Conserving Compartments in Pollination Webs

Society for Conservation Biology

Conserving Compartments in Pollination WebsAuthor(s): Sarah A. CorbetSource: Conservation Biology, Vol. 14, No. 5 (Oct., 2000), pp. 1229-1231Published by: Wiley for Society for Conservation BiologyStable URL: http://www.jstor.org/stable/2641768 .

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Conserving Compartnents in Pollination Webs

Pollination systems in European agricultural landscapes are threatened by changes in land use, and some components of those systems are more vulnerable than others. Pollina- tion webs can be compared with food webs (Memmott 1999), and it has been suggested that food webs contain subwebs or compartments, such that there are more interactions within a compartment than across its bounds. Pimm and Lawton (1980) and Raffaelli and Hall (1992) tested food webs for compartmentalization. Both found it, but in only a small proportion of the food webs they analyzed. Compartmentalization is expected to be more common in webs involving mutualisms, and it was clear in a system of ant-plant mutualisms studied by Fonseca and Ganade (1996).

Compartmentalization is expected in pollination systems too. The nebu- lous concept of pollination syndromes recognizes functional groups of plant species that share similar floral attributes and visitor spectra, a puta- tive legacy of coevolution, however diffuse. Further, morphological and physiological constraints must limit the plant-pollinator species interactions that increase the fitness of either partner: a given pollinator can forage profitably only on plants that offer adequate, accessible rewards, and a given plant can be pollinated only by visitors with appropriate morphological characteristics (Corbet et al. 1995; Corbet 1997).

Compartments within a web may differ in ways relevant to conservation. Small insects such as flies, beetles, and small bees visit plant species with small, open flowers, in- cluding many annuals, whereas large insects such as large bees and butter-

flies, like flower-visiting birds, have higher energy demands and must visit flowers offering substantial energetic rewards-commonly, perennials (in- cluding monocarpic perennials, also known as biennials) (Brown et al. 1978; Bond 1994; Corbet et al. 1995).

Annual plowing may promote the small-insect/small-flower compartment but destroy the perennials that sustain the large-bee/large-flower compartment. The large-flowered perennials visited by large bees are often self-incompatible (Bond 1994; Cor- bet 1995). If pollinators are lost, plant species dependent on cross-pollinated seed for recruitment will be progres- sively eliminated, and persistence of self-incompatible plant species will depend on vegetative reproduction and on the longevity of individuals, clones, and habitats (Olesen 1992, 1994; Eriksson 1993; Bond 1994; Ole- sen &Jain 1994). Large bees are then expected to depend increasingly on nectar-rich plants that persist without cross-pollination, notably perennials with clonal growth. In disturbed landscapes where dispersal is neces- sary, they have to rely on plants that set seed without pollination, such as the apomictic bramble Rubus fruticosus agg.

In central and northern Europe, large bees are particularly impor- tant pollinators because of the endo- thermy that allows them to forage at low ambient temperatures (Stone & Willmer 1989; Corbet et al. 1995) and because of their ability to fly long distances between remnant habitat patches (Saville et al. 1997; Osborne et al. 1999). The first bees to invade isolated habitat islands remote from other "partial habitats" (Westrich 1996) such as nesting places, are the larger species (Gathmann et al. 1994;

Steffan-Dewenter & Tscharntke 1999). These, like some large butterfly species, can carry pollen substantial distances, helping to ameliorate the ad- verse genetic consequences of habitat fragmentation (Jennersten 1988; Kwak et al. 1991; Kunin, 1993).

The large-bee/large-flower compartment can be further subdivided ac- cording to the length of the tongue and corolla. Unlike North America, which lacks longer-tongued bumblebee species (Inouye 1976, cited in Brown et al. 1978), Europe has bumblebee species with a wide range of tongue lengths. Shorter-tongued bumblebee species differ from longer-tongued bumblebee species in the plant species visited; the longer- tongued species generally visit deeper flowers (Fussell & Corbet 1992). Fre- quent disturbance has resulted in re- placement of deep-flowered perennials by annuals, selectively affecting the longer-tongued bee compartment (Corbet 1995). The decline of Bom- bus species in western Europe, for example, has involved several longer- tongued species listed by Skovgaard (1936) as major visitors to red clover (Trifoliuumpratense) (Williams 1986; Rasmont 1988). In eastcentral En- gland, the loss of longer-tongued species in recent decades leaves the long- tongued Bombus bortorum and the moderate-tongued B. pascuorum as virtually the only effective pollinators in the longer-tongued-bumblebee/deep-corolla flower compartment (Williams 1986).

The two large-bee compartments may also differ in the extent to which honeybees can compete with, or substitute for, wild pollinators. Honey- bees, native to Europe but proba- bly not coevolved pollinators for its flora (Westerkamp 1991), may be

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1230 Issues in International Conservation Corbet

expected to compete with shorter- tongued bumblebees, which they re- semble in tongue length. Possible effects of competition, however, may also extend to the longer-tongued compartment if corolla biting by shorter-tongued bumblebees, such as Bombus terrestris, allows honeybees to deplete deep flowers, as in the red clover (Fussell 1992; Proctor et al. 1996). It will be interesting to see whether robbing by the recent immigrant B. terrestris in Tasmania similarly exacerbates competition between honeybees and flower-visiting birds (Paton, this issue).

The effects of biodiversity loss on ecosystem processes depend in part on attributes of individual species, particularly when species numbers are low (Lawton 1994; Anderson 2000). The degree of dependence of pollinators on individual plant species is generally low (Jordano 1987), but some plants may be regarded as keystone species. "Cornucopia" species such as bramble (Yeboah Gyan & Woodell 1987) or thyme (Thymus capitatus; Petanidou & Smets 1996) supply - abundant nectar to a range of insect species. Bluebells (Hyacin- thoides non-scripta) flower in spring when there is little alternative forage for longer-tongued bumblebee queens establishing nests (Corbet & Tiley 1999).

In some compartments, crops may be keystone species. Red clover, with its extended flowering season, is a major forage source for many Eu- ropean longer-tongued bumblebee species (Skovgaard 1936; Rasmont 1988). Rasmont (1988) attributed loss of bumblebee species in France and Belgium to the decline in forage legume crops as tractors replaced horses on the land. In contrast, ento- mophilous crops with brief flowering periods, such as flax or oilseed rape, are unlikely to be fully exploited by wild bees.

Triage is a system for allocating limited medical resources to those moderately serious cases that will benefit most from treatment, rather than to those that will die or survive

whether treated or not. Although an analogy is often drawn between triage and the setting of priorities for conservation, priorities are not cur- rently set this way. Instead, attention is focused on rare species. Little is known about the functional role of rare species in ecosystems (Lawton 1994), but they are likely to be involved in fewer interactions than common keystone species. Conser- vation effort in the United Kingdom is directed toward the now rare Bom- bus sylvarum but not toward B. hortorum, now almost the sole pollinator in the longer-tongued-bee/ deep-corolla compartment in some ar- able regions. By the time B. hortorum is rare enough to receive conservation attention, this pollination compartment may not be retrievable. Perhaps a higher proportion of conservation effort should go to keystone plant species on which many pollinators depend and to maintenance of particularly vulnerable compartments such as the longer-tongued-bee/deep-corolla flower compartment (McIntyre et al. 1992; Corbet 1995).

The research needed to underpin this approach will involve identifying compartments in pollination webs and the keystone species that sustain them and assessing their vulnerability. This can be done with existing research techniques. For example, the seed set of standard plants set out in the field has been used to evaluate pollinator availability, and the provi- sioning time of sofitary bees in standard trap nests has been used to evaluate forage availability (Gathmann et al. 1994; Steffan-Dewenter & Tscham- tke 1999). The age structure can help in identifying populations of perennial plants in which recruitment by seed has declined (Olesen 1994). A similar approach might be used to monitor the status of selected compartments. There have been few comprehensive studies of pollination webs (Jordano 1987; Herrera 1990; Elberling & Olesen 1999; Memmott 1999), and the search for compartments in pollination webs has hardly begun. In theoretical studies of food

webs, compartments are assumed absent unless they can be demonstrated rigorously (Pimm & Lawton 1980). In the context of conservation of pollination systems, however, it may be safer to assume that they are present unless shown to be absent.

Acknowledgments

I thank L. Dicks, C. Kremen, J. Os- borne, T. Ricketts, and T. Tscharn- tke for helpful suggestions.

Sarah A. Corbet*

Department of Zoology, Downing Street, Cambridge CB2 3EJ, United Kingdom, email [email protected] *Address correspondence to: 1 St. Loy Cottages, St. Buryan, Penzance TR19 6DH, United Kingdom.

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Conserving Compartments in Pollination Webs