INTRODUCTION

Suburban development has been implicated in the invasion of exotic plants in nearby native vegetationthrough an increase in soil nutrient levels. Studies ofthe effects of urbanization on terrestrial vegetation oron the riparian vegetation of streams have been conducted (e.g. Clements 1983; Lambert & Turner1987; Leishman 1990; Medina 1990; Rose &Fairweather 1997), but the impacts on within-streamvegetation have not previously been addressed. As noted by Leishman (1990), streams receive highinputs of nutrients from stormwater runoff and sewageoverflows during storms. They are thus prone to regular hydrological disturbance and receive highnumbers of propagules. Streams may therefore be particularly susceptible to weed invasion. Many streamsin the northern Sydney region flow from urban areas into remnant bushland or national parks, and represent sources of nutrient enrichment, subsequentweed invasion and degradation of conservationreserves.

It is probable that the variables that influence terrestrial plants will be largely similar to those affect-ing rooted aquatic plants and riparian vegetation, withthe obvious exception of increased disturbance fromflowing water. The invasion of exotic terrestrial plants

in northern Sydney has mainly been attributed to anincrease in soil phosphorus concentrations. It is widelyaccepted that phosphorus is a limiting nutrient inHawkesbury sandstone communities (Beadle 1953;Hannon 1961). Beadle (1953, 1966) suggested thatmesomorphic or annual exotic species cannot invadesoils derived from Hawkesbury sandstone withoutnutrient enrichment, specifically phosphorus enrich-ment. Clements (1983) attempted to explain the differences in urban and non-urban Hawkesbury sandstone vegetation communities by the increase inthe level of soil phosphorus. Three main plant group-ings in the area were identified (dry sclerophyll, wetsclerophyll/rainforest and urban) that were explainedon the basis of total soil phosphorus.

However, while soils derived from Hawkesburysandstone are low in total phosphorus concentration,they are also low in many other nutrients (Lambert &Turner 1987). Le Brocque and Buckney (1994) founda simple nutrient hypothesis inadequate to explain the variation in Hawkesbury sandstone vegetation.Instead, vegetation gradients were best explained bycomplex combinations of environmental variables, bothchemical and physical. It is therefore possible that,rather than phosphorus alone, a more complex combination of nutrients (or non-nutrient variables)may better explain both the distribution of native plantsin this region and their invasion by exotic species. Inorder to examine the relationships between vegetationpatterns and sediment characteristics in within-stream

Austral Ecology (2000) 25, 455–461

Urbanization and exotic plants in northern Sydney streams

SCOTT A. KING* AND ROD T. BUCKNEYDepartment of Environmental Sciences, University of Technology, Sydney, PO Box 123, Broadway,NSW 2007, Australia (Email: Scott.King@uts.edu.au)

Abstract The vegetation and sediment of urban and non-urban streams in the northern Sydney region werecompared to examine the possible effects of urbanization on within-stream vegetation. Many sediment character-istics were significantly different in urban streams. At least one exotic plant species was found in each urban streamsampled, but none were found in the non-urban streams. The presence of exotic species led to the overall num-ber and abundance of plant species being significantly higher in urban streams. Interestingly, the number and abun-dance of native species at the urban sites were the same as non-urban sites, but a different suite of species wasusually present. This suggests that urban streams favour exotic plants and certain native plants that are adaptedto the modified conditions. The differences between the plant communities in the urban and non-urban streamsappeared to be associated with the increased level of nutrients in the urban stream sediment. Several multivariatetechniques were used to assess the relative importance of individual nutrients, but no nutrients were directly asso-ciated with the observed differences. In particular, total phosphorus levels were less important in explaining thevegetation patterns than a combination of nutrients. It is therefore likely that the general increase of nutrients instream sediment has enhanced exotic invasion and altered stream plant communities in Sydney streams.

Key words: exotic plants, invasion, nutrient enrichment, stream vegetation, urbanization.

*Corresponding author.

communities, this exploratory study compared urbanand non-urban areas north of Sydney.

METHODS

Study region

The study streams were on the Hornsby Plateau, innorthern Sydney, New South Wales, Australia. Themajor outcropping rock formation forming the plateauis Hawkesbury sandstone that has been eroded to formdeep valleys, ragged narrow ridges, and tree-coveredslopes (Fairley & Moore 1989). The Hawkesbury sand-stone is mostly horizontally bedded, with occasionalminor shale and clay layers (Beadle 1962; Buchanan& Humphreys 1980; Rice & Westoby 1983). Theannual precipitation of the northern Sydney region isapproximately 1300 mm, and is not strongly seasonal(Rice & Westoby 1983).

Sampling

Thirty streams were sampled between April and July1996: 15 in catchments that had a substantial degreeof urbanization and 15 in non-urban (or minimally disturbed) catchments (Fig. 1). A catchment was con-sidered to be non-urban if the natural vegetation waslargely intact and disturbance was limited to occasionaltracks or roads. Streams were of first, second or thirdorder. Plant communities were sampled using a 1 m 321 m quadrat divided into seven 1 m 3 3 m sub-quadrats. The quadrat was placed along the edge of thestream, extending one metre into the water, startingfrom a randomly determined point. The quadrat wasnot strictly linear, but followed the curvature of thestream. Plants were only recorded if they were rootedin saturated sediment (no free-floating plants wereencountered in this study). Plants received a frequencyscore of 1–7 corresponding to the number of sub-quadrats they were found in. A quadrat was also established from the opposite bank of the stream. Thepair of quadrats was then moved up or down stream anominal distance of 50 m. Four quadrats were thusestablished at each stream. The abundance values ofthe species sampled from the four quadrats at eachstream were summed.

Most aquatic plants take the majority of their nutrients from the sediment (Best & Mantai 1978).Only in hypereutrophic waters is there significant phosphorus uptake from the water (Carignon & Kalff1980). For this reason, water samples were notanalysed. Four sediment samples of 120 mL were collected at each stream, one sample corresponding toeach of the quadrats. Samples from the top 10 cm ofthe sediment were collected from several locations

along the quadrat and bulked. These samples were thencombined with the sample from the opposite quadrat.All sediment samples collected at each stream werecombined before analysis.

Sediment analysis

The pH of the samples was determined on the saturated sediment. The aluminium, calcium, iron,potassium, sodium and magnesium cations wereextracted from the soil by washing with 1 mol L–1

ammonium acetate. The concentration of the ions wasthen determined using atomic absorption spectropho-tometry (AAS). The soil samples for the total phos-phorus analysis were digested in 100 mL tubes in adigestion block, using the single digestion method ofAllen (1989). The phosphorus concentration was thendetermined colourimetrically. The organic content ofthe sediment was determined by loss-on-ignition at550°C for 2 h. A particle size fractionation methodusing wet sieving was modified from Allen (1989). The particles were fractionated into two classes: sand,100 mm–2 mm; and silt and clay, <100 mm.

Statistical analysis

The soil parameters, species richness and abundancewere compared between urban and non-urban

456 S. A. KING AND R. T. BUCKNEY

Fig. 1. Location of study streams in the northern Sydneyregion. N, non-urban sites; U, urban sites.

areas using ANOVA after heterogeneity of variance wasevaluated by Levenes test. Calcium, aluminium, totalphosphorus, and total abundance were found to haveheterogenous variances and were log transformed andvariances re-checked prior to analysis. The trans-formation was successful in each case. Ninety-five per cent confidence intervals were determined on thetransformed data then back-transformed. All analyseswere conducted at the P = 0.05 level of significance.

Indicator species analysis, a divisive polythetic hierarchical clustering technique, was used to identifyspecies clusters [Hill et al. 1975; TWINSPAN (Tablefit Programs, Cambridgeshire, UK), Hill 1979]. Indirect ordination [correspondence analysis, CA; programCANOCO v4.0 (Microcomputer Power, New York,USA), ter Braak 1987] was used to identify vegetation gradients. Detrended correspondence analysis (DCA; program CANOCO v4.0 , ter Braak 1987) withdetrending-by-segments was used to give a quantitativemeasure of species turnover (beta diversity). The directordination technique of canonical correspondenceanalysis (CCA; program CANOCO v4.0, ter Braak 1987)was used to relate the environmental variables to thefloristic data. CCA directly relates the community variation to a set of environmental variables (ter Braak 1986). Species abundances were square-roottransformed for CA and CCA.

The BIO-ENV subroutine of the program PRIMER

(PLYMSOLVE, Plymouth, UK; Clarke & Ainsworth 1993)was used to identify the subset of environmental vari-ables that gave the best match between the floristic andenvironmental data sets. This is achieved by correlat-

ing the species-by-site matrix with the environmentaldata-by-site matrix, and choosing that subset of envi-ronmental variables that gives the highest rank correla-tion (harmonic (weighted Spearman)). BIO-ENV has thebenefit of extraneous variables reducing the correlation,unlike multiple regression. Following the suggestions ofClarke and Ainsworth (1993), total phosphorus, potas-sium, calcium, magnesium, iron, and aluminium werelog(x) transformed before using BIO-ENV.

RESULTS

Vegetation

Seventy-one species were encountered in this study, ofwhich 11 (15%) were exotic. Only native species werefound at the non-urban streams, but every urbanstream had at least one exotic species present. Oneurban stream had no native species.

Table 1 shows the frequency and number of nativeand exotic species found in the urban and non-urbanstreams. The mean number of species at urban streamswas significantly greater than at non-urban streams.However, the mean number of native species at urbanstreams was not significantly different from the meannumber at non-urban streams. The total frequency (theaddition of the frequency values of all species in thefour quadrats at each stream) was significantly greaterat urban streams than at non-urban streams. No difference was found between the abundance of nativespecies at urban and non-urban streams.

EXOTIC PLANTS IN SYDNEY STREAMS 457

Table 1. Number of species and frequency of within-stream vegetation in urban and non-urban streams

Mean no. of Mean no. of native Total frequency Total native species (SE) species (SE) (95% CI) frequency (SE)

Urban (n = 15) 10 (1.3) 4.9 (0.8) 44.4 (27.9–74.1) 18.4 (5.1)Non-urban (n = 15) 5.6 (0.8) 5.6 (0.8) 13.5 (7.7–23.0) 18.3 (3.1)P 0.008* 0.526 0.002* 0.982

*Significant at P = 0.05 (ANOVA).

Table 2. Sediment characteristics of urban and non-urban streams (n = 15 for both groups)

Variable Non-urban mean (95% CI) Urban mean (95% CI) P

pH 6.0 (5.8–6.3) 6.2 (6.1–6.5) 0.087Organic content (%) 1.3 (0.9–1.7) 1.8 (0.9–2.6) 0.338Silt (%) 1.0 (0.5–1.4) 2.6 (0.4–4.8) 0.124K (mg/100 g) 1.8 (1.5–2.1) 3.2 (2.0–4.5) 0.028*Ca (mg/100 g) 1.8 (0.9–3.1) 20.7 (13.4–31.7) 0.000*Mg (mg/100 g) 2.0 (1.1–3.0) 3.7 (2.4–5.0) 0.029*Fe (mg/100 g) 1.9 (1.5–2.4) 1.5 (1.0–1.9) 0.104Al (mg/100 g) 4.3 (3.4–5.4) 2.2 (1.8–2.7) 0.000*Total P (mg/100 g) 3.1 (2.2–4.2) 10.3 (6.8–15.2) 0.000*

*Significant at P = 0.05 (ANOVA).

Sediment analysis

The sediment analysis results are shown in Table 2.Sodium was excluded from all analyses as the concen-trations were too low for the AAS to give a reliable reading. The sediment concentrations of total phosphorus, calcium, potassium, and magnesium weresignificantly higher in the urban streams. The concen-tration of aluminium was significantly higher at thenon-urban streams.

Vegetation patterns

The ordination of the floristic data by CA is shown inFig. 2. Two main groups are separated on the first axis.One group consists solely of the non-urban streams,and the other solely urban streams. The non-urbanstreams are more variable in floristic composition thanthe urban streams, as shown by the greater spread onthe second axis.

The principal division of the species by TWINSPAN

(Table 3) produced one group containing only nativespecies that occurred at non-urban streams, andanother of ‘urban species’ that included both native andexotic species found at urban streams. Many of thenative species at urban sites were not found at non-urban sites.

The length of the first DCA axis was about 6.5 SD(standard deviation units) indicating complete speciesturnover along the axis. Sites that differ by four or more

SD normally have no species in common (ter Braak1995).

Vegetation-environment relationships

The CCA ordination diagrams (Figs 2 & 3) show sitesand species as points, and represent the environmentalvariables as arrows. Important environmental variableshave longer arrows than the less important (ter Braak1995). The first axis extracted (horizontal) representsthe linear combination of environmental variables thatexplains the main variation in vegetation composition.

Figure 3 shows the CCA ordination diagram for allstreams, and Fig. 4 shows the ordination of species.Forty-three per cent of the variation in the vegetationdata is explained by the measured environmental variables. The first two axes explain 19%. The samepattern as shown on the CA of sites (Fig. 2) is seen,with urban streams and non-urban streams being separated on the first axis. This shows high negative correlation with pH, aluminium and calcium. Totalphosphorus is shown to be relatively less important thanmost of the other parameters by the short arrow andweak correlation with the first axis (r = –0.272). Thustotal phosphorus is less successful at explaining the separation of urban and non-urban sites on the first axis than pH, aluminium, calcium or magnesium.Magnesium and calcium are also well correlated withthe second axis. The ordination of species (Fig. 4)shows the separation of the two groups identified bythe indicator species analysis (Table 3) on the primaryaxis. The native species identified as ‘urban species’ areintermingled with the exotic species.

458 S. A. KING AND R. T. BUCKNEY

Fig. 2. Ordination axes 1 and 2 from correspondenceanalysis of stream vegetation. Eigenvalues for axes 1 and 2are 0.869 and 0.485, respectively. s, urban streams; h, non-urban streams.

Table 3. Species groups identified by the indicator speciesanalysis

Urban species Non-urban species

Ageratina adenophora* Actinotus minorAgeratina riparia* Baumea rubiginosaAster subulatus* Empodisma minusCallitriche stagnalis* Gahnia sieberianaColocasia esculenta* Leptospermum polygalifoliumCyperus polystachyos Lepyrodia scariosaHydrocotyle bonariensis* Restio complanatusIsolepis inundata Schoenus melanostachysIsolepis prolifera* Schoenus paludosusLudwigia peruviana* Tristaniopsis laurinaPersicaria decipiensPersicaria strigosaPotamogeton tricarinatusRanunculus repens*Rorippa nasturtium-aquaticum*Tradescantia albiflora*

*Exotic species. Only species occurring at more than onesite shown.

The BIO-ENV routine found a combination of calcium and aluminium to give the highest correlation(r = 0.470) with the vegetation data. Calcium gave thehighest single correlation of 0.447. Phosphorus wasranked third after calcium and pH with r = 0.178.

DISCUSSION

The results show that streams in urban areas are susceptible to both the invasion of exotic plants and thealteration of native plant communities. The presenceof exotic species has led to the number and abundanceof plant species being significantly higher at urbanstreams. The number and abundance of native speciesat the urban sites was not different from non-urbansites, but the species have been replaced by differentnative species (Table 3). Many of the urban and non-urban sites had few or no species in common, leadingto high beta diversity. This suggests that exotic speciesare not simply utilizing under-exploited habitat, butthat urban streams have been modified by developmentin a way that favours exotic plants and certain nativeplants. The results also show that a general increase ofnutrients in urban stream sediment, with the exceptionof aluminium, has occurred. Similar results werefound by Preston (1995) and Hayes and Buckney(1995), although neither study examined aluminium.

It is likely that the increase in urban sediment nutrient levels has allowed the establishment not onlyof exotic species that are adapted to higher nutrient levels, but also the establishment of native species withsimilar characteristics that might not normally occurin the area. Plants identified as ‘urban species’ by theindicator species analysis may thus be indicators of highnutrient levels, either growing in streams with naturallyhigh nutrients or in streams with artificially enrichedsediment. Clements (1983) also found some nativespecies in urban areas to be grouped by clusteringanalysis with exotic species, due to similar preferencesfor moisture and nutrients.

Although CCA and BIO-ENV are based on differentunderlying algorithms, there was good agreementbetween their results. Both identified calcium and aluminium as good correlates of the vegetation pat-terns, although CCA also emphasized pH and mag-nesium. However, both techniques found totalphosphorus to be relatively less important. This seemssurprising because the importance of total soil phosphorus in the invasion of terrestrial Hawkesburysandstone communities was emphasized by Clements(1983) and Leishman (1990). The discrepancy may bedue to differences between stream and terrestrial com-munities; however, Clements (1983) and Leishman(1990) only examined total phosphorus, which was alsofound in this study to be significantly higher in theurban sediment. However, the differences in vegetation

EXOTIC PLANTS IN SYDNEY STREAMS 459

Fig. 3. Site-environment biplot of axes 1 and 2 from canonical correspondence analysis of stream vegetation andsediment data. Eigenvalues for axes 1 and 2 are 0.813 and 0.343, respectively, and together they explain 19% of the variation in the vegetation data. s, urban streams; h, non-urban streams. Arrows represent environmental variables: Al, aluminium; Ca, calcium; Fe, iron; K, potassium; Mg,magnesium; P, total phosphorus.

Fig. 4. Species–environment biplot of axes 1 and 2 fromcanonical correspondence analysis of stream vegetation andsediment data. d, native species; h, exotic species. Arrowsrepresent environmental variables: Al, aluminium; Ca, calcium; Fe, iron; K, potassium; Mg, magnesium; P, totalphosphorus. Only species with occurrences at more than onesite are shown.

could be better explained by a combination of nutrients. Other studies have also shown multiple environmental variables to be better correlates of vege-tation patterns than single nutrients (e.g. Margules et al. 1987; Le Brocque & Buckney 1994). In addition,Lyon and Sagers (1998), in a study of riparian vege-tation, found pH and elevation above water level to bewell correlated with a primary CCA axis. Soil particlesize was well correlated with the second axis.

It is unlikely that exotic plants occur in the urbanstreams simply because of the availability of propagules,independent of nutrient enrichment. Though this cannot be conclusively tested without experimentalmanipulations, the patterns found by the multivariateanalyses suggest that the plant communities were influ-enced by the sediment. If the distribution of plants wasnot related to the sediment nutrient levels, we wouldnot expect to find interpretable patterns. The use ofexploratory techniques, and the observational natureof the study, points to the environmental variables thatcorrelate best with the observed patterns. Variables thatwere not measured, such as flow or cover, wouldundoubtedly contribute, either on their own or becauseof correlation with the measured variables (Clarke1993). Stream communities are exposed to regularhydrological disturbance, so there would always besome variation that cannot be explained on the basisof sediment characteristics. Further studies will berequired in order to quantify the relative importanceof factors such as sediment and water chemistry, and stream physical characteristics, in determiningvegetation patterns.

Regardless of the effects of nutrient enrichment,exotic species cannot invade a stream unless viablepropagules of the species are present. The presence ofpropagules is often dependent on human habitation innearby areas (Clements 1983). Arthington andMitchell (1986) stated that people are usually the mainagents for dispersing exotic aquatic plants within andbetween continents. Within urban areas, propagulesmay be spread by vehicles (Schmidt 1989; Lonsdale &Lane 1994), particularly construction machinery, and propagules may also be present in topsoil used forlandscaping. Gardens and stormwater are also richsources. Indeed, the number of possible sources ofexotic propagules in urban areas means that invasionof nutrient-enriched streams is almost inevitable. Thissuggests that even if exotic plants could be removed orcontrolled, the original suite of native species may not tolerate the altered conditions. They may bereplaced with ‘weedy’ natives or natives adapted tohigher nutrient concentrations.

Urbanization has led to an increase in nutrients inthe sediment of urban streams, and it is likely that thealteration of the natural conditions has allowed exoticand native plants that are adapted to high nutrient levels to invade. Urbanization may thus affect the plant

communities of streams by favouring species adaptedto higher sediment nutrient levels than are naturallypresent.

ACKNOWLEDGEMENTS

Narelle Richardson and the technical staff of theDepartment of Environmental Sciences providedinvaluable help with the sediment analysis. The NSWNational Parks & Wildlife Service permitted samplingin Garigal and Ku-Ring-Gai Chase National Parks.Peter Fairweather and Jann Williams gave valuablecomments on the manuscript.

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