Soil biodiversity and its implications for ecosystem functioning in a heterogeneous and variable environment

This paper evaluates theoretical arguments and empirical evidence on the interrelation of soil biodiversity and ecological functioning, in order to identify appropriate methodologies and promising fields of investigation. Predictions from a range of general theories are evaluated against the background of empirical findings and results from models on specific soil ecosystems with reference to the functional effects of species losses (1) within trophic levels, (2) in food-webs, and (3) those that depend on the size of the spatial scale. Difficulties in proving species richness effects empirically arise because (i) a range of mechanisms can compensate species richness effects, (ii) predictions on the effects of species richness are context-dependent, which renders valid generalisations less likely, and (iii) species richness effects are of a probabilistic, rather than a deterministic nature. Generally, the effects of changes in species richness are likely to be stronger in species-poor communities than in species-rich communities. The authors deduce the hypotheses that (a) the species richness within trophic levels is likely to reduce functional gaps in space and time, (b) the species richness across trophic levels of the decomposer food web can enhance synchronisation of subprocesses of nutrient cycling, and (c) differences in species richness are likely to be more pronounced on larger spatial scales. The authors infer that an explicit consideration of spatiotemporal variation is essential in experimentation, as well as in modelling, in order to analyse species richness–function relationships.