Biodiversity and litter decomposition: a case study in a Mediterranean stream

The importance of riparian diversity for the functioning of headwater streams has been demonstrated repeatedly. For example, mixing litter from different riparian tree species can influence their decomposition rates, an effect that is contingent on both the litter assemblage and the detritivore community. However, the effects of mixing litter species have been studied mostly in temperate streams, and very few studies have been done in non-temperate streams. Mediterranean streams are often subjected to recurrent flow intermittency, and their riparian tree and stream detritivore communities have unique sets of species and traits, which probably influence the effects of litter mixtures on decomposition. We hypothesized that high dissimilarity in litter traits could promote effects of litter mixtures on decomposition that would be counteracted by the low abundance and small body size of detritivores in Mediterranean streams. We manipulated litter diversity and the size-class and presence/absence of detritivores in a 2(nd)-order Mediterranean stream in a 46-d experiment and found substantial but contingent effects on litter mixtures. Mixture effects were not significant on average, but both negative and positive effects of litter mixtures occurred. For instance, mixing soft and nutrient-rich litter species led to up to 9.6% increase in leaf mass loss. Microbial activity accounted for 85% of total leaf mass loss, and no effect of litter mixture was observed when detritivores were excluded. In contrast, the presence of detritivores, despite their relatively low abundance and diversity, was a key factor for litter decomposition and promoted effects of litter mixture. These results suggest that the extinction of a few key taxa (riparian tree species or large detritivores) could impair nutrient and C cycling in Mediterranean streams with potential consequences for stream food webs.