Disturbance can alter the diversity of an ecosystem (Atlas, 1984) directly by affecting survivorship of individuals or indirectly by changing resource levels (Hobbs and Huenneke, 1992). Sometimes, diversity measurements reflect the result of disturbance caused by pollution and/or stress. For example, taxonomic diversity of microinvertebrate communities was less in polluted or disturbed than in unpolluted or undisturbed agricultural sites (Atlas et al., 1991). Pollution eliminates sensitive species, reducing competition so that tolerant species proliferate (Atlas, 1984).
The successional status of a soil community may also reflect the history of disturbance. Succession in cropped agricultural fields begins with depauperate soil which acts like an island to which a series of organisms immigrate. First, opportunistic species, such as bacteria and their predators, are colonists of soil. Subsequently, fungi and their predators migrate into the area (Bostrom and Sohlenius, 1986). Microarthropods, such as collembolans, mites, and fly maggots, can colonize nearly bare ground and rise quickly in population density. Top predator microarthropods, such as predaceous mites and nematodes, become established later and may have a function similar to keystone predators in other community food webs (Elliott et al., 1988). Finally, macro- and megafauna, such as earthworms, millipedes, slugs, centipedes, wood lice, sow bugs, and pill bugs, join the soil community (Strueve-Kusenberg, 1982).
Succession can be interrupted at various stages by agricultural practices, such as cultivation and applications of fertilizer and pesticide (Ferris and Ferris, 1974; Wasilewska, 1979). Such interruptions reduce diversity and successional "maturity." Maturity indices are based on the principles of succession and relative sensitivity of various nematode taxa to stress or disruption of the successional sequence (Bongers, 1990). Indices that describe associations within biological communities, such as a maturity index, are less variable than measures of abundance of a single taxonomic or functional group and are, thus, more reliable as measures of ecosystem condition (Neher et al., 1995).
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