Decomposition of terrestrial resource subsidies in headwater streams: Does consumer diversity matter?

Leaf litter mixtures alter decomposition rate, nutrient retention, and bacterial community composition in a temperate forest

Litter decomposition is a key step in global biogeochemical cycling. In forest ecosystems, litter from different tree spec1ies often decompose together. Although species diversity is widely acknowledged to accelerate decomposition through the regulation of nutrient transfer between litter and decomposer communities, the underlying mechanism remains unclear. To explore the association between the bacterial community and mixed-litter chemical transformation, we conducted a one-year litter mixing decomposition experiment using leaf litter from four dominant tree species in Mount Tai (Eastern China), Robinia pseudoacacia, Quercus acutissima, Pinus tabulaeformis, and Pinus densiflora. Our results showed that: 1) Mass loss of leaf litter mixtures was significantly faster than that of leaf litter monocultures, except for R. pseudoacacia. Litter mixtures without R. pseudoacacia showed non-additive synergistic effects, whereas litter mixtures with R. pseudoacacia exerted additive effects; 2) Litter species in the absence of R. pseudoacacia significantly decreased the nutrient retention rates of litter mixtures compared to those of monocultures; 3) Litter mixtures with or without R. pseudoacacia showing additive and non-additive effects in monocultures had a distinct bacterial community structure; 4) Bacterial community structure was also modified by initial litter traits; carbon (C), nitrogen (N), and phosphorus (P) concentrations in monocultures; N/P and C/N ratios of mixtures with R. pseudoacacia; and the lignin/N ratio of mixtures without R. pseudoacacia. Overall, these findings indicate that tree species diversity controls decomposition and nutrient cycling, implying that an appropriate species community composition is beneficial to maintaining forest ecosystems.

Species identity drives ecosystem function in a subsidy-dependent coastal ecosystem

Declines in species diversity carry profound implications for ecosystem functioning. Communities of primary producers and consumers interact on evolutionary as well as ecological time scales, shaping complex relationships between biodiversity and ecosystem functioning. In subsidized ecosystems, resource inputs are independent of consumer actions, offering a simplified view of the relationship between species diversity and function for higher trophic levels. With food webs supported by substantial but variable inputs of detritus from adjacent marine ecosystems, sandy beaches are classic examples of subsidized ecosystems. We investigated effects of consumer species diversity and identity on a key ecological function, consumption of kelp wrack from nearshore giant kelp (Macrocystis pyrifera) forests. We assessed effects of species richness on kelp consumption by experimentally manipulating richness of six common species of invertebrate detritivores in laboratory mesocosms and conducting field assays of kelp consumption on beaches. Consumer richness had no effect on kelp consumption in the field and a slight negative effect in laboratory experiments. Kelp consumption was most strongly affected by the species composition of the detritivore community. Species identity and body size of intertidal detritivores drove variation in kelp consumption rates in both experiments and field assays. Our results provide further evidence that species traits, rather than richness per se, influence ecosystem function most, particularly in detrital-based food webs with high functional redundancy across species. On sandy beaches, where biodiversity is threatened by rising sea levels and expanding development, our findings suggest that loss of large-bodied consumer species could disproportionally impact ecosystem function.