Highly diverse urban soil communities: does stochasticity play a major role?

Urban soil biota can be surprisingly diverse and recent studies hypothesize that such biodiversity is partly due to stochastic community dynamics caused by fragmentation and high environmental variability. We aimed to quantify the relative effects of these factors on the community structure of soil oribatid mites inhabiting holm oak woodlands in two Mediterranean cities. We partitioned the community variation into fractions uniquely attributable to gradients in soil abiotic properties, pollution, microbiological properties, and spatial and temporal variation that could not be related to measured soil factors. As we found strong spatial structure at the local scales, a neutral model was fitted to test whether beta diversity patterns of relatively isolated communities were consistent with a purely stochastic assembly process. The overall diversity of the assemblage was remarkable: a total of 124 species were identified and rarefaction curves demonstrated that urban parks are as rich in species as their counterpart woodlands in suburban areas. The fraction of variation explainable in terms of soil properties, microbiology and pollution was statistically significant but surprisingly low. At the site scale, patterns of beta diversity (i.e. spatial turnover in species composition and relative abundances) were not significantly different from those predicted by neutral models. Stochastic models can parsimoniously predict background levels of urban soil biodiversity at local scales, while disturbance and environmental variation still play some significant but not major role at broader scales.