Oribatid mites reveal that competition for resources and trophic structure combine to regulate the assembly of diverse soil animal communities

The role of niche partitioning in structuring species-rich soil animal communities has been debated for decades and generated the enigma of soil animal diversity. More recently, resource-based niche partitioning has been hypothesized to play a very limited role in the assembly of soil animal communities. To test this hypothesis, we applied a novel combination of stable isotopes and null models of species co-occurrence to quantify the extent of resource niche partitioning on a diverse oribatid mite community sampled from mature oak woodland. We asked whether species aggregate or segregate spatially and how these patterns correlated with the abundance of estimated trophic guilds. We also estimated the effects of environmental variables on community structure. All measured environmental variables accounted for 12% of variance in community structure, including 8% of pure spatial structure unrelated to measured environmental factors and 2% of pure environmental variance unrelated to spatial variation. Co-occurrence analysis revealed 10 pairs of species that aggregated and six pairs of species that were spatially segregated. Values of delta N-15 indicated that five out of the 10 pairs of aggregated species occupied the same trophic guild, while values of delta C-13 indicated that species in these five pairs consumed resources of different quality, supporting a significant role of resource-based niche partitioning. Also, one of the five pairs of segregated species occupied the same trophic guild but had overlapping delta C-13 values suggesting that these species do not co-occur locally and thus minimize competition for shared resources. Partitioning of resources plays an underestimated role in soil microarthropod communities and different local communities consisted of the same trophic guilds with species identity changing from place to place. The sum of resource partitioning, multi-trophic interactions, and microscale environmental variability in the environment is a viable solution to the enigma of soil animal diversity.