Oribatid Mite Communities in Soil: Structure, Function and Response to Global Environmental Change.

Abstract

Little is known regarding the relative role of stochastic and deterministic forces in the community assembly of soil oribatid mites, which are species-rich and fulfill important roles in ecosystem functioning as detritivores. This dissertation investigates the mechanisms underlying the community assembly of soil oribatid mites and addresses how global environmental change influences their community structure and function. I hypothesized that dispersal limitation was a central mechanism structuring oribatid mite communities, due to their low locomotive activity through the soil. However, a study of the geographic distribution of oribatid mites along a chronosequence in a deglaciated region demonstrates that soil/litter dwelling oribatid mites are not limited by dispersal in their re-colonization of deglaciated areas, as evidenced by an overall high similarity in the species richness and composition of oribatid mites along the chronosequence. In contrast, climatic, biogeochemical and biotic factors explained significant amounts of variation in the species composition of soil oribatid communities, indicating that environmental filtering and competition for food resource are more important than dispersal limitation in structuring soil oribatid mite communities. Moreover, stable isotope (15N) analysis reveals that over half of soil oribatid mites are fungal feeders and that their trophic positions are stable among diverse environments, indicating a high level of feeding specialization among soil oribatid mites. Feeding specialization and competition for food resources, especially for fungal hyphae, among soil oribatid mites provides a potential mechanism underlying the observed decline in the density of soil oribatids under chronic N deposition. Furthermore, a litterbag experiment reveals that the decline in microarthropod abundance in forest floors is associated with a reduction in the mobilization of newly-added C into fungal biomass. Microcosm experiments further illustrate that a decline in microarthropod abundance alters fungal communities through microarthropod-fungi interactions. Although such changes in fungal communities do not influence subsequent litter decay during the early stages of litter decomposition, changes in the abundance of microarthropods and subsequent feedback on fungal communities may have important influence on the decay of recalcitrant organic matter.