Effects of habitat complexity and composition on a diverse litter microarthropod assemblage

Oribatid mites (Acari: Oribatidae) are the most diverse arthropod group in forest titter and soil, and they make significant contributions to decompos ition as microbial grazers and saprophages. As is true for all the hyperdiv erse soil taxa, the determinants of their diversity and species composition are virtually unexplored. This experiment tests whether heterogeneity of t he litter habitat is a determinant of their local diversity, and whether li tter 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 fr om a series of 42 1-m(2) plots and, for three consecutive years, replaced w ith treatment litters that varied in their composition and complexity. Plot s of pure yellow birch (Betula alleghaniensis), sugar maple (Acer saccharum ), and red oak (Quercus rubra) litter comprised the monotypic or simple lit ter treatments. Two complex litters included a mixture of these three litte r species and a mixture of seven litter species with pieces of small woody debris. Monotypic litters developed profiles of reduced thickness that contained lo wer 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 spe cies composition increased markedly among replicates within each litter tre atment for two sectors of the assemblage: the large, litter-dwelling specie s and the endophagous and wood-associated species. Species composition amon g small litter-dwellers was unresponsive to litter type. Several characteristics of monotypic-litter habitats potentially contribute d 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 m aterial in the litter layer. Each monotypic litter contained only a subset of the structural microhabitats that serve as refugia for eggs and juvenile s. Finally, the synchronized decomposition of uniform substrates could have led to a "boom-bust" economy in microbial resources that was unfavorable t o oribatid mites and their conservative life histories.