Communities are composed of the species occupying a site. Identification of patterns in community structure has been a major goal of ecological research. However, no standard approach for delimiting a site and describing or comparing community structure has been adopted. Indices of species diversity, food web structure, and functional group organization are three methods used to facilitate comparison among communities.
Species diversity has two components: richness and evenness. Richness is the number of species in the community, whereas evenness is a measure of relative abundances. These two components can be represented by rank-abundance curves and by diversity indices. Geometric rank-abundance curves characterize harsh or disturbed habitats with a limited number of adapted species and strong dominance hierarchy, whereas log and broken stick models characterize more stable habitats with higher species accumulation and greater evenness in abundance among species. A number of diversity indices and similarity indices have been developed to integrate richness and evenness in a variable that can be compared among community types.
Food web structure represents the network of pairwise interactions among the species in the community. A number of food web attributes have been proposed, based on limited taxonomic resolution of insects and other arthropods. These hypotheses are being challenged as greater resolution of arthropod taxonomy reveals networks of interactions within this diverse group.
Functional group organization reflects combination of species on the basis of functional responses to environmental variables or effects on ecological processes, regardless of taxonomic affiliation. This approach has become popular because it simplifies species diversity in an ecologically meaningful way. However, the allocation of species to functional groups is based on particular objectives and is therefore arbitrary to the extent that each species represents a unique combination of functional responses or effects.
The noncomparable descriptions of communities based on these three approaches, compounded by the variety of arthropod sampling techniques, each with its unique biases, have hindered comparison of community structure among habitat types. Many taxa show latitudinal gradients in abundance, with species richness increasing toward the equator. However, the climate gradient thought to underlie this trend is correlated with latitudinal gradients in habitat area and productivity. Some taxonomic groups are more diverse within biogeographic realms of origin or where resources have been available over longer time periods. Some functional groups are more abundant in certain biomes (e.g., pollinators in diverse tropical habitats and detritivores and wood borers in habitats with greater organic matter or wood accumulation).
Habitat area and stability, resource availability, and species interactions are major factors that affect community structure. Habitat area affects the pool of species available and the heterogeneity of habitat conditions and resources. Habitat stability determines the length of time available for species accumulation, assortment, and species packing. Species richness generally increases with resource availability, up to a point at which the most adapted species competitively suppress other species. Species interactions often affect persistence in a particular habitat. Colonists cannot survive unless their host resources are available. Competition, predation, and mutualism also affect species directly and indirectly. Indirect effects often are at least as important as direct effects. Keystone species have effects on community structure or ecosystem processes that are disproportionate to their numbers or biomass. Keystone species include predators that focus on the most abundant prey species, thereby reducing competition among prey species and maintaining more species than would co-exist in the absence of the predator. Some herbivorous insects function as keystone species by selectively reducing abundance of dominant host species and facilitating persistence of nonhosts. Trophic cascades reflect top-down effects of predators reducing prey abundance, thereby increasing abundance of the trophic level supporting the prey.
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