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Research in low biodiversity extreme environments allows separation of the climatic, soil and biological interactions that determine soil biodiversity and community structure. Studies focused on the response of low diversity communities in soils of the Antarctic Dry Valleys and the Chihuahuan Desert of the southwestern USA, to manipulations of soil resources and climate, offer the best opportunity to learn about the environmental controls on soil biodiversity and the role of biodiversity in soil functioning. We propose that insights based on research in these extreme environments should be applicable to understanding soil biodiversity in more complex, temperate and tropical ecosystems. The study of extreme soil ecosystems may also provide information on the response of soil biodiversity to increasing occurrences of environmental extremes that are predicted to occur from global change models. Studies from hot and cold deserts show that decomposition-based food webs can be very simple, that aridity produces similar mechanisms for survival and dispersal of organisms in temperate and polar systems, that suitable soil habitats are patchily distributed in arid environments, and the low biodiversity of extreme soil ecosystems creates little or no functional redundancy making these systems susceptible to disturbance. We suggest that species within the same functional group can have small differences in ecology that are sufficient to affect ecosystem processes. When this occurs, differential responses of species to disturbance within a functional group will not stabilise the soil ecosystem, but rather lead to dramatic changes in community composition and ecosystem process rates. 1999 Elsevier Science B.V. All rights reserved.