Weak effects of plant diversity on leaf-litter breakdown in a tropical stream

Abstract

The relationship between biodiversity and ecosystem functioning is incompletely understood, particularly the relative influence of species richness and composition on functioning. We investigated the effects of plant species diversity on litter breakdown rate (a measure of functioning) and the associated detritivorous macroinvertebrates in a tropical stream in Hong Kong by using single-species and three- and four-species mixtures of leaves with contrasting breakdown rates. We hypothesised that the scarcity of shredding detritivores in Hong Kong streams would limit the scope for diversity effects attributable to differences in leaf composition. Nonetheless, breakdown rates were affected by differences in composition (constituent species within a mixture) rather than richness (number of species in a mixture). Ficus fistulosa (Moraceae) was a key species in mixtures, producing non-additive effects through acceleration of breakdown of other constituent species. There were no other positive (or any negative) non-additive effects of mixing. Leaf diversity did not affect macroinvertebrate functional organisation or taxonomic composition. Assemblages on all litter types were dominated by collector-gatherers (mainly chironomids, >70% of total abundance), and shredders were scarce (<5%). Although diversity had no obvious bottom-up influence on detritivores, non-additive effects of mixing may arise from the influence of litter quality on microbially mediated breakdown in this tropical stream.