Biodiversity of soil animals and its function

Current knowledge suggests a high redundancy of soil organism communities, i.e. saturation of function at low levels of species richness. This does not imply, however, that research on soil organism biodiversity and its function is irrelevant. First, it is well established that several species of the decomposer community are functionally more important than others. The 'step' hypothesis developed in this paper shows that under these conditions random loss of species is much more likely to impact ecosystem processes than expected from the redundancy hypothesis. Second, redundant species may gain functional significance by interacting with functionally important species. Third, the number of 'important species' is increased by the multiplicity of functions carried out by soil biota. And finally, alteration in species composition will not be a random process. In fact, functionally important soil biota might be among the first to be affected by large-scale changes in land use. Even if we were to accept that conservation efforts should be confined to the functionally most important species, we would have no idea how to do so. Crossing the 'spatial barrier' seems to be the biggest challenge for future investigations on soil biodiversity, because traditional approaches of community ecology will not be sufficient to answer the questions originating from large-scale impoverishment of the soil fauna. Some examples of promising macroecological topics are discussed: (i) the impact of the regional species pool on local species richness, (ii) the relationship between alpha- and gamma -diversity, and (iii) abundance-occupancy relationships. It is argued that research in this direction will be essential for answering the question of how populations and communities must be organised to resist alterations of the soil habitat at the landscape-level. (C) 2001 Editions scientifiques et medicales Elsevier SAS.