Getting there is half of the fun: Using species traits to infer mechanisms driving metacommunity patterns in stream networks

The “metacommunity” is a series of local communities connected by the dispersal of organisms. A primary goal of metacommunity ecology is to understand the relative contributions of local processes (i.e., environmental filtering, species interactions) vs. regional processes (i.e., dispersal) in dictating community structure. One empirical pattern used to examine these relationships is the distance-decay relationship (DDR) that describes changes in similarity between local communities across space, with a strong distance-decay relationship generally indicating dispersal-driven dynamics. However, because environmental conditions also vary across space, correctly inferring process from DDRs can be difficult. We incorporated species traits—in particular, traits related to dispersal—into an analysis of DDRs using a large dataset on stream macroinvertebrate communities. We predicted that communities characterized by strongly dispersing species would produce weaker DDRs than communities characterized by poor dispersers. We also predicted that results would rely on proper identification of dispersal pathway, either within the stream network (e.g., drift) or outside of the stream network (e.g., overland flight). Here we describe empirical results that generally support our predictions and suggest that communities in different sections of stream networks (i.e., upstream vs. downstream) may be shaped by drastically different processes.