SOIL MICROARTHROPOD CONTRIBUTIONS TO DECOMPOSITION DYNAMICS: TROPICAL–TEMPERATE COMPARISONS OF A SINGLE SUBSTRATE

Abstract

This study examined the effect of soil microarthropods on the decomposition of a single substrate (Quercus prinus L.) at two humid tropical forests (La Selva, Costa Rica (LAS), and Luquillo Experimental Forest, Puerto Rico (LUQ)) and one temperate forest (Coweeta Hydrologic Station, North Carolina, USA (CWT)). In this litterbag ex- periment, naphthalene was applied to reduce the microarthropod population density from half of three replicate plots established at each site. This enabled us to quantify the mass loss contributed by the fauna (MLCF) at each site and permitted an analysis of the influence of site-specific differences in the composition of the microarthropod assemblages on de- composition rates. We hypothesized that microarthropod regulation of the microbial pop- ulations involved in leaf litter decomposition would be stronger in humid tropical forests, which experience conditions of low climatic variability. In these conditions, there can be an enhanced degree of biotic interactions between microarthropods and their microbial food sources. The elevated extent of these interactions should be expressed as a greater influence of microarthropo ds at the tropical sites and could result in a site-specific effect of faunal assemblages on decomposition . Decomposition of the oak litter proceeded faster in Puerto Rican and Costa Rican forests than in a temperate forest in North Carolina, USA. Microarthropods had little effect on decomposition in the temperate forest, whereas their influence was pronounced at tropical sites. Mass loss of litter from plots with reduced microarthropod populations was similar at the tropical sites. When plots with intact faunal communities were compared, differences in the tropical sites were apparent, suggesting that there was a site-specific faunal contri- bution to decomposition at these sites. Oribatid mites constituted a dominant component (41-64%) at each of the sites. Species richness of oribatids and Fisher's alpha diversity were similar in each of the three sites. The Shannon index revealed a lower diversity at LUQ. Abundance of microarthropods was lowest at LAS. Species accumulation curves for each site, though similar in form, were distinctive, as were diversity accumulation patterns in samples of increasing size. There was a positive relationship between species richness and the contribution of the fauna to litter mass loss within each site. Thus, species diversity of decomposer fauna may have important ecosystem consequences, particularly in warm moist tropical forests.