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the impacts of human activity on ecosystems effectively and ensure soil sustainability.

Species in soils perform ecological services that directly control the sustainability of human life. Soil microorganisms and invertebrates (such as fungi, bacteria, nematodes, and earthworms) provide for the purification of air and water, for the decay and recycling of organic matter and hazardous wastes, and for soil fertility. Soil organisms mediate critical ecosystem processes, particularly those in biogeochemical cycling (Swift and Anderson 1994; Matson and others 1987). Soils store vast amounts of carbon, and it is the biota in soils that most influences local and global processes involving the cycling of carbon and nitrogen, including several greenhouse gases (Coleman and Crossley 1996; Huston 1993). The organisms in soil—through their direct, indirect, and modifying effects on these ecosystem processes (Lavelle and others 1995)—provide humans with numerous services (table 1). Pimentel and others (1997) valued the function of soil biodiversity at $25 billion per year on the basis of the contributions of soil biodiversity to topsoil formation in agricultural lands; this value would increase considerably if natural terrestrial systems were included.

A single ecosystem service, such as the generation and renewal of soil and soil fertility (table 1), involves many ecosystem processes and countless organisms representing diverse phyla. These range from large vertebrates to invertebrates and smaller macrofauna such as earthworms and ants that channel through the soil, algae living on the soil surface, and microorganisms involved in the decay of organic matter (Pankhurst and Lynch 1994). The decay of a small animal (such as a piglet) in the soil requires many phyla and can involve 100–500 species of Arthropoda (Richards and Goff 1997). Knowledge of the succession of species participating in the decay of humans is used in forensic medicine to determine the time of death (Goff 1991). Information on the number and types of soil species and phyla required to decompose plant material or invertebrates might be avail-