Specialization of oribatid mites to forest microhabitats—the enigmatic role of litter

Abstract The degree of ecological specialization influences the biological performance of species in their natural environment and affects the coexistence of different taxa. However, on a small scale, the diversity of microarthropods that coexist in forest soils and leaf litter seems inordinately high, a situation known as the “enigma of soil animal species diversity”. Since recent studies point to the importance of small‐scale heterogeneity to explain this phenomenon, we use interaction networks between microhabitats and their inhabitants to resolve and quantify the community structure (species composition, richness, and diversity) of oribatid mites (Oribatida) in five discrete, patchy substrates—dead wood, lichens, mosses, sod, and tree bark—and in the general leaf litter. Since oribatid mites are ubiquitous in all these microhabitats in temperate forests, the analysis of their community structure in the light of generalization and specialization might help us understand the ecological role of litter. We investigated whether litter acts as a specific microhabitat with the intrinsic characteristics that enable the “enigmatic” high diversity of oribatid mites (Habitat‐Hypothesis), if litter acts as a source from which oribatid mite species more or less randomly invade different associated microhabitat‐patches (Source‐Hypothesis), or if litter only connects patchily distributed microhabitats with specific species compositions (Connector‐Hypothesis). In total, 25,162 adult oribatid mite individuals were analyzed, most belonging to the derived group Brachypylina. Species richness, density, and diversity differed among microhabitats with highest values found in mosses and dead wood and lowest on tree bark. In general, specialization of oribatid mite species was low—highest on tree bark and in grass sod—but differed slightly among oribatid mite taxa (Enarthronota, Mixonomata, Nothrina, Brachypylina). The Connector‐ and Habitat‐Hypotheses can explain the distribution of most oribatid mite species but the Source‐Hypothesis explains the distribution patterns for only a few species.