Functional macroinvertebrate diversity stabilizes decomposition among leaf litter resources across a river network

Biodiversity underpins the functional integrity of ecosystems. At present, our understanding of the relationship between biodiversity and ecosystem functioning (BEF) is essentially based on manipulative experiments. Compelling data at large spatial scales are scarce, especially for river networks. BEF patterns across landscapes are complex because they unfold in the context of environmental gradients and compositional turnover of natural communities. Leaf litter decomposition, a pivotal ecosystem process in streams, is no exception to this context dependency. The dendritic structure of river networks plus the unidirectional water flow shape both environmental conditions and the distribution of leaf resources and consumers. However, it is difficult to predict how spatial gradients of resource and consumer composition can overlap across a river network, and thus govern spatial patterns of decomposition. Here, we investigated the capacity of macroinvertebrate biodiversity to control decomposition rates of heterogeneous leaf resources at the river-network scale. We deployed five litterbags containing either one of four single leaf species or a mixture of all species at 51 sites across the Thur River network (Switzerland). We measured litter decomposition rates, variation of decomposition among leaf resources, and the effect of leaf litter diversity on decomposition. We found that decomposition rates decreased from headwaters to downstream reaches mainly due to the parallel decrease in the abundance of key shredder taxa (namely, Amphinemura, Nemoura, Leuctra, Habroleptoides, and Stenophylacini). Macroinvertebrate diversity had a minor, negative effect on decomposition rates. However, high functional macroinvertebrate diversity at the reach scale reduced the variation of decomposition among leaf resources, thus alleviating nutritional constraints exerted by nutrient-poor leaf resources. Furthermore, litter mixtures were preferably decomposed by communities with low evenness and dominated by a few taxa. These findings point to a critical role of macroinvertebrates in controlling litter decomposition at the network scale beyond environmental effects. While shredder abundance and community composition are key to determining decomposition rates across the river network, functional diversity is important in decreasing the variation of decomposition rates among leaf resources. Our results stress the importance of biodiversity controlling ecosystem functioning not only at the local but also at the river network scale.