The role of branching in the maintenance of diversity in watersheds

Stream systems are characterized by a hierarchical dendritic branching pattern, which is thought to have important consequences for stream community structure. However, branching is associated with many other changes in stream condition, which make disentangling the effects of branching from associated environmental changes challenging. Moreover, much difficulty arises in scaling up local effects of branching within a watershed to the watershed as a whole, which is where diversity is arguably maintained and management and restoration efforts are assessed. Here, we show that branching can be present with no associated changes to community dynamics, either locally or in the watershed as a whole when branching affects only dispersal patterns and not environmental structure. We explore violations of these conditions to identify how branching can modify environmental conditions affecting coexistence in the watershed. We show that the strength of coexistence in the watershed depends in all essentials on the amount of environmental heterogeneity with only a small role for other factors. Only when streams are small relative to the scales of environmental change or dispersal can branching affect the strength of watershed-scale coexistence independently of changes to environmental heterogeneity in the watershed, but these effects are small and depend on interactions with environmental variation. We suggest that focusing on environmental heterogeneity rather than branching patterns will yield the greatest understanding of the drivers of stream community structure and the greatest benefits for management and restoration of stream communities.