Invertebrates as determinants and indicators of soil quality Nigel E. Stork and Paul Eggleton Abstract Invertebrates are an integral part of soils and are important in determining the suitability of soils for the sustainable production of healthy crops or trees. We discuss the importance of the soil invertebrate fauna in relation to terrestrial habitats and global biodiversity as we understand it We describe the role of the main invertebrate groups in soils, including earthworms y termites y springtails, andnematodes, and how they determine soil quality. Practical problems in dealing with the invertebrate fauna include sampling, taxonomy and availability of biological information on species. Various measures are available that use invertebrates to assess soil quality, each with its advantages and disadvantages. They include abundance, biomass, density, species richness, trophic/guild structure, food web structure, keystone species and ecosystem engineers. We propose the three most useful and practical of these as suitable to be combined with other biological (microbial) and non-biological (hydrological, physical, chemical) criteria into a single index of soil quality that might be used on a regional, if not international basis. Keywords: biodiversity, soil invertebrates, species richness, keystone species, earthworms, springtails, nematodes, termites Introduction The soil is among the most complex habitat systems on the globe, yet its biological systems are poorly understood. It provides a living place for at least part of the life cycle of many animals, and the highly connective nature of foodwebs means that most, if not all, terrestrial or- ganisms depend directly or indirectly on biological processes in the soil. Many small organisms, such as insects and other invertebrates, play a vital role in the production and maintenance of healthy soils, and therefore are key elements in the development of sustainable agriculture and forestry. The central tenet of this paper is that in- vertebrates are an integral part of soil sys- tems and that soil quality results at least in part from the interactions of soil with its Biodiversity Division, Department of Entomology, The Natural History Museum, London SW7 5BD, United Kingdom. biotic community. To many soil inver- tebrate biologists this will seem a truism, given the extensive literature on this sub- ject (e.g., Fitter et al., 1985; Edwards et al., 1988; Spence, 1985; Eselbeis and Wichard, 1987). To other biologists, how- ever, and to most non-biologists, it is not so obvious. We review the ways in which invertebrates determine the nature of soil, and show how invertebrates can be used to assess human-induced changes in soil quality. We define "soil quality" as "the fitness of soils for the sustainable produc- tion of healthy, agriculturally important plants." This paper outlines the role of in- vertebrates in soil processes, suggesting how this role might be included in a general index of soil quality. This index would in- clude physical and chemical measures, as well as other biological measures such as microbial activity. Consequently, we first outline the contribution of the soil fauna to biodiversity in general, emphasizing inver- tebrates. Second, we discuss the impor- tance of the soil fauna in relation to the fauna of other terrestrial and aquatic ecosystems. Third, we describe the most important invertebrate components and their role in soil processes. Fourth, we dis- cuss problems of sampling, identification, and geographical distribution. Fifth, we consider how different measures of inver- tebrate biodiversity (biomass and abun- dance; species richness) and ecological complexity (trophic group analysis; food web studies; recognition of keystone species/ecosystem engineers) have been or could be used to indicate management-in- duced changes in soil quality. Finally, we suggest which of these measures are most practical. Because the field we cover is wide, we take a conceptual view and sug- gest broad approaches based on ecological and systematic theory. Invertebrate Diversity— the Baseline Only a fraction of the species on earth have been named, but what fraction is a matter of great debate. We are not even sure how many species have been named already, although estimates range from 1.4 to 1.8 million (Stork, 1988; Stork and Gas- ton, 1990; May, 1990a,b). Insects com- prise the largest single group of named species and attention has therefore focused on this group for estimates of total number of species on earth. Until the 1980s, most estimates for the number of insect species were between 1 and 3 million. Against this baseline, the 30 million tropical species suggested by Erwin (1982) created consid- erable renewed controversy. His method of estimating this figure, and the assump- tions and extrapolation involved have been questioned by several authors. More recent estimates have been below 10 mil- lion (Stork, 1988; May, 1990a,b; Thomas, 1990; Gaston, 1991; Holloway and Stork, 1991; Hodkinson and Casson, 1991). Less 38 American Journal of Alternative Agriculture