A third approach to community description is based on the guild, or functional group, concept (Cummins 1973, Hawkins and MacMahon 1989, Körner 1993, Root 1967, Simberloff and Dayan 1991). The guild concept was originally proposed by Root (1967), who defined a guild as a group of species, regardless of taxonomic affiliation, that exploit the same class of environmental resources in a similar way. This term has been useful for studying potentially co-evolved species that compete for, and partition use of, a common resource. The largely equivalent term, functional group, was proposed by Cummins (1973) to refer to a group of species having a similar ecological function. Insects, as well as other organisms, have been combined into guilds or functional groups based on similarity of response to environmental conditions (e.g., Coulson et al. 1986, Fielding and Brusven 1993, Grime 1977, Root 1973) or of effects on resources or ecosystem processes (e.g., Romoser and Stoffolano 1998, Schowalter et al. 1981c, Siepel and de Ruiter-Dijkman 1993). This method of grouping is one basis for pooling "kinds" of organisms, as discussed in the previous section.
Pooling species in this way has been attractive for a number of reasons (Root 1967, Simberloff and Dayan 1991). First, it reflects the compartmentalization of natural communities (see previous section) and focuses attention on sympatric species that share an ecological relationship (e.g., competing for a resource or affecting a particular ecological process), regardless of taxonomic relationship. Second, it helps resolve multiple usage of the term "niche" to refer both to the functional role of a species and the set of conditions that determines its presence in the community. Use of guild or functional group to refer to species' ecological role(s) permits limitation of the term niche to refer to the conditions that determine species presence. Third, this concept facilitates comparative studies of communities that may share no taxa but do share functional groupings (e.g., herbivores, pollinators, detritivores, etc.). Guild or functional groupings permit focus on a particular group, with specific functional relationships, among community types. Hence, researchers avoid the necessity of cataloging and studying all species represented in the community, a nearly impossible task, before comparison is possible. Functional groupings are particularly useful for simplifying ecosystem models to emphasize effects of functional groups with particular patterns of carbon and nutrient use on fluxes of energy and matter. Nevertheless, this method for describing communities has been used more widely among aquatic ecologists than among terrestrial ecologists.
The designation of functional groupings is largely a matter of convenience and depends on research objectives (e.g., Hawkins and MacMahon 1989, Körner 1993, Simberloff and Dayan 1991). For example, defining "same class of resources" or "in a similar manner" is ambiguous. Each species represents a unique combination of abilities to respond to environmental conditions and to affect ecosystem processes (i.e., species within functional groups are similar only on the basis of the particular criteria used to distinguish the groups). Characterization of functional groups based on response to climate change, response to a disturbance gradient, effect on carbon flux, or effect on biogeochemical cycling would involve different combinations of species.
Insects are particularly difficult to categorize because functional roles can change seasonally (wasps switching between predation and pollination) or during maturation (e.g., sedentary herbivorous larvae becoming mobile pollinating adults, aquatic larvae becoming terrestrial adults, etc.), and many species are too poorly known to assign functional roles. All Homoptera can be assigned to a plant sap-sucking functional group, but various species would be assigned to different functional groups on the basis of the plant part(s) affected (e.g., foliage, shoots, or roots, xylem or phloem). Clearly, functional groups can be subdivided to represent a diversity of responses to different gradients or subtle differences in ecological effects. For example, a stress-adapted "functional group" could be divided into subgroups that tolerate desiccation, physiologically prevent desiccation, or avoid desiccation by feeding on plant fluids. Similarly, a foliage-feeder guild can be divided into subgroups that fragment foliage, mine foliage, or suck cellular fluids; feed on different plant species; etc., each subgroup affecting energy and matter fluxes in a different manner. Luh and Croft (1999) developed a computer algorithm to classify predaceous phytoseiid mite species into functional groups (specialist vs. generalist predators). The computer-generated classification confirmed the importance of the combination of life history traits used previously to distinguish functional groups.
Species included in a particular functional group should not be considered redundant (Beare et al. 1995, Lawton and Brown 1993), but rather complementary, in terms of ensuring ecological functions. Schowalter et al. (1999) reported that each functional group defined on the basis of feeding type included species that responded positively, negatively, or nonlinearly to moisture availability. Species replacement within functional groups maintained functional organization over an experimental moisture gradient.
Changes in the relative abundance or biomass of functional groups can signal changes in the rate and direction of ecological processes. For example, changes in the relative proportions of filter-feeder versus shredder functional groups in aquatic ecosystems affect the ways in which detrital resources are processed within the stream community and their contribution to downstream communities. Similarly, changes in the relative proportions of folivores versus sap-suckers affect the flux of nutrients as solid materials versus liquid (e.g., honeydew) and their effect on the detrital community (e.g., Schowalter and Lowman 1999, Stadler and Müller 1996, Stadler et al. 1998).
The functional group concept permits a convenient compromise in dealing with diversity (i.e., sufficient grouping to simplify taxonomic diversity while retaining an ecologically relevant level of functional diversity). Therefore, the functional group approach has become widely used in ecosystem ecology.
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