Dendritic network location mediates detritivore community structure and associated processing of leaf litter in a riverine ecosystem

Historically, studies have examined how local habitat, resources and species interactions influence community structure in stream ecosystems. Increasingly, though, attention has turned to understanding how regional factors (e.g. dispersal) interact with local conditions to influence communities. Often dispersal of organisms occurs in spatially constrained habitats, which can drastically influence community assembly. Dendritic networks are an example, and have a branching spatial configuration with some branches of the system more connected to others, making dispersal easier, while other locations are more isolated. As interest in multi-scale community assembly mechanisms has increased, less work has focused on the relationship between community assembly and ecosystem processes. Here, we sought to understand how consumer-resource interactions unfold in river networks. We predicted that stream network location would mediate detritivore (shredder) richness and abundance, and in turn would be associated with a shift in decomposition of organic matter (leaf litter). To examine this, we manipulated leaf litter species in isolated (headwaters) and connected (mainstem) stream reaches. We found that shredder richness and abundance were influenced by both leaf litter quality and network location. Headwater environments supported a stronger consumer-resource relationship, and shredder communities were further richer and more abundant. This was not the case in mainstem locations. In these relatively harsher environments, we offer that shredders did not appear to be actively feeding on the resources, but rather utilizing leaf litter more for habitat. Our results suggest river network position has important implications for how ecosystem function changes across spatially constrained environments.