Diversity in form and function of biotic communities results in the formation of spatial and temporal heterogeneity of organisms that contributes to the overall function of the ecosystem. Individual taxa may have multiple functions, and several taxa may appear to have similar functions. However, function may not necessarily be redundant, because taxa performing the same function are often isolated spatially, temporally, or by microhabitat preference (Beare et al., 1995). Biodiversity allows organisms to avoid intense competition for food or space, decrease invasion and disruption, and maintain constancy of function through fluctuating environmental conditions.
Various measures of diversity are available to describe soil invertebrate communities including abundance, biomass, density, species richness, species evenness, maturity indexes, trophic/guild structure, and food web structure. Indexes of diversity, which include elements of richness (number of taxa) and evenness (relative abundances), can be applied at scales ranging from alleles and species to regions and landscapes. Diversity indexes do not reveal the taxonomic composition of the community. For example, a community composed entirely of exotic species could have the same index value as a community composed entirely of endemic species. Therefore, a diversity index, by itself, does not predict ecosystem health or productivity.
Debates concerning relationships between biodiversity and ecosystem stability became popular in the 1960s and 1970s. MacArthur (1955) was the first to argue that complex systems are more stable than simple systems. In the early 1970s, May (1972; 1973) used mathematical models to argue that diverse communities were less stable than simple systems. Today, some conclude that relatively simple, short food webs that exhibit little omnivory or looping are more stable than longer food webs with much omnivory or looping. A short food web is one with few trophic levels (Polis, 1991). Others hypothesize that high linkage is responsible for making food webs unstable, i.e., stability can develop if numbers of species increase but not if omnivory increases (Pimm et al., 1991; Lawton and Brown, 1993). It is clear from this ongoing debate that it is impossible to generalize the relationship between biodiversity and ecosystem stability. Besides, none of the theories has been tested adequately for application to soil communities.
Factors affecting diversity within trophic groups of the detritus food web include altitude, latitude (Procter, 1984; Rohde, 1992), predation in the presence of strong competitive interactions (Petraitis et al., 1989), and disturbance (Petraitis et al., 1989; Hobbs and Huenneke, 1992). For example, the pervading theory is that the greatest species diversity is found in the tropics and that diversity decreases with increasing latitude (Rohde, 1992). However, the opposite is true for free-living nematodes. Free-living nematodes are more diverse and abundant in temperate than in tropical regions (Procter, 1984; 1990). Nematodes are tolerant of harsh conditions at high latitudes but are not competitive against more-specialized soil fauna in the tropics (Petraitis et al., 1989).
At smaller scales, predators may promote species diversity among competing prey species when they feed preferentially on exceptionally competitive prey (Petrai-tis et al., 1989). Disturbance also plays a role with the "intermediate disturbance hypothesis" suggesting that taxonomic diversity should be highest at moderate levels of disturbance (Petraitis et al., 1989; Hobbs and Huenneke, 1992). Disturbance is defined as a cause (a physical force, agent, or process, either abiotic or biotic) that results in a perturbation (an effect or change in system state relative to a reference state and system) (Rykiel, 1985). If disturbance is too mild or too rare, then soil communities will approach equilibrium and will be dominated by fewer taxa that can outcompete all other taxa. However, attainment of steady-state equilibria in agricultural or natural ecosystems is uncommon (Richards, 1987). If disturbance is common or harsh, only a few taxa that are insensitive to disruption will persist, therefore decreasing biodiversity (Petraitis et al., 1989). For example, Prostigmatid mites in the Eupodidae, Tarsonemidae, and Tydeidae are among the most abundant in cultivated agroecosystems and their numbers increase rapidly in response to disturbances such as cultivation (Crossley et al., 1992).
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