Contribution of macroinvertebrate shredders and aquatic hyphomycetes to litter decomposition in remote insular streams

Shredders play a crucial role in litter decomposition in streams. However, in oceanic islands, many streams have low shredder density and richness, and microbes seem to be the main litter decomposers. Here, we evaluate the effects of shredders and aquatic hyphomycetes on litter decomposition in insular streams. Three leaf species differing in physical and chemical characteristics, Alnus glutinosa, Clethra arborea, and Cryptomeria japonica, were enclosed in bags of coarse and fine mesh to allow and avoid macroinvertebrate access to the litter, respectively, and incubated in six streams along a gradient of Limnephilus atlanticus (Trichoptera) density in Sao Miguel Island. In streams with higher L. atlanticus density, leaf mass loss was higher in coarse than fine mesh bags. However, no difference in litter mass loss was found between bag types in streams with no L. atlanticus, despite the presence of other shredder taxa. These results suggest that when L. atlanticus are present at relatively high densities, they significantly contribute to litter decomposition, while litter decomposition is mainly driven by microbes when L. atlanticus density is low, or they are absent. Moreover, litter decomposition depends on litter quality, with leaves with high nutrient concentration and low concentration of secondary compounds being preferred by shredders.