of the government Strategy is to address weed problems, which threaten the profitability or sustainability of Australia’s principal primary industries, conservation areas, environmental resources of national significance and biodiversity. Whilst, the National Strategy does touch on why some plants have become undesirable in landscapes, under certain conditions, there is no recognition of the beneficial impacts of some colonising species on Australia’s overall biodiversity and landscapes, and on people’s lives. By focusing wholly on waging a war against weeds, perhaps an opportunity has been lost to influence the way land managers, including farmers and private landholders think about weedy taxa, and their potential value as biological resources, at least under certain circumstances.

10 While Australian agriculture no longer contributes a large share to GDP, it utilises a large proportion of natural resources, accounting for 70% of water consumption and almost 60% of Australia's land area (Australian Bureau of Statistics, 2007). 11 In 1996, the Australian federal government endorsed a National Weeds Strategy (NWS) with a promise of $19 million (AU) over five years for its implementation. Developed with the involvement of federal and State governments, technical experts, industry and the public, the NWS is the formal outcome of an extensive social process.

Weeds or Useful Plants- a ‘matter of opinion and circumstances’ Grice and Brown (1996) highlighted the dilemma of labelling a weed in relation to managing Australian rangelands. From a conservation perspective, a species may be called a weed because it is non-native; from a land use perspective, a native or an exotic species may be labelled a weed because it is toxic to livestock, or reduces productivity from pastoral agriculture. From an ecological point of view, a species may be called a weed because it changes the structure of a plant community, or modifies the hydrology of an ecosystem. The same species may be identified in another situation, by a different group of users, as a useful plant, and in another, as a weed.

Many species, previously introduced for beneficial purposes by land management authorities are now classed ‘undesirable’ and are controlled at large costs. Among examples are several legumes, such as Prickly Acacia [Acacia nilotica (L.) Delile], Mesquite (Prosopis spp.)12, and Parkinsonia (Parkinsonia aculeata L.). These were introduced to grasslands in Central-West Queensland for their nitrogen-fixing capacity, fast growth, and provision of fodder. The grass plains had been cleared of native trees during the previous 150 years or so, and revegetation from native perennials was poor, because of factors related to the shrinking and expanding of clay soils. The legumes were probably thought of as ‘saviours’, which could tolerate these harsh conditions, and transform the landscape to a more productive state.

Without competition from native trees, the legumes expanded their range in the new habitats, particularly in the arid and semi-arid grass plains. Their obvious strengths: nitrogen-fixing capacity and high fecundity, would have contributed to rapid growth, and subsequent expansion in relatively poor soils. Furthermore, both Prickly Acacia (CRC, 2003a) and Mesquite (CRC, 2003b) became problems when animal farming shifted from sheep to cattle. While sheep digest a majority of these seeds they consume, cattle do not, and so act as major vectors. The extent of Prickly Acacia in the Mitchell Grasslands Bioregion in Central-West Queensland is estimated as 7 million ha, and the economic costs of this invasion have been high, with recent estimates that $5 million is lost per year in productivity, and a further $4 million spent on control. All three species, once thought to make the grasslands more productive, have had the opposite effect as they formed extensive impenetrable thickets. Economic costs to landholders stem from increased difficulty in mustering stock, a reduction in stock access to watering points and a decrease in carrying capacity of the grasslands invaded by the species. Additionally, Parkinsonia infestations are known to provide refuges for feral pigs (CRC, 2003c). These negative impacts have resulted in costly management programmes, and the listing of the species as Weeds of National Significance (WONS)13.

The European Olive (Olea europaea L.) further exemplifies the dilemma. It was introduced to the continent in 1805, and was first planted in a farm near Sydney (Dellow et al., 1987). At that time, Olives were highly regarded for its edible fruit and oil, a view that prevailed for more than 150 years. In recent times, there has been a significant expansion of Olive plantations in South Australia. However, ‘feral olives’- those that spread outside

12 Four Mesquite species P. pallida (Humb. & Bonpl. ex Willd.) Kunth, P. velutina Wooton, P. glandulosa Torr. and P. laevigata (Humb. & Bonpl. ex Willd.) M. C. Johnst. and their hybrids are recognised as having a serious adverse impact on the grasslands. 13 Weeds Of National Significance (WONS) is a list of 20 species, nominated under the National Weeds Strategy of 1997, which require a national (trans-boundary) effort to tackle their management. These weeds affect extensive land use systems, such as conservation areas and grazing systems, rather than cropping systems. The major criteria used in listing species are: (1) Invasiveness, (2) Impacts, (3) Potential for Spread, and (4) Socioeconomic and Environmental Values (Virtue et al., 2001).

plantations, are regarded as a major threat to native vegetation. The spread of feral olives is largely attributed to birds like starlings and changes in grazing management. There are now several large-scale projects, such as in the Adelaide Hills in South Australia, to contain the spread of ‘feral olives’ (Crossman, 2002).

Other taxa, much valued in the past, but now outlawed, include Bitou Bush [Chrysanthemoides monilifera ssp. monilifera (L.) Norl] and Willows (Salix spp.). Bitou Bush was accidentally introduced to Australia with ships’ ballast. However, land managers, such as the NSW Soil Conservation Service and companies extracting Titanium from sand, saw its potential for stabilising sand dunes. The species was extensively planted in Eastern Australia during 1946-1968. Spread by birds, wind, and human vectors, Bitou Bush colonised vast areas of the eastern seaboard, adversely impacting on native vegetation. Large-scale control programmes have been on going since the 1980s (Holtkamp et al., 1999). Bitou Bush is now a declared WONS species in Australia.

Similarly, Willows (Salix spp.) were imported from temperate Europe and planted widely across the States of NSW, Victoria, and Tasmania to prevent stream bank erosion. At least 32 naturalized and 47 cultivated species of Willows are now present in Australia, and many are seen growing along watercourses. The negative impacts of Willows on riparian ecology are poorly understood, but adverse effects on the abundance and diversity of woodland-dependent native bird species have been highlighted. Once considered extremely useful in the Australian environment, all species of Willows except Salix babylonica L. (Weeping Willow), S. x calodendron Wimm. and S. x reichardtii A. Kern. are now declared as WONS. These include the most invasive ones: Crack Willow (S. fragilis L.), Black Willow (S. nigra Marshall), and Grey Willow (S. cinerea L.). The eradication of Willows and their replacement with native vegetation along watercourses in NSW, Victoria, and Tasmania is now well underway (Ladson and Gerrish, 1996).

These examples demonstrate the tenuous nature of the human judgement on the virtues of a species, whether it is a weed or a useful plant. Clearly, this is a matter of opinion, largely based on human needs, wants, and perceptions, at a particular time, place, or circumstances. Such opinion is easily swayed by the needs of a situation, short-term gains, and profit motivation, and there is room for significant error.

Indigenous Australians and Uses of Weeds Aboriginal Australians have occupied the continent since about 50,000 years (Horton, 2000). Their relationship with plants, and indeed with the landscape, is acknowledged as unique. For these hunter-gatherers, the use of plant resources was not a matter of choice. They would have used the broadest possible range of plants, primarily as food, followed by use as tools, weapons, medicines, and shelter materials.

There is no evidence that any plant was regarded as particularly undesirable by Indigenous Australians. On the other hand, there are many examples of ‘weedy’ species that they have used widely. For instance, Common Reed [Phragmites australis (Cav.) Trin. ex Steud.], often described as troublesome in wetlands, has been referred to by various Aboriginal names in different languages, which reflect the different uses of the plant14. Stems of Phragmites were highly prized for spears, and were cut into short pieces for necklaces or nose ornaments (Australian National Botanic Gardens, 1998). Sharp edges of the reed provided ready cutting tools and the leaves were used for making bags and baskets.

14 Some Aboriginal names for Phragmites and language/region: Charr-ak [Djadja wurrung], Kaerk [Djab wurrung], Taark [Gunditjmara], Jaark [Lake Boga], Djarg [Wemba Wemba], Kowat [Gunai/Kurnai]

Stems were also bound together for making large rectangular rafts used for mussel collecting on inland lakes. Tasmanians also ate the shoots of the underground stems (rhizomes).

Bull Rushes or Cumbungi (Typha spp.) in Australia comprise three species: broad-leaved Cumbungi (Typha orientalis C. Presl), narrow-leaved Cumbungi (Typha domingenis Pers.) and Cumbungi (T. latifolia L.). All are wetland species, which are generally regarded as undesirable. However, for Aboriginal Australians, who called all Cumbungi Poorteetch [Gunditjmara], these were important plants. Their extensive root systems and rhizomes were cooked by steaming, and chewed to remove starch. The remaining fibre was used to make string (Australian National Botanic Gardens, 1998). The young shoots were eaten raw as a salad. Records indicate that Typha was the most important food for people living along the Murray Darling River systems. Common Nardoo (Marsilea drummondii A. Braun) is another species commonly called a wetland weed. Indigenous Australians called this species Dullum Dullum [Wemba Wemba], and used it extensively as a food plant. The root masses were made into dough and cooked; Spores were either roasted or ground to make flour (Australian National Botanic Gardens, 1998).

As the oldest continuous culture on earth, Aboriginal Australia contains significant empirical knowledge and sophisticated understanding of plants as a whole. Much of this herbal knowledge may have been lost after colonisation of the continent in late-18th Century. However, many species with the label ‘weed’ have been used by Indigenous Australians for medicinal purposes. Some examples are provided in Table 1.

Table 1. Some examples of weeds used as medicines by Indigenous Australians1

Species & Common Name Recorded Uses

Centipeda cunninghamii (DC.) A.Braun & Asch. (Common Sneeze-weed)

This aromatic species has an objectionable odour. Aborigines placed the plants around campsites to repel ants, and used it for bad colds.

Centipeda thespidiodes F. Muell (Desert Sneeze-weed)

Aborigines have used this aromatic species from arid areas, to prepare a decoction for colds, sore throat and sore eyes, and as a poultice for sprains.

Cleome viscosa L. (Tick-weed)

Aborigines used Cleome to relieve headaches, and sometimes, to treat wounds and ulcers.

Pseudognaphalium luteoalbum (L.) Hilliard & B.L. Burtt (Jersey Cudweed)

A drink prepared from this Composite has been used by Aborigines for general sickness.

Pterocaulon serrulatum (Mont.) Guill. (Toothed Ragweed)

Aborigines have used the sticky, aromatic leaves of this herb to treat head colds. The scent was inhaled by chewing, by preparing a decoction or simply by crushing. Some tribes have used the leaves for dressing wounds.

Urtica incisa Poir. (Stinging Nettle, Scrub Nettle)

Urtication, flogging with nettles, is a traditional European method of treating rheumatism. Aborigines have done the same, beating affected parts with a bunch of nettle leaves.

1Source: Cribb and Cribb (1981)

Uses of Weeds as Medicinal Plants and Source of Pharmaceuticals

‘Weeds are not a pain, but a blessing’ is the by-line of one website (www.detox.net.au), which promotes the consumption of a variety of herbaceous weeds for their therapeutic values. However, the practice is not very common, despite the fact that more than 500 Australian plant species growing in the wild have been used as medicines, either in Australia or overseas (Cribb and Cribb, 1981). Early settlers introduced and cultivated many for herbal remedies or for culinary purposes, and some of these may have later escaped to become weeds or wild plants (Dwyer, 2006)15. However, examples of plants considered ‘weeds’ used as medicines are not as numerous in Australia as in Chinese Medicine or in Ayurvedic Medicine. The predominant Australian medicinal ‘weeds’ are mostly derived from Europe, China, and other Pacific Rim countries, and there are a very large number of species (see Cribb and Cribb, 1981). Table 2 provides some outstanding examples, to illustrate the point.

The subject of weeds as medicinal plants for various traditional and modern cultures, all over the world is vast and a review is beyond the scope of this essay. However, there is strong recent evidence, discussed below, that weeds are major sources of pharmaceuticals in the modern sense. Stepp and Moerman (2001), and Stepp (2004) showed that weed species form a substantially higher proportion of source plants in pharmacopoeias than would be expected from their proportion in the general flora. These studies provide valuable insights of the potential of weedy taxa and the promise they hold for drug discovery. Secondary compounds in colonising species are important chemical defenses against herbivory, and are implicated in allelopathy as well, where chemicals released by one plant affect the growth of other plants (Rice, 1984; Putnam, 1994; Rizvi et al., 1992).

Feeny (1976) and Coley et al. (1985) attributed the presence of biologically active compounds in different plants to two major anti-herbivory chemical defense strategies. The first strategy is the reliance of long-living plants to concentrate metabolically inactive immobile compounds (quantitative defenses), such as tannins and lignins, which reduce digestibility. These compounds are not biological toxins, and anti-herbivory is largely dependent on having a sufficiently high quantity, to make leaves unpalatable. The second strategy, used by ephemeral, successional, short-lived, or colonising species is one that relies on low molecular weight, mobile (qualitative defenses), such as alkaloids, cardiac glycosides or terpenoids (Feeny, 1976). For these fast-growing, often herbaceous plants with short-lived leaves, deterrence of herbivores is achieved by small amounts of highly potent, biologically active compounds, and many species have been found to possess such chemicals.

The evidence of allelochemicals in plants has generated considerable interest in colonising species as sources of compounds of high pharmacological value. Stepp (2004) draws attention to the well-known example of a plant from disturbed habitats- Catharanthus roseus (L.) G. Don, (Holm et al., 1977)16, which is a weed of exceptional value to modern pharmacology. This plant provides two of the most important cancer treatment drugs in the world, vincristine and vinblastine.

15 Apart from Prickly Pear (Opuntia sp.), which is listed in the manifest of the First Fleet, ‘Jalap’ is one of the plants taken on board at Rio de Janeiro in August 1787 (Collins, 1971, quoted by Dwyer, 2006). It is probably related to blue-morninglory (Ipomoea indica Burm. Syn. I. congesta R.Br.). The colonial use of Ipomoea as a purgative may have contributed to their widespread cultivation. Several Ipomoea species are now environmental weeds. 16 In Sri Lanka Catharanthus roseus is often found growing in disturbed habitats, such as cemeteries; the local Sinhalese name ‘Mini Mal’ reflects this, meaning ‘Plant found associated with cemeteries’.

Table 2. Other examples of weeds used as medicinal herbs in Australia

Species & Common Name

Recorded Uses

Centella asiatica (L.) Urban (Gotu Kola)

As in Asian countries, Gotu-Kola is valued in Australia as a medicinal herb, and rarely treated as a weed. Leaf extracts and ‘Gotu Kola Tea’ products are available in herbal shops at $ 10 for 50 tea bags, an indication that users value the product.

Hypericum perforatum L. (St. John’s Wort)

This European herb is valued as a remedy for mild depression and nervous exhaustion. It is harvested from the wild because of the large market in Europe and North America. Hypericin, an alkaloid in the plant, causes photosensitisation and blisters in cattle and sheep (CRC, 2000).

Taraxacum officinale Webb (Dandelion)

Dandelion, a troublesome weed1, is a well-known medicinal plant. Drugs from its root are sold as a diuretic, and used in treating disorders related to the liver, gall bladder, and digestive system, or in treatments of chronic conditions such as arthritis, skin disorders, upper respiratory mucus, and gout.

Silybum marianum (L.) Gaertn. (Variegated Thistle

A widespread, common weed in pastures or home gardens, Silybum is used in treating liver related disorders, or gastro-intestinal problems. It is the seed of the milk thistle that is used medicinally. Seeds can be consumed raw, usually freshly milled, made into a tea and used as a hydro-alcoholic extract. Extracts of the seed are also presented in tablet form.

Capsella bursa-pastoris (L.) Medik (Shepherd’s Purse)

An extract of the plant has been used widely to arrest bleeding during the First World War (Dwyer 2006). Auld and Medd (1992) suggested that this and other medicinal uses might be the reason for its widespread occurrence throughout Australia.

Commelina cyanea R. Br.) (Scurvy Weed)

Early European settlers used Scurvy Weed, a widespread species in NSW, to avoid or alleviate scurvy.

1Source: www.yourhealth.com.au; www.herbsarespecial.com.au

In the last few decades considerable attention has been focused on the medicinal plants used by traditional peoples. There is evidence that many cultures in the Brazilian Amazon preferred secondary forest and disturbed habitats for procuring medicinal plants (Voeks, 1996; Stepp and Moerman, 2001 and references therein). The list of species used as medicines by highland Tzeltal Maya of Chiapas, Mexico (Stepp and Moerman, 2001) includes many common weeds. Some cosmopolitan species, such as Fennel (Foeniculum vulgare Mill.), Lantana (Lantana camara L.), Mint (Mentha spicata L.), Plantain (Plantago major L.), Castor-Oil-Plant (Ricinus communis L.), Sorrel (Rumex crispus L.), Salvia (Salvia lavanduloides H.B.K.), Nightshade (Solanum americanum Miller), Dandelion, and Wild Sunflower [Tithonia diversifolia (Hemsl.) A. Gray], are common in disturbed habitats and roadsides. The list also contains genera such as Ambrosia, Bryophyllum, Erigeron, Ocimum, and Verbena, which have species implicated in therapeutic medicines of many cultures. Other taxa in the list, such as Tobacco (Nicotiana tabacum L.), Nectarine [Prunus persica (L.) Batsch], and Guava (Psidium guajava L.), are not considered as weeds by most people.

The dandelion is an example of a well-known cosmopolitan colonising species. Schütz et al. (2006) compiled its phytochemical and pharmacological profiles, in order to ascertain which compounds may be linked to known therapeutic effects. They concluded that the traditional use of dandelion to treat digestive disorders is supported by its pharmacological profile. However, they did not find a good correlation of known properties of various polyphenolics and sesquiterpenes with other supposed health-promoting properties of dandelion extracts from leaves or roots, e.g. anti-inflammatory, anti-carcinogenic and anti-oxidative activities. They suggested further studies of the plant’s constituents, and assessment of their pharmacological activities in humans, as future priorities.

It is clear that new pharmaceuticals are likely to be found in colonising plants, and, as Stepp (2004) suggested, weeds need to be given more attention as potential sources of phytomedicines. This is important because 80% of the world population continue to rely mainly on traditional medicines for their health care (Gurib-Fakim, 2006). Health authorities world wide, therefore, have a responsibility to understand better both the positive and negative properties of the active constituents in these plants, since mis-use or over-use could compromise public safety. Investigating therapeutically or allopathically active compounds in colonising plants also presents the scientific challenge of elucidating their chemistry and identifying opportunities for the future development of medicines (Schütz et al., 2006). Another, reason to encourage further study is the possible relationship of human ecology, and biochemical defenses that may have evolved in colonising plants. It is possible to hypothesize that the spectrum of bioactive compounds may be wider in certain taxa, and the compounds could be concentrated in leaves and shoots, as a response by weeds to increasing grazing pressure in landscapes modified by man for animal farming.

Edible Weeds As in most countries, Australians use a variety of plant species as food. However, the use of weeds as food is no longer common among those of European descent. On the other hand, most Australians of South and South-East Asian ethnic origins, attach a high value to several ‘weedy’ species as foods. ‘Kang Kung’ (Ipomoea aquatica Forssk.), several amaranths (Amaranthus spp.), including Amaranthus viridis L., Marsh Water Cress [Rorippa palustris (L.) Besser], Fennel, and Purslane (Portulaca oleracea L.) are examples of edible herbage weeds valued by such communities. All of these are commonly available for purchase in markets and shopping centres in major Australian cities, albeit in small quantities.

Records indicate that early settlers in Australia used a variety of wild or feral plants as vegetable foods. Examples are: Cobblers pegs (Bidens pilosa L.), Dandelion, Plantain (Plantago major L.), flick weed [Cardamine hirsuta (DC.) Hook. f.], Chickweed [Stellaria media (L). Vill.], Wild Carrot (Daucus carota L.), Wild Mustard (Sinapsis alba L.), Scurvy Weed, Sorrel (Rumex spp.), Clover (Trifolium spp.), Variegated Thistle, Fat hen (Chenopodium album L.) and Purslane. Such edible weeds are promoted for consumption by various web sites, magazine articles and books, although their actual use is insignificant, except for a few species seen in markets.

The importance of the food value of many colonising species cannot be ignored. The case of the Sri Lankan community contributing to the spread of Alligator Weed [Alternanthera philoxeroides (Mart.) Griseb] in Australia through mis-identification is a classic case (Gunasekera and Bonilla, 1999). New immigrants from Sri Lanka mistook Alligator Weed for a similar species, a popular vegetable in Sri Lanka: “Mukunuwenna” [Alternanthera sessilis (L.) R. Br. ex DC.]. This example demonstrates how the fondness of immigrant groups for edible herbs, which are traditional foods in their home countries, could lead to increased spread of a species. Successful educational campaigns have arrested the problem. Sri Lankan Australians, nowadays eat the native Alternanthera species- A.

denticulata R. Brown and A. sessilis itself, which have been distributed as substitute vegetables. Both species are sometimes regarded as weeds.

Taro (Colocasia esculenta) is another valuable food plant with strong colonising attributes. Nutritionally, Taro compares favourably with other major root crops of the world. The U.S. National Academy of Science (1975) named Taro as a potential food plant of the future. Taro’s significance as a food crop is well recognized in Africa and Asia-Pacific countries (Matthews, 2004) and in Central America (Gomez-Beloz and Rivero, 2006). It is a common food found in Australian markets yet the plant has been considered to pose an ecological threat to tropical Queensland and parts of NSW (Groves et al., 2005).

The problems posed by plants with colonising abilities are recurrent themes in developed countries. Many edible ‘weedy’ species are no longer valued as food, possibly because no high yielding varieties have been developed for commercial use. The issue though, can be resolved by a balanced point of view, which recognises the high economic value and utilization potential of species like Taro to some societies, while acknowledging the unsuitability of certain taxa for particular landscapes, at a particular time. Taxa like Taro are too important for human welfare to be summarily dismissed as ‘invasives’.

Weeds, allelopathy and allelochemicals Harper defined plant ‘interference’ as any physical or chemical mechanism that results in the reduction of the growth of one plant, over time, due to the presence of another plant (Harper, 1977, p 151). One component of this interference is competition, which is the process whereby plants interfere with the growth of neighboring plants by competing for growth-limiting resources, including space, light, nutrients, and moisture. The other major component of plant interference is allelopathy, mediated through the release of chemicals by one species into the environment of another (Putnam and Duke, 1974; 1978; Rice, 1984; Putnam, 1986; 1988a; 1988b; 1994; Rizvi et al., 1992). Plants release chemicals through roots, rhizomes, leaves, stems, and litter, and these could have beneficial or deleterious effects on other organisms. However, allelopathic mechanisms are difficult to separate from interference by competition under field conditions, because interactions usually occur through the complex chemical matrix of soil; hence, it has been difficult to demonstrate a causal relationship (Zimdahl, 1999). The literature on the subject is vast and growing.

The compounds involved are secondary metabolites of plants and include a variety of alkaloids, terpenes and steroids, flavonoids, organic acids, aldehydes, aromatic acids, simple saturated lactones and tannins. The possible role of such compounds as chemical defences of plants has been discussed (Feeny, 1976; Coley et al., 1985). As Whittaker (1970) suggested, some of the repellents and toxicants could be seen as “evolutionary expressions of quiet antagonism of a plant to its enemies”. Lovett suggested (1989) that allelochemical signals could be a part of a plant’s defences, serving to offset their sedentary habit. They could also be a means of conveying messages- as part of a network of communication in which disparate organisms give similar responses to similar compounds, or families of compounds.

During the last 30 years, the potential impacts of allelopathy on agriculture have been described and discussed in detail (Putnam and Duke, 1978; Rice, 1984; Putnam, 1986; Putnam and Weston, 1986; Qasem and Foy 2001; Weston and Duke 2003). More recently, evidence was presented that allelopathy may explain how certain colonising taxa become ‘invaders’ in distant ecosystems. For example, Callaway et al. (2004) reported much stronger allelopathic effects in invaded than in native ranges for Centaurea maculosa Lam. through root exudates. These exudates, largely containing oxalic acid, appeared to be more harmful to species that had not experienced them before, and perhaps, had not evolved tolerance.

Putnam and Duke (1974) first explored the possibility of utilizing allelopathic cucumber varieties or accessions to suppress weed growth in fields. Since then, the

possibility of developing weed-suppressive crops has been explored. The allelopathic potentials of numerous crops such as barley (Lovett and Hoult, 1995), oats (Fay and Duke, 1977), sunflower (Leather, 1983), sorghum (Nimbal et al., 1996), Rice (Dilday et al., 1998; Olofsdotter et al. 1999) and wheat (Wu et al., 1999) have been reported. Allelopathic crop cultivars are likely to be developed further, for potential use in the field of sustainable, low input weed management. However, these may require genetic engineering, by insertion of genes that produce allelochemicals, rather than conventional breeding, because of the multi-genic nature of allelochemical biosynthesis (Kim and Shin, 2005).

There is considerable interest in utilising allelopathy phenomena as a management tool, and there is evidence that colonising species could be useful in this regard. An early example of this is Slootweg’s discovery in 1956 that marigold (Tagetes erecta L.) planted into soil, subsequently provided control of the lesion nematode Pratylenchus penetrans in Daffodil (Narcissus tazetta L.) (Slootweg, 1956). Since then, a large number of publications have reported the suppressive effects of marigolds on nematode populations, whether utilized as a cover crop, rotation crop, green manure, or source of nematode-antagonistic extracts (Miller and Ahrens, 1969; Chitwood, 2002). One of the more successful field trials showed that rotations of Tagetes patula L. or T. erecta provided economic control of P. penetrans on tobacco (Nicotiana tabacum) for two successive years (Reynolds et al., 2000).

Recent research by Ploeg (1999; 2000) concluded that marigolds consistently suppressed Pratylenchus spp., Meloidogyne incognita and M. javanica than M. hapla and M. arenaria. Examination of nine cultivars of three Tagetes species indicated that the four major Meloidogyne species reproduced on T. signata Bartl., but not on some varieties of T. erecta and T. patula. The most conspicuous differences among the four nematode species was in their reproduction on tomato (Lycopersicon esculentum L.), transplanted into soil containing marigolds that had had their shoots removed. A few cultivars suppressed all four species; one suppressed only M. hapla and another suppressed all four species except M. hapla. Control provided by growing T. patula for eight weeks before transplanting tomatoes was more effective than control achieved by amending soil with incorporated roots or shoots. The general conclusions from Ploeg’s studies are that the nematicidal effects of marigold may be not as strong, or as specific, as has been claimed in the past. Incorporating marigold plants into the soil may not have a substantial effect on M. incognita infestation of a subsequent crop, but may give non-specific growth benefits to the crop. Ploeg (2000) pointed out that marigolds may be used as a component of an integrated nematode management strategy, and this would be useful, particularly when cropping is conducted under sub-optimal conditions.

Along similar lines, Anver and Alam (2000) described the application of oilseed cakes containing extracts of Neem (Azadirachta indica A. Juss), Castor (Ricinus communis L.), Mustard (Brassica campestris L.) and Rocket salad (Eruca sativa Mill.) to amend soil under controlled conditions. This application reduced the multiplication of two species of nematodes, producing effects similar to those of commercial nematicides. Practical applications may be in protecting several leguminous crops in the Indian sub-continent. With the increasing costs of commercial nematicides, the development and implementation of such strategies are becoming ever more urgent, particularly in developing countries.

In another example, Tsuzuki and Dong (2003) reported that pellets of Buckwheat (Fagopyrum spp.), containing allelochemicals (ferulic and caffeic acids), were inhibitory to several plant species under controlled conditions. Most importantly, in the field, these pellets inhibited weeds in rice fields, without adverse effects on the crop. Weed suppression activity of Buckwheat has been long known to farmers. Recently, Fuji et al. (2005) confirmed strong weed suppression activity in Common Buckwheat (Fagopyrum esculentum Moench), and Tartary Buckwheat (Fagopyrum tataricum Gaertner). Their field studies showed that up to 75% weed suppression could be obtained by using Buckwheat as a cover crop, compared

with Buckwheat free plots. Gallic acid, rutin, and fagomine have been identified as the most important allelochemicals from Buckwheats.

The possible utilization of allelopathic weeds for control of rice weeds in Vietnam was demonstrated recently by Xuan et al. (2005). These researchers established that nineteen species of common weeds, applied at 1-2 tons ha-1, caused a 70% reduction in rice weeds and 20% increase in rice yield. Among the most effective residues, which caused >80% reduction of rice weeds and simultaneously increased rice yields by ≈20-80%, were shoots of Alfalfa (Medicago sativa L.), Jerusalem Artichoke (Helianthus tuberosum L.), Oleander (Nerium oleander L.), Billy Goat Weed (Ageratum conyzoides L.) and Leucaena leucocephala. (Syn Leucaena glauca Benth.).

Leucaena is considered undesirable in Australia. As a fast growing nitrogen-fixing legume tree that produces cheap fuel wood and fodder, it has been a species of considerable interest to sustainable cropping and agroforestry systems for at least three decades. Budelman (1988) reported that Leucaena mulch broke down too quickly and did not provide effective weed suppression in fields. In the same studies, leaf residues of two other fast-growing legumes, Gliricidia sepium (Jacq.) Steud (Syn Gliricidia maculata H.B.K.) and Flemingia macrophylla (Willd.) Merr., provided effective weed suppression. Chandrasena et al. (1989) also reported on the allelopathic weed suppression by Gliricidia mulch. Multi-purpose trees legumes like Leucaena, Gliricidia and Flemingia are sometimes branded as ‘invasives’ that could damage ecosystems. Yet, they are highly valued taxa in many tropical and subtropical regions of the world.

These studies, and many others, indicate that colonising plants would be important in the search for sustainable and low input agricultural systems. Continuing to explore allelopathic phenomena for practical weed suppression in fields, as well as to identify novel bioactive compounds is clearly a worthwhile activity.

Colonising Aquatic species and their potential uses Compiling the world’s worst weeds, Holm et al. (1977) listed only ten aquatic species as posing serious problems. These included Water Hyacinth [Eichhornia crassipes (Mart.) Solms], Salvinia (Salvinia molesta D. S. Mitchell), Alligator Weed, and Water Lettuce (Pistia stratiotes L.). The number of species of concern has now grown to about three dozen (Charudattan 2001), and includes Phragmites australis and Typha spp. Despite the challenges posed by many aquatic weeds, the question arises as to whether they have only negative effects. Usually, aquatic weeds are the symptoms of degradation of the quality of water bodies and waterways that has been caused by human activities.

The beneficial effects of aquatic species, and opportunities for their utilization has been recognised in many parts of the world (Mitchell 1974; Gaudet 1974; Soerjani 1977; Charudattan 2001). Most people would agree that excessive growth of aquatic weeds is unacceptable, but there is enough evidence to indicate that because of the crucial roles they play, some level of aquatic macrophyte presence is necessary for the health of water bodies. While emergent, submerged, and floating macrophytes may be the main primary producers in shallow or clear aquatic ecosystems, phytoplankton serve that role in deeper or more turned water bodies. Aquatic plants contribute to maintaining ecological stability in the water environment by not only being primary producers, but also by their important roles in nutrient recycling. They are also food for consumers, and their organic matter feed detrital food chains. Submerged and emergent macrophytes trap, consolidate, and stabilize sediments, reducing water turbidity. Through photosynthesis, they increase dissolved oxygen in the water column. A variety of invertebrates, including snails and crustaceans, and vertebrates- small fish and fingerlings, and amphibians, depend on the shelter, cover and food associated with aquatic plants and their roots. The relationship of maintaining healthy

food chains is reflected in increased abundance of aquatic birds associated with water bodies that have a diverse aquatic macrophyte component.

However, the aquatic species, which provide the afore-mentioned benefits, can also be a nuisance. Prevalence of aquatic weeds with colonising abilities is associated, in most cases, with disturbance of natural ecosystems by human activities, such as creation of large reservoirs or irrigation canal networks. Adverse human impacts on creek systems, rivers, wetlands, and water bodies occur largely through direct sewage effluent pollution, or indirect nutrient enrichment because of intensive farming and development-related activities. Increased urbanization has caused pollution of waterways, particularly in major population centres. In human-modified environments, it is common to find one kind of aquatic species, or another, beginning to dominate over other biotic control factors in that system. In developed countries, vast resources are expended by authorities to fix this symptom, although the cause is commonly human disturbance. Developing countries often lack the resources necessary for such action.

Most specialists would agree that the management of aquatic species anywhere in the world has not been very successful, even with the integration of preventative control programmes with mechanical, chemical, biological, and other control methods. This leads to the question, can the utilization of aquatic weeds be integrated into management, so that the overall process of integrated control becomes environmentally, socially, and economically, more responsible? In the following sections, three well-known colonising taxa are discussed, to explore possibilities of utilization, instead of control alone.

Water Hyacinth- Beautiful Blue Devil?

Although Water Hyacinth is now widely distributed, it is outlawed in Australia because it can interfere with navigation and with water use in dams and irrigation schemes. Most Australians would reject any suggestion for the utilization of Water Hyacinth, despite evidence of the potential of this extraordinary plant, which has been known for decades.

In many parts of the world, Water Hyacinth infestations have caused significant economic costs, by reducing access to water and navigation (Vietmeyer 1975, Holm et al. 1977; Charudattan 2001). Nevertheless, uses of Water Hyacinth around the world are substantial. For instance, Vietmeyer (1975) reported its use to create floating vegetable gardens by peasant farmers in Bangladesh and Burma. Heaping fertile bottom sediments and organic muck on top of densely packed stands of Water Hyacinth and reeds, allows crops like Eggplant, Cabbage, Cauliflower, Cucumber and Beans to be grown in the presence of ample nutrients and moisture. Its other uses- as forage, silage, compost, green manure and in commercial potting mixes, as well as raw material for paper, handicrafts and various other chemicals, are also well known (Vietmeyer, 1975).

Water Hyacinth has been long used to remove pollutants from water. In some early studies, under Indonesian conditions, Soerjani (1977) reported that the potential for annual removal of N and P by Water Hyacinth from water was 313-777 kg N/ha and 96-238 kg P/ha with a population equivalent of 78-190 and 70-175 persons, respectively. More recently, Ingole and Bhole (2003) and Tiwari et al. (2007) have discussed the potential of using Water Hyacinth as a biofiltration method to remove heavy metals- lead (Pb), chromium (Cr), zinc (Zn), manganese (Mn) and copper (Cu) from polluted water bodies in India. Other studies (Giraldo and Garzo´n, 2002; Misbahuddin and Fariduddin, 2002; Trivedy and Pattanshetty, 2002; Williams, 2002; Singhal and Rai, 2003; Jayaweera and Kasturiarachchi, 2004) also demonstrate its potential for use in wastewater treatment ponds to remove organic and inorganic pollutants from agricultural and domestic wastes, and industry effluents.

The study from Bangladesh (Misbahuddin and Fariduddin, 2002) indicates how Water Hyacinth can be used as a natural and cheap means of removing arsenic from contaminated

drinking water at the household level. The effectiveness of removal depended on factors, including the amount of Water Hyacinth available for uptake, amount of arsenic present in the water, duration of exposure, and presence of sunlight and air. Ebel et al. (2007) also identified an important practical application for Water Hyacinth, to clean up cyanide (CN) from effluents produced in small-scale gold mining in South America- an industry where the use of CN is poorly regulated and waste treatment is insufficient. The authors demonstrated that Water Hyacinth could be used to remove CN, because of its high biomass production, and tolerance to cyanide and metals. They argued that, despite the species causing major problems in some parts of the world, the risk of using it in closed and controlled treatment ponds in areas where the plant is already present should be acceptable (Ebel et al., 2007).

Once called the ‘Beautiful Blue Devil’ (Vietmeyer, 1975), possibilities with Water Hyacinth seem endless. The plant’s remarkable capacities to convert solar energy to plant biomass efficiently, propagate itself, and maintain populations are the reason why it is so successful. They are the same reasons that make it amenable to easy manipulation. The luxuriant growth of Water Hyacinth in polluted water bodies is often an indication of eutrophication and unbalanced ecosystem processes. Ghabbour et al. (2004) isolated humic acids from leaves, stems and roots of Water Hyacinth growing in the Nile Delta in Egypt, and suggested that these acids confer the strong metal and organic solute binding capacity to the species. They reported that higher concentrations of the acids in the plant tissues are directly correlated with its metal-binding functions.

However, in putting Water Hyacinth to practical uses, it is probably necessary to think beyond the traditional way, and design engineering systems that can contain, as well as use the power of the plants to absorb and retain urban and industrial pollutants. Instead of focusing only on control, a changed emphasis like utilization is likely to lead to effective management of the species, where its extensive populations are undesirable.

Common Reed- Phragmites australis

The cosmopolitan Common Reed- Phragmites australis is sometimes regarded as a weed, but at other times, recognized as a highly beneficial plant. In a recent ethnobotanical study, Kiviat and Hamilton (2001) established the importance of Phragmites to a large number of Native American Indian tribes. The technological importance of the species in non-industrial settings is related to the characteristics of the plant, especially the culms, which are strong, hollow, lightweight, buoyant, rapidly growing, and slow-decomposing. Some of Phragimites’ uses (i.e. matting, basketry, young shoots as food, boat construction) are similar to uses of Scirpus spp. and Typha spp., whereas other uses (arrowshaft, fire drill shaft, splint) are related to its special qualities, which are rarely found in other robust marsh graminoids (Kiviat and Hamilton, 2001). The multiple uses of the species to Indigenous Australians are also well recognised.

Following European settlement of the North American continent, factors related to human disturbances appear to have contributed to the expansion of Phragmites (Kiviat and Hamilton, 2001), bringing the species into conflict with humans. Among the factors, which may have caused spread are intensive agriculture, introduced livestock, diminished numbers of free-ranging animals, hydrological disruption of rivers, increased salinity in tidal wetlands, and general pollution of freshwater marshes by de-icing salts, fertilizers, sewage, and sediments. Reduced competition from other wetland species and woody plants, which are less abundant due to land clearing, may have also contributed to greater invasiveness. Recent introductions of more invasive genotypes from other continents may have also played a role (Marks et al., 1994). These themes are common in many parts of the world, including Australia, although discussions on the ‘human factor’ in causing disturbances that create conditions favourable to colonising species, such as Phragmites are often muted.

In Australia, Phragmites is abundant everywhere- in marshes and swamps, along streams, lakes, ponds, and ditches. There is significant interest in using it to control shore erosion due to wave action, and to stabilise river and canal banks that have been degraded by past land-use practices. In addition, Phragmites is also a favoured species for use in Constructed Wetlands, which have been gaining in popularity as a reduced cost and low maintenance technology for treating wastewater from urbanised areas. Macrophytes, such as Phragmites, are an essential component of these systems for removing polluting wastes at sources in a catchment, particularly in cases where total elimination of the problem is not possible. Phragmites’ colonising attributes- i.e. fast growth, profuse seed set, vegetative reproduction from rhizomes that could reach up to 10 m in length, and wide ecological amplitude, i.e. tolerance of heavy metals, moderate salinity, water level fluctuations from 15 cm below soil surface to 15 cm above; and burning) are the reasons why it is favoured. Many wildlife enthusiasts also appreciate the habitat and food value provided by Common Reed, particularly for birds and other animals. The importance of Phragmites reed beds for roosting and breeding of many of the world’s threatened and endangered birds, is also well documented within ornithology literature.

Cattails or Cumbungi- Typha spp.- Weed Problem or Useful Plant?

In 1975, Morton, writing in Economic Botany, posed the above question. He suggested that Typha spp., subject to costly control measures in many parts of the world, deserve study in relational to potential for industrial utilization, as an alternative to destruction (Morton, 1975). Various uses of Typha spp. reported from around the world have been recently reviewed by Mitich (2000). The following is a brief summary.

Cattail stands provide food and cover for wildlife, and several species have been grown as ornamental plants in water landscapes, and to control erosion and stabilise banks. As construction material, Cattail stems and leaves have been used to make various objects ranging from sandals to sailboats; the dried and flattened stem has been used for weaving. Fibres from leaves and stems have been used in papermaking to produce a strong paper. Cattail floss has a wide array of uses including stuffing for cushions and use as an insulating and sound proofing material. Young plants, rootstocks, flower spikes, pollen and seeds have been used as human and animal foods, and various parts of the plant have been used for medicinal purposes. Cattail seeds yield a drying oil. On the negative side, Cattails have been reported as toxic to humans and animals on some occasions, and to offer breeding ground for mosquitoes. They have also been implicated in causing increased silting, obstruction of fishing and recreation, and increased water loss from reservoirs (Holm et al., 1977).

That Cattails accumulate heavy metals has been known for a long time (Taylor and Crowder, 1983), and their applications in wastewater treatments have been widely reported (Lan et al., 1992). Using Typha orientalis in wastewater treatment experiments in Australia, Cary and Weerts (1984) reported uptake values of about 60 g nitrogen (N), 8 g phosphorus (P), 73 g potassium (K), 12 g calcium (Ca), 6 g magnesium (Mg), and 1 g sodium (Na) per m2 of Typha. However, because of the ‘weedy’ nature of the species, the authors avoided recommending the species for Constructed Wetlands in Australia.

The capacity of Typha spp. to withstand changes in water level and produce extensive biomass is well known (Grace, 1989). There are also indications of different ecotypes based on latitudinal differences. For instance, under Australian conditions, Typha orientalis grew up to 3 m and produced 33.4 shoots m-2, (Roberts and Ganf, 1986), while a study in Japan (Inoue and Tsuchya, 2006) reported that it grew up to 2 m and produced 328 shoots m-2. Research in growth strategies of Typha spp., biomass and resource allocation, under different water depths (Grace 1989) or under hypoxic conditions (Matsui and Tsuchiya, 2006), continue to be of interest, because of the potential to use the species in diverse applications.

Freshwater biodiversity represents the most highly threatened global ecosystem, with over one-half the wetlands already lost to agriculture or urban conversion, and reductions in flows from irrigation diversions or hydropower. The colonising abilities of both Phragmites and Typha spp. are much needed attributes in restoring wetlands globally, and for creating sustainable freshwater ecosystems, to redress the imbalance.

As pollutant removal from wastewater occurs, great quantities of macrophyte biomass are produced. In general, the consensus is that plants must be harvested for the most effective removal of pollutants. This leads to large amount of biomass being available for different uses like conversion to compost or feed supplements for animals. Recent research (Ciria et al., 2005) has highlighted the potential for using the biomass produced by Phragmites and Typha spp. as bio-fuel, and this may provide a good example of utilization of aquatic species on a scale that is large enough to be sustained.

All three species, discussed in this Section epitomize the dilemma we have. In both developing and developed countries, there are many situations where Phragmites and Typha spp. will come into conflict with human interests; but their utilitarian values cannot be doubted. In well-managed and sustainable landscapes, they would be invaluable; and societies, which value plants, will recognize and use their potential.

Use of ‘weedy’ species in Phytoremediation Soils frequently receive a wide range of contaminants from industrial activities, sewage sludge disposal, metal processing, and energy production and, in many cases, remediation by chemical or physical means can be both expensive and intrusive to ecosystems. Phytoremediation (the use of plants and plant processes to remove, degrade, or render harmless hazardous materials, such as nonvolatile hydrocarbons and immobile inorganic matter, including heavy metals, present in the soil or groundwater) offers the possibility of alternative treatments. Many plant species, including trees, crops, grasses, and weeds, accumulate a wide range of heavy metals, and this makes them suitable for in situ cleanup of large volumes of low to moderately contaminated soils (Baker and Walker, 1990; Cunningham et al., 1995; Khan et al., 2000; Kumar et al., 1995). The attributes of ‘pioneering’ species- i.e. fast growth and biomass production, wide tolerance of environmental stresses, and capacity to maintain high population densities, make them particularly attractive for use in phytoremediation.

There are many examples of phytoremediation to eliminate pollutants and restore polluted soil or degraded land (Migliore et al., 2000; Chen et al., 2003), and there are potential applications both in developed countries, and in rapidly industrializing countries, such as China, where increasing heavy metal contamination of soil is common due to the rapid urbanization, mining, and industrialization. The use of weedy species to eliminate heavy metals from contaminated sites has received significant attention (Guo and Huang, 2002; Huang et al., 2001). For instance, the aggressive weed Reynoutria japonica Houtt was shown to have considerable ability to accumulate the heavy metals copper (Cu), zinc (Zn), lead (Pb), and cadmium (Cd) (Hulina and Dumija, 1999). Wang and Liu (2002) also reported that species like Water Hyacinth, Redroot Amaranth (Amaranthus retroflexus L.), and Maiden’s Tears [Silene vulgaris (Moench) Garcke] had a strong tendency for uptake and accumulation of Cu, Zn, and chromium (Cr) in heavy-metal polluted environments. In a similar study, Wei and Zhou (2004) showed that Dandelion (Taraxacum mongolicum Hand-Mazz.), Nightshade (Solanum nigrum L.) and Canadian horseweed [Conyza canadensis (L.) Cronq.] strongly tolerated pollution by cadmium, either alone or together with lead, copper and zinc and exhibited characteristics of hyper-accumulators.

Wu et al. (2005) recently demonstrated the ability of 17 individual weed species and five species combinations to accumulate lead from soil. Lead concentrations in soil did not affect the biomass production of any weed species under the two different growth environments. The implication was that this tolerance of lead by weed species could be used for cleanup of lead-polluted soil. The study also recorded differences between species in their ability to accumulate lead. For instance, Japanese Clover [Kummerowia striata (Tunb.) Schindl.], Chinese ixeris (Ixeris chinensis Thunb.), Rye grass (Lolium perenne L.), Wild Oat (Avena fatua L.), and Plantain (Plantago virginica L.) accumulated more lead than others, demonstrating that it would be possible to select weed species, and use species mixes and combinations to remediate Pb-polluted soil. These studies are an indication that weedy taxa differ in their efficiency in acquisition of heavy metal elements (Guo and Huang, 2002; Huang et al., 2001). The way forward may be to use species combinations of weedy species, with their non-weedy relatives, but phytoremediation certainly shows promise for wider exploitation of weedy characteristics in programmes for restoring ecosystems.

Weeds and Biodiversity The term ‘biodiversity’ was coined in U.S.A. to bring political attention to the goal of protecting species (Sagoff, 2005). It is now widely used in biology and ecology to stress that nature is a complex matrix of species-interactions between all living forms. However, the tendency of some authors to exclude non-native or ‘alien’ species in a given area, including cultivated or engineered ones, from ‘biodiversity’, has led to confusion (Sagoff, 2005). Are weeds not part of the same biodiversity?

Biotic interactions between weeds and their environment, particularly the dependence of animals on weeds, have been of interest for several decades (Altieri, 1999). Fifty years ago, John Harper used the example of Ragwort (Senecio jacobaea L.) to conceptualise the biotic relationships a weed has in food chains, and expressed the view that Ragwort control in agricultural fields ‘might affect all the organisms in the food chain’ (Harper, 1957). Of late, in Britain and other European countries, interest in weeds as important components of the biodiversity of farmlands and the landscape has been heightened. This is due to recognition that, by focusing only on reducing weeds in arable farms, irreparable damage is being done to the biological diversity in vast areas of agricultural and semi-rural landscapes. Robinson and Sutherland (2002) reported that most arable weeds in British farms have declined since the 19th century, and losses have accelerated towards the end of the 20th century with intensification of agriculture. A number of farmland birds have declined (Siriwardena et al., 1998), and invertebrate animals have shown similar declines. Changes in agriculture, such as the intensity of farming, drainage, introduction of new crops and monocultures, changes in the sowing season, and increased use of agrochemicals (fertilizers, herbicides and insecticides) have all been implicated (Krebs et al., 1999). The development of more ecologically sustainable production systems, maintaining some weed species within crops is seen as a new challenge (Marshall, 2001; 2002; Marshall et al., 2003; Storkey, 2006; Storkey and Westbury, 2007).

In the present discourse, weeds are recognised as not just a part of biological diversity of nature, but as critical components of ecosystems, as suggested by Harper (1957). Managing weeds within crops to support biodiversity inevitably involves the risk of reducing crop yields and the long-term build-up of problem weed communities. However, a pragmatic approach to reconciling biodiversity with crop production is to manage low populations of

‘beneficial’ weed species17. Marshall et al. (2003) recently identified a range of such plant species on the basis three attributes: (a) the number of insect species associated with particular weeds; (b) the number of and the importance of weed seeds in the diet of farmland birds; and (c) a competitive ability index. As an example, their evaluation of several weeds resulted in some species, such as Poa annua L. and Polygonum aviculare L., as being more important for biodiversity in arable systems than others like Alopecurus myosuroides Huds. and Veronica persica Poiret. The challenge will be to apply such methods to select species and populations that may be tolerated, in order to achieve a sustainable ecological balance (Marshall et al., 2003).

Storkey (2006) extended this approach to identify groups of weeds that are similar in terms of the balance between their competitive ability and biodiversity value. This ‘trait-based analysis’ identified two beneficial groups that could potentially be managed to reconcile biodiversity with crop production. To illustrate, these were: (a) spring germinating species, including Chenopodium album L., Persicaria maculosa Gray and Polygonum aviculare L., and (b) autumn germinating species including Poa annua L., Senecio vulgaris L. and Stellaria media (L.) Vill. Species in this latter group have a growth form that is complementary to the crop. That is, they generally grow below the crop canopy, forming an understorey, and mature early, avoiding crop competition late in the season. As a result, they utilize, in part, resources that the crop is unable to capture, and the total productivity of the system is increased. Storkey suggested this as a possible ‘win–win’ situation, where a certain amount of weed biomass is maintained, and is associated with a minimal yield loss.

Recognizing beneficial impacts of weeds, and tolerating some weed biomass is not new to traditional farming systems, including ‘slash and burn’ agriculture, which is still common in many parts of rural South and South-East Asia, Africa and Central America. In many situations, rural farmers do not see the need to clear vast areas of vegetation for farming. This may reflect, in most cases, avoidance of spending energy, which they can ill afford. However, there is enough anecdotal evidence from many countries that rural farming systems place significant values on plant as resources, although these cases are poorly documented at present. Of those that are recorded (Chacon and Gleissman, 1982, quoted by Altieri, 1988), corn farmers in lowland tropics of Mexico are known to leave some areas of weeds unattended in their farmlands. The basis of this intentional ‘relaxed’ weeding is a classification of non-crop plants according to their use potential and effects on soil and crops. The Mexican farmers recognized 21 plants as ‘bad weeds’ and 20 as ‘good weeds’ that serve as food, medicines, ceremonial materials, teas, and soil improvers, etc.

In Britain, the impact of declining populations of arable weeds on farmland biodiversity is being widely acknowledged. Government-sponsored schemes encourage the provision of plant communities as a resource for higher trophic groups (Storkey and Westbury, 2007). The aim is to strike a balance between adequate weed control, including the prevention of weed seed build-up, and the requirement for some plants to support biological diversity. The challenges are related to managing problem weeds (mostly perennials), while sustaining beneficial species (mostly annuals) at economically acceptable levels within a functionally diverse farming landscape. Integrated weed management, including the use of selective herbicides, such as foliar-acting graminicides, are important tools in meeting this challenge as they control many perennial grasses, while leaving most broadleaf species (Marshall, 2001).

17 As opposed to ‘invasive’ weeds, beneficial weeds are defined as species that provide low levels of competition with an arable crop and have potential values as resources for higher trophic consumer groups.

Positive impacts of colonising species on biodiversity in Australia are not well researched, at least for the time being. However, there are some significant studies (Date et al., 1996; Buckley et al., 2006; Gosper and Vivian-Smith, 2006), which indicate the benefits of introduced and ‘invasive’ species to birds. For instance, Date et al. (1996) reported that in Northern New South Wales, several indigenous fruigivores rely heavily on the fruits of Camphor laurel [Cinnamomum camphora (L.) Nees], one of the plants introduced by European settlers. Similarly, Gosper and Vivian-Smith (2006) established the dependence of fruigivorous birds on Lantana (Lantana camara L.), a widespread, aggressive scrub, well known in the tropical and sub-tropical regions of the world18. The general view is that ‘alien’ species may have helped buffer some fauna populations from the broad-scale destruction of their natural habitats. Studies, like those of Gosper and Vivian-Smith (2006) have focused on examining Australian native fruit-bearing species, which might be promoted as replacements for Lantana in management programmes.

Whilst future research and discussions on weeds and their role in biodiversity will continue, introduced species, including ‘invasive’ species, stimulate new species and community interactions through antagonistic or mutualistic relationships with soil biota all the time Reinhart and Callaway (2006). The potential enriching effect of colonising species on biotic communities should also be recognised, just as much as negative impacts, as some of the interactions and impacts of ‘new immigrants’ are bound to be beneficial.

Role of Weeds in Agro-ecology and Permaculture Apart from the potential to utilize weeds in ways discussed previously, there are other perspectives, which need recognition. For instance, most farmers recognize the value of perennials weeds, with their extensive tap roots and rhizome systems that penetrate deep into the subsoil, breaking it up and enabling the less vigorous roots of other plants, particularly annuals to penetrate further into the soil. Breaking up of the subsoil also improves drainage and creates microsites for other biota. Many weeds protect topsoil from the eroding forces of rain, wind, and sun, especially when the vegetation cover is poor, and this is widely recognised in landscapes. After disturbances caused by flooding or bushfire, colonising species are the first to grow, providing the first cover of vegetation, which enables biological activity and renewal processes to continue.

Colonising species play an integral part in establishing biological diversity, the keystone in the field of agro-ecology. In this field, the fundamental principle is to reduce monoculture cropping, and to encourage the development of self-sustaining systems based on the understanding that biodiversity regulates the functioning of agro-ecosystems. Plant and animal interactions provide essential ecological services, such as recycling of nutrients, and breakdown of organic matter. These are important in securing soil fertility. Plants also regulate the microclimate of agro-ecosystems, and their complex interactions lead to suppression of undesirable organisms, and detoxification of noxious chemicals.

Altieri (1999) recently reviewed various options of agro-ecosystem design to enhance functional biodiversity in agricultural landscapes. Essential features include incorporating windbreaks, shelterbelts, and living fences in crop/field boundaries to improve habitat for wildlife and beneficial insects. Such areas also provide sources of wood, organic matter, resources for pollinating bees, and, in addition, modify wind speed and the microclimate. Establishing such ‘biological corridors’ allows for the circulation of biodiversity across large-scale agricultural landscapes. Colonising species are very much a part of the biological integrity and stability sought within such systems.

18 Lantana is also a Weed of National Significance in Australia.

Altieri (999) also suggested that, in order to improve biodiversity within annual or perennial cropping systems, there should be better utilisation of crop diversification options that have been known since the early days of agriculture. Rotation of crops and multiple cropping systems are effective management strategies for annual monocultures. For perennial cropping systems, much research suggests that cover crops transform orchards and vineyards into agro-ecosystems of increasing ecological diversity and stability by influencing key processes and system components. Many cover crops are fast growing colonising species. They provide habitat for beneficial insects, activate soil biology, add organic matter continually to soil, modify microclimate within cropping systems and some can fix atmospheric nitrogen. In such agro-ecological approaches, no species is regarded as particularly undesirable. Species with colonising attributes are essential, because of their capacity for nutrient recycling, biomass production and accumulation, and soil amendment through both physical breaking up and the chemical changes in soil they sponsor.

The above concepts are reflected in the Permaculture movement, which originated out of Landcare movements in the 1970's. The permaculture strategy, accepts a greater role for trees, perennial plants and fast growing species in the stabilisation of degraded, human-modified landscapes. In that sense, it is the equivalent of a large-scale revegetation strategy. As in agro-ecology, the primary agenda of the permaculture movement is to assist people to become more self reliant through the design and development of productive and sustainable gardens and farms. This agenda includes mimicking what nature does, producing food locally with minimal outside inputs, creating healthy ecosystems, building soil, constructing housing based on local, renewable resources, ending pollution, erosion and degradation of landscapes. Colonising plants are generally not condemned within this framework; instead, the permaculturist’s view is that every plant has its uses and weeds are no exception. An often-used slogan in the movement is ‘one person's weed is another's medicine or building material’. Although the number of people committed to the austere lifestyle promoted by the permaculture movement is still minuscule, its attitudes, favouring sustainable land use thinking, resonate with the view that plant resources should not be devalued.

Agro-ecological or permaculture designs are intended to reach noble goals, namely, improved living systems, which are both economically and ecological sustainable, and are ‘in tune’ with the locally available biodiversity. However, to be sustainable in developed countries, such as Australia, these approaches need primarily to meet the aspirations of landholders and farmers and, secondly, to contribute to meeting the broader environmental and socio-economic and political agendas of governments.

Weeds as Fertilizing material, Compost and Fodder Charles Frederick Milspaugh (1892) highlighted the long-known potential of weeds as fertilizing material. He examined the potential for composting 49 common weeds as fertilizers by analysing the nitrogen, phosphorus, and potassium contents (NKP ratios) of the species and converting those figures to money values, ‘according to the commercial value of the minerals at the time’ and showed that properly composted weeds (dry matter only) were worth on average, $ 9.60 per ton. He encouraged proper composting and argued that farmers would profit by utilizing weeds, without wasting them.

In general, conversion of green waste- leaves, branches and grass-clippings from parks, gardens, roadsides and homes, is common in Australia, and colonising species make up a substantial part of this material. Although composting is a sustainable practice, ineffective commercial composting has been implicated in spread of certain weeds through seeds and propagules, which are not dead.

Aquatic weeds harvested from water bodies have also proven useful as compost. In one recent example from Australia, a large Salvinia infestation in the Hawkesbury-Nepean

River in the Sydney basin, NSW, was mechanically harvested over several months prior to December 2004. Approximately 70,000 m3 of Salvinia was removed. Composting was an option to utilize this large biomass. Dorahy et al. (2007) reported that applying the composted material as a soil conditioner, at a depth of 20 mm, supplied an equivalent of 700 and 95 kg of nitrogen and phosphorus per ha, and suggested that the material could be used in establishment of pasture. Whilst promoting the benefits, Dorahy et al. (2007) also identified the potential risk of spreading Alligator Weed onto land with the composted Salvinia, and recommended ways by which this threat could be managed.

Pasture non-availability or poor pasture are serious problems in many parts of Australia, especially during winter and in drought periods. This is common in other parts of the world as well. In such situations, colonising plants will continue to provide fodder for herbivorous animals. Farmers in Western Australia, surveyed by the Agricultural Bureau, reported that several colonising taxa are important food for grazing animals, and in non-cropping situations, they are actually beneficial (Williams et al., 1987).

Discussion

‘Weediness’ is in the eye of the beholder’

"There is nothing either good or bad, but thinking makes it so." - W. Shakespeare19; a famous quote, which resonates with the current dilemma regarding weeds. Most people, growers, farmers, biologists, and even politicians will agree that weeds are useful resources, as demonstrated by the examples discussed above. This word ‘weed’, an epithet of human invention and a dubious ‘cultural construct’, has caused so much confusion within the field of Weed Science. In the world of plants, it simply does not exist.

After much debate in the 1980s, the Weed Science Society of America adopted the definition of a weed as: ‘a plant growing where it is not desired’. The European Weed Science Society extended this idea to include: ‘any plant or vegetation, excluding fungi, interfering with the objectives or requirements of people’. The Australian definitions have a strong slant towards the European version, i.e. ‘a weed is a species that adversely affects biodiversity, the economy or society’ (Groves et al 2005) or ‘a weed as a plant, which has, or has the potential to have, a detrimental effect on economic, conservation, or social values in Australia’ (ARMCANZ 1999). These definitions have effectively removed the accepted view that weeds are generally a symptom of a man-made crisis, but not the cause of it.

As discussed in this essay, even plants with strong colonising attributes may be of value in various situations, at different times, or to different people. It may be worthwhile to broaden the common definition that ‘a weed is a plant growing where it is not desired’, to capture the idea that weeds present problems to some people, and certainly not at all times or at all places. In that regard, Kloot’s definition from Australia (Kloot 1987) that a weed, ‘is a plant that may interfere with human activity in one way or another and, thus, has come to be regarded negatively by at least part of the society’ is a reasonable one to consider.

Bunting (1960) pointed out that that ‘weeds are pioneers of secondary succession, of which the weedy arable field is a special case’ Baker (1965) defined a weed as ‘a plant …if, in any specified geographical area, its populations grow entirely or predominantly in situations markedly disturbed by man, (without, of course, being a deliberately cultivated plant)’. Zimdahl (1999) favoured the view that weeds are: ‘those plants that are successful in disturbed environments, are fast growing, and, are often, but not always herbaceous’.

19 This famous quote, spoken by Hamlet in a conversation with a friend, Rosencrantz, is from William Shakespeare’s Hamlet (Act II, Scene 2).

Definitions of a similar nature are common within the Weed Science ‘body of knowledge’. Part of the negative attitude towards weeds, is due to the lack of clarity on man’s own role in creating disturbed habitats. As Bunting said, ‘an essential feature of all of man’s activities, in agriculture or otherwise, is the production of open, or at least disturbed, habitats’. Downplaying man’s part in creating much of the disturbance to which ‘colonising plants’ naturally respond, has led to misconceptions, and, over time, to the hardened attitude towards ‘weedy’ taxa.

Until about the 1970s, weed issues were discussed only from the perspective that they were problems to crop production. In subsequent decades, attention turned to weeds as environmental problems affecting landscapes. Weeds are now projected as a major problem affecting all aspects of our daily lives, and much energy and resources are spent fighting them. However, given the evidence, is the problem really weeds or is it our perception of them? As Auld (2004) states, weed occurrence is inevitable. This is because man’s activities will continue to disturb environments, and movement of people across continents will exacerbate introductions into new areas. There is no simple remedy for the weed problem in its many manifestations. Therefore, whilst continuing to study the reasons why colonising species come to dominate landscapes, the best management strategy would be to use several control tactics in an integrated manner. Management approaches need to be designed to prevent new introductions to disturbed areas, and to minimise the undesirable impacts where the conflict exists between man and the colonising species. However, this must be done with a proper ecological understanding, and with a balanced view of economic implications, but without dramatising weed issues, and certainly avoiding messages that create a visceral dislike for some plant taxa.

Conservation of biodiversity

In essence, biodiversity is a term to describe the assemblages of organisms that have evolved together to exploit the resources of particular areas or environments in ways that maximise the cycling of energy and nutrients within a certain area- an ecosystem. Plants are key components of such systems, with different species filling a variety of roles. By their nature, ecosystems are dynamic, and change in response both to environmental changes and to the adaptive evolution of their constituent species.

We also need to realise that the aim of mechanized, large-scale, modern agriculture, as opposed to subsistence agriculture, is to export nutrients and energy from an area. Therefore, there is a natural antithesis between agriculture and conservation of biodiversity- we can never completely reconcile the two, but can we minimise the conflict? Humans clearly present the greatest threat to nature, wilderness and biodiversity, of which both people and colonising species are constituent parts. This message needs to be given much more intense publicity, to achieve a better balance between human greed, genuine development aspirations of nations, and global biological diversity. There are strong moral, aesthetic, social and economic reasons for protecting biodiversity. Marshall, (2001), points out that, “a culture, which encourages respect for nature and wildlife is preferable to one that does not”.

Nowadays Australians value the preservation of what remains of the continent’s native landscapes. They recognise that much damage has been done to native vegetation since European settlement in the 18th century and that farming practices of the past, particularly large-scale land clearing for pastoralism, have caused enormous losses of native vegetation communities, and native animals.

It has been argued strongly in numerous publications that colonising species threaten biodiversity. However, it should be evident that they also provide benefits that are not yet fully understood. A key message from agro-ecology is that, if correctly assembled in time and space, biological diversity is capable of repairing landscapes, sponsoring soil fertility,

protecting crops, and increasing productivity. Given the opportunity, colonising species will be at the forefront of remediation of land that had once supported large forests. Australian studies on such possibilities are almost non-existent, whereas much has been written about the negative impacts of ‘invasives’. To redress the imbalance, we need to reappraise our approach to the discipline of Weed Science.

The search for self-sustaining, low-input, diversified, and energy-efficient agricultural systems is now a major worldwide concern. A key strategy in sustainable agriculture is to restore both the structural heterogeneity at the different spatial scales of field, farm, and landscape (Krebs et al., 1999) and the functional biodiversity of the landscapes (Altieri, 1999). This can be achieved in time through age-old practices like crop rotations and sequences, and in space in the form of cover crops, intercropping, agroforestry, and crop/livestock mixtures. Plants with colonising abilities are very much a part of biodiversity in arable fields, or in non-cropped areas; they add much to biotic interactions by way of their highly developed chemical defences, and they perform a variety of ecosystem services. Creation of appropriate biologically diverse cropping and non-cropping landscapes is likely to result in increased pest regulation through restoration of natural control of insect pests, nematodes and diseases caused by fungi, bacteria and viruses, and to produce optimal nutrient recycling and soil conservation by activating soil biota. All of these factors should lead to more sustainable yields, better energy conservation, and less dependence on external inputs. However, adoption of such approaches will depend on demonstration of the synergies of biodiversity conservation and the economic profitability of farming.

Weeds will always present a stimulating challenge

Without any doubt, weeds have contributed enormously to the development of the ‘innovative thinkers’ amongst humans. From the earliest development of subsistence agriculture to the modern agricultural revolutions, they have challenged our way of living. This has led to inventions from the earliest digging sticks to sophisticated machinery and to the development of agronomy, irrigation, and surveying and of the agrochemical industry.

The development of genetically modified crops by multi-national biotechnology companies during the last two decades also falls into this category. The advances in biotechnology that have created modified organisms that can be grown and harvested on a large scale to feed growing populations must rate high in the continuum of human innovation. The stimulus that profits can be made from such innovations came from the challenge offered by weeds.

Colonising species will always be the ultimate survivors in the conflict with man. Rather than a zero tolerance towards particular taxa, it would seem reasonable to propose ‘ecological management’ of problematic populations, with an eye on their potential benefits, on a ‘case-by-case’ basis. This requires synecological20 models that capture all of the key factors that govern the dynamics of populations in a given location. These differ from autecological, ‘species-led’ approaches that are more concerned with the reactions of single species. The agro-ecology approaches, extensively covered by Altieri (1999), are invaluable ecological risk management models, in the sense that the practices promoted have long-proven benefits in ecosystems. They also encourage positive thinking, linking people with nature, and stimulate people to closely integrate with all components of biological diversity, including ‘colonising species.

20 Synecology is the branch of ecology that deals with the structure and development of entire ecological communities, their interactions with the chemical and physical environment, and the complex interrelationships between all plants and animals within them.

Will there be a change in attitude?

The hardened attitude towards colonising plants in Australia, and perhaps in other affluent countries, is related to the profits that can be made by individual landholders through farming. Despite the fact that agricultural production represents only a declining 3% of the nation’s GDP, Australian farmers form a powerful lobby, whose concerns receive the attention of many politicians. Many farmers resist change because of personal learning experiences, property-related factors, and economic factors. Shifting the emphasis of weeds from ‘foe’ to friend’ will require strong campaigning, both from governmental and non-governmental initiatives. Relaxing the attitude towards colonising species may come with time, and this can be hastened by economic incentives to manage biodiversity within farmlands, and landscapes, as has been done in European countries.

Given that colonisation of the continent is only two centuries old, there may also be a lack of collective ‘traditional wisdom’ upon which sustainable societies and cultures are usually based. For early Australians of European origins, much of their ‘traditional knowledge’ of wild plants, herbs and weeds came from European farming. Many growers and farmers, with deeply entrenched perceptions, often mistrust alternatives. The collective wisdom of all weed scientists and weed managers in Australia may be required to bring about a change in farmers’ mind-set, as well as an attitude change amongst landholders.

Pratley (1995) noted that the discipline of Weed Science in Australia is poorly developed in terms of educational programmes, and that there has been limited support by government agencies and funding bodies for weed research. He was enthusiastic about the establishment of the government-sponsored Cooperative Research Centre (CRC), for Weed Management, the first of its kind that came into being around that time. The CRC has improved the status of knowledge about invasive species. It has also increased public awareness in Australia of potential undesirable effects on ecosystems of unmanaged population explosions. However, perhaps because it was focused on the waging a war against weeds, the CRC has not investigated the potential value of investment in their utilization. This probably reflects the prevailing lack of discussion on possibilities, and the commonly held view that most weeds are ‘alien’ species of no value. There is also a general view that utilization of weeds, such as harvested aquatic weeds, is costly and, therefore, not economically worthwhile (Howard and Harley, 1998). Nevertheless, not everyone will agree that human endeavour should always be measured in monetary terms. Investment in utilization of weeds is one area, which is justified not just because it is a good management practice, but also because provides a positive message for the public.

An Ethno-biological perspective- Link between Plants and Humans

In discussing the relative variety and intensity of uses of Phragmites by human groups, Kiviat and Hamilton (2001) suggested that the utility of a plant to humans is related to: (1) abundance and distribution of the plant; (2) length of time the plant and a human group have been in contact; (3) invention or transmission of traditional ecological knowledge of the plant; (4) ease of managing, acquiring, and processing the plant; (5) physical and chemical qualities of the plant (e.g. pharmaceutical or toxicological properties, fiber characteristics, nutritional composition); and (6) availability and quality of alternate taxa. These ideas reveal why some taxa are much valued, and others much disliked. Discussion of such ethno-biological perspectives is important in building better relationships of plants by humans, particularly in developed countries where the conflicts between the two are more profound.

The importance of traditional cultures, their wisdom and sustainable interactions with plants and animals is a routine subject in Anthropology, and Social Science. Interactions between the humanities and a discipline like Weed Science are not strong and hence both

sides may gain from a closer exchange of views. Journals dedicated to Ethnobotany and Economic Botany often carry articles relating to human uses of colonising plants. Increased appreciation of plant resources can be achieved by studying these ethno-biological appraisals, as well as by exercising more common sense. Improved understanding of plants of value to humanity may assist those who consider ‘weed risk assessments’ when deciding whether or not to list particularly resourceful taxa as ‘invasives’ that should be controlled at any cost. The presumption here is that ‘branding’ of taxa tends to stick in the minds of the public. Applying ‘a guilty until proven innocent’ approach to taxa with colonising abilities, as has been widely practiced in Australia, belies common sense, is disrespectful to nature, and may not be tenable for long.

The Way Forward

Making the case for utilization of weeds is not difficult. The compilation of existing knowledge from different cultures should assist this task and, in that sense, there is much to learn from the existing Economic Botany and Ethnobotany ‘body of knowledge’. Ishizuka (2001) stated that the biodiversity of weeds should be investigated in terms of both constant and changeable species-specific adaptive traits. For this reason, Weed Science should pay attention to the development of new aspects of research. His view was that disciplines such as agronomy, horticulture, agroforestry, ecology, biology, genetics, physiology, biochemistry, gene technology, and even economics may be useful for the clarification of biodiversity of weeds and should be integrated into Weed Science. It is hard to argue against these views, given the evidence presented. Harada (2001) pointed out that humans have for long used colonising species as foods, medicinal plants, animal feeds, housing materials, raw materials for handicrafts, ornaments, and so on. He considered that, before they are forgotten, priority should be given to the investigation and recording of the ways in which traditional cultures in the Asia-Pacific region have used weeds. He strongly argued for more cooperative research on utilization of weeds in the future.

This essay highlights the indifferent attitude in countries like Australia, towards the use of colonising plant species as resources. This may be a sign of the times we live in. On the other hand, traditional societies, including indigenous Australians, have used plants wisely and have ‘co-existed’ with them. Are there not lessons from previous generations, that plant resources should be respected rather than maligned? In the attempt to maximise agricultural production in developed countries, anything other than the crop plant whose yield brings profit, is regarded as undesirable. This view is seriously flawed and is not sustainable under the commonly accepted principles of Ecologically Sustainable Development, to which many affluent nations have committed.

Perhaps the ‘paradigm shift’ required in the field of Weed Science in the 21st Century is to recognise the potential of colonising plants and to find ways to integrate them into our lifestyle. The vast amount of literature on Ethnobotany and Economic Botany, some of which is referenced in this essay, seems to point to possibilities that cannot be ignored. Rather than over-dramatising the negative aspects of plants regarded as weeds, the Weed Science community, in Australia and elsewhere, needs to bring about a balance and to emphasize the utilitarian value of colonising plants, with their tenacity and vitality, and to reconsider the advantages of putting these into practical use. Utilization, instead of attempts to eradicate, these maligned taxa will lead to their effective management in most situations, where the potential undesirable effects of a large population are untenable.

By 2025 AD the global population is expected to reach 8.5 billion, of which 83% will be living in developing countries. Most would agree that one of the greatest challenges facing mankind is to increase food production in landscapes where productivity has been generally declining. The challenge is to increase food and fibre production in a sustainable

manner, while maintaining systems and landscapes for future generations. Therefore, particularly in the less affluent countries, a negative attitude towards any group of plants, including those that sometimes interfere with human affairs is unwarranted, and making such a mistake is not affordable. The examples discussed show why this is so.

In order to alleviate socio-economic hardships, and to conserve biological and cultural diversities, it is necessary to build on existing links between people and biological resources. The level of success of this, however, is dependent on accommodating local knowledge, aspirations and priorities of communities, including indigenous people and farmers, with some trade-offs between development and conservation initiatives. The ultimate goal must be for the present generation to be ‘custodians of landscape’, as indigenous Australians see themselves to be, instead of being exploiters, and this task requires positive messages and a proper appreciation of plant resources.

Conclusions Weeds are ‘colonising plants’ or ‘pioneers of secondary succession’ and need to be understood in this way. Wherever or whenever man disturbs a habitat, they will be among the first to make use of the opportunity of space. This ecological emphasis has been downplayed in a large number of publications, perhaps inadvertently, because the focus has been on weed control. Weeds are not the culprits; they are just a symptom of the real cause, which is ecologically destructive land-use practices by humans, including land clearing, overgrazing, and introductions of species for short-term profit. In natural or man-made ecosystems, many weeds serve valuable ecological functions that need more recognition. Examples of their complex biological interrelationships, such as providing resources for wildlife, slowing erosion, building soil, and generally enriching biological diversity need to be studied and given wider publicity.

Many species of plants are currently considered as invasive weeds. Much of the time publicity in developed countries is a blitz against weeds, often over-emphasizing the conflict man has with some species. The fact that so many colonising species grow and coexist in the same environments with native species and crops tends to be overlooked. Because of their adaptability, weeds will always compete with other plants, like crop plants, or slow-growing perennials. Whilst the economic consequences of this interference with crops have been clarified, the ecological consequences of colonising species are not well understood. Some generalities have been made, such as that weeds reduce biodiversity and the regeneration of native species, but these views overlook the fact that weeds themselves are part of biological diversity, enriching nature and stimulating biotic interactions all the time. Given that most weed invasions can never be reversed, they can only be controlled by reducing their populations to a below a level that might be construed as unacceptable, the challenge is to better understand the negative impacts of weeds on the environment.

In a strategic approach to managing weeds, the utility of these plant resources needs to be highlighted, and people should be encouraged to look at different ways of using them. The summary condemnation of plant taxa because we may not like to have them in particular situations or enterprises is not a sensible way to approach a s complex man-made problem. A much broader appreciation of the useful attributes of plants and their applications in improving the human condition is a high priority. Weeds are clearly highly successful plants, largely due to characteristics that confer superior colonising ability and competitiveness. As demonstrated in this essay, these attributes can be very useful, not just in repairing damaged ecosystems, but also in providing food and fibre for all animals, including humans.

As human enterprises expand, population increases and pursuit of material wealth continues, the mode of existence of some colonising plant taxa will increasingly clash with our existence. It is through no intrinsic fault of the plants themselves. The same attributes

that make a plant ‘invasive’ will be sought after under a different set of circumstances. The examples discussed demonstrate the two sides of the argument. The way forward is to broaden our understanding of plant resources, and their crucial role as integral parts of biological communities. In the case of taxa with strong colonising abilities, their resilience, tenacity, and capacity to adapt to environmental disturbances need to be recognised. Perhaps this understanding would help modify our attitudes, or allow us to avoid creating conflicts with plant taxa, and getting into situations from which we cannot win.

As in many other fields, it is necessary from time to time, to realign the focus of a scientific discipline, and Weed Science may have reached that stage. Whilst there is a vast amount of disparate literature, some of which has been reviewed in this essay, the future requires a ‘body of knowledge’ of utilization of colonising species to be established, so that present and future generations could benefit from that knowledge.

Acknowledgements

I thank Professor K. U. Kim for the invitation to write this essay. Dr. Peter Michael and David Eccles critically reviewed the document and provided valuable comments. Dilsiri Chandrasena is acknowledged for assisting with the review of literature.

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