Background/Question/Methods   A metacommunity is a collection of local communities connected by dispersal, challenging the view that communities are localized and isolated by placing them on a landscape and connecting them via a common regional species pool. In this context, structure of local communities can be due to local environmental constraints and species interactions, or regional effects driven by dispersal and factors such as habitat connectivity. Even though the metacommunity concept inherently involves space, the direct influence of the configuration or shape of the metacommunity is a factor that has received less attention. If dispersal emerges as a key regional process important to assembly, and if dispersal patterns correlate with habitat geometry, understanding spatial structure is integral to understanding community assembly at the local scale. Stream drainage networks are an ideal system to test how habitat configuration influences the relative role of local versus regional effects on community structure. Here, we used both a null model analysis and trait-based approach to learn whether deterministic versus stochastic processes structure fish assemblages at different locations in a drainage network.

Results/Conclusions   Riverine ecosystems are a common, spatially structured ecosystem on the landscape, and a classic example of a dendritic network. We predicted that local effects should be more important in low order locations (i.e., headwaters), owing to high isolation from the regional species pool and adaptation to strong environmental gradients. However, regional effects should prevail in higher order locations given closer proximity to the regional species pool and higher potential dispersal between local communities. Using a dataset of 185 fish communities comprising 50 species from the mid-Atlantic Piedmont region of the U.S., we learned that local effects were important across all orders studied (1st – 4th), but strongest in 2nd order streams. Furthermore, trait analysis revealed that environmental filtering, not interspecific interactions, were responsible for this pattern. We conclude that environmental constraints structure fish communities across drainage networks, but that dispersal plays a smaller role in lower order streams than in more well-connected, higher order streams. Therefore, different metacommunity processes are likely occurring simultaneously in dendritic networks.