EFFECTS OF HABITAT COMPLEXITY AND COMPOSITION ON A DIVERSE LITTER MICROARTHROPOD ASSEMBLAGE

Oribatid mites (Acari: Oribatidae) are the most diverse arthropod group in forest litter and soil, and they make significant contributions to decomposition as microbial grazers and saprophages. As is true for all the hyperdiverse soil taxa, the determinants of their diversity and species composition are virtually unexplored. This experiment tests whether heterogeneity of the litter habitat is a determinant of their local diversity, and whether litter composition is a determinant of their species composition. At a single site of temperate deciduous forest at the Coweeta Hydrological Laboratory in the mountains of North Carolina, USA, natural litterfall was excluded from a series of 42 1‐m 2 plots and, for three consecutive years, replaced with treatment litters that varied in their composition and complexity. Plots of pure yellow birch ( Betula alleghaniensis ), sugar maple ( Acer saccharum ), and red oak ( Quercus rubra ) litter comprised the monotypic or simple litter treatments. Two complex litters included a mixture of these three litter species and a mixture of seven litter species with pieces of small woody debris. Monotypic litters developed profiles of reduced thickness that contained lower numbers of invading roots and less humic and arthropod fecal material. Over 3 yr, oribatid abundance and richness declined substantially and to a similar degree in all simple litter treatments, though the dominant species, Oppiella nova, was unaffected by litter simplification. Similarity of species composition increased markedly among replicates within each litter treatment for two sectors of the assemblage: the large, litter‐dwelling species and the endophagous and wood‐associated species. Species composition among small litter‐dwellers was unresponsive to litter type. Several characteristics of monotypic‐litter habitats potentially contributed to the erosion of the oribatid assemblage. Loss of structure in monotypic litter likely led to reduced and less hospitable physical living space. It appeared to reduce recruitment of roots and retention of humic and fecal material in the litter layer. Each monotypic litter contained only a subset of the structural microhabitats that serve as refugia for eggs and juveniles. Finally, the synchronized decomposition of uniform substrates could have led to a “boom–bust” economy in microbial resources that was unfavorable to oribatid mites and their conservative life histories.