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.

The dilution effect was originally proposed to describe the negative effect of increased host diversity on parasite abundance; with greater host diversity, parasite levels per host are predicted to be lower due to a higher probability of dispersing parasites encountering non-competent hosts. Dilution effects could also occur in many mutualisms if dispersing symbionts encounter hosts that vary in their competency. The introduction of non-native hosts can change community competency of a local group of host species. Crayfish introductions are occurring world-wide and these introductions are likely disrupting native crayfish-symbiont systems. Branchiobdellidan symbionts declined on native Cambarus crayfish occurring in the presence and absence of non-native Faxonius crayfish in the New River, USA. We performed an experiment investigating the effect of host density (1 vs 2 native hosts) and host diversity (1 native host and 1 introduced host) on branchiobdellidan abundance. The introduced F. cristavarius is a non-competent host for these worms. Six C. ingens were stocked on a C. chasmodactylus in each treatment and worm numbers were followed over 34 days. Worm numbers decreased over time on C. chasmodactylus alone and in the treatment in which a C. chasmodactylus was paired with an F. cristavarius. Worm numbers remained highest in the 2 C. chasmodactylus treatment . There was no significant effect of host diversity on worm reproduction. Crayfish invasions may have negative effects on mutualistic symbionts depending on the competence of introduced hosts. Loss of native symbionts is one of the potential hidden, negative effects of invasions on native freshwater diversity.