Studies on grasslands (McNaughton, 1977; Tilman, 1996) show that biodiversity stabilizes community and ecosystem processes, although individual species within the system may fluctuate considerably. Tilman (1996) found wide variations in the biomass of the 24 most abundant species of plants in an 11-year study. In a 6-month study of soil ciliates under a spruce stand, Lehle (1992) found that the proportions of the three dominant ciliate species fluctuated widely: Cyclidium muscicola ranged from 8 to 75% of the total populations and two colpodid species varied from 4 to 45%. The different responses in these two studies may reflect changes in the realized niches of species; thus, biodiversity furnishes a reservoir of biotic abilities contributing to ecosystem sustainability (Bamforth, 1995b). Biodiversity, like the comparison of nontillage to conventional agriculture, may not produce noticeable increases in crop production, but maintaining biodiversity can retard the deterioration that has characterized agroecosystems through 4000 years of human history. Protozoa can serve as bioindicators of ecosystem conditions, and warn of soil impoverishment.
The appeal of protozoan bioindicators is their environmental sensitivity due to their delicate cell membranes, their rapid growth rate, restricted movement in soil, ubiquity, and wide range of morphologies in ciliates and testacea, providing a multispecies approach enabling community analyses to indicate soil conditions (Foissner, 1994). Difficulties arise in the taxonomy and time needed for identification and enumeration, but, as the following applications illustrate, protozoa convey valuable information about agroecosystems because of their pivotal position in the nutrient cycling that all terrestrial ecosystems depend upon.
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