Advancing theory of community assembly in spatially structured environments: local vs regional processes in river networks

The most basic ecological questions are frequently the most difficult to answer. The foundational questions of community ecology are prime examples: Why are these species found in these abundances at this location? Why does the assemblage at site X differ from that at site Y? How and why do these assemblages change through time? The questions are simple, but the answers have proven to be multifaceted, complex, and often elusive. However, community ecologists recently have made large strides in their ability to answer these fundamental questions. Much of this progress has resulted from recognition of a simple principle: processes that influence the structure and dynamics of communities occur on multiple spatial scales and, for many communities, focusing on processes at a single spatial scale will not provide understanding of the factors that shape communities. In particular, recent research has focused on the interaction between local and regional forces in structuring communities, where local refers to species interactions and environmental filtering, whereas regional refers to processes primarily driven by the dispersal of organisms. Community ecologists have long realized that both local and regional processes can structure communities, but the recent emphasis on the interactions between scales has led to progress in both theoretical and empirical community ecology. This change in perspective was implicit in the controversial Neutral Theory (Hubbell 2001), is explicit in the emerging body of theory known as Metacommunity Ecology (Leibold et al. 2004, Holyoak et al. 2005), and continues to produce a groundswell of innovative community ecology research. Embracing the interaction between local and regional factors as an organizing feature of communities has obvious significance for the study of riverine communities and ecosystems. Streams and rivers are, by their nature, spatially structured systems with a very particular dendritic (i.e., linear branching), hierarchical architecture (Fagan 2002, Grant et al. 2007). The directional flow of rivers strongly affects the movement of organisms through bulk flow, which transports both organisms and materials, and the life histories of many organisms reflect evolutionary responses to a flowing-water environment that leads to directional rather than diffusive dispersal patterns. The pervasive and sometimes extensive movement of organisms in riverine landscapes suggests that understanding community assembly and dynamics in riverine systems will require knowledge of regional dispersal-driven processes in addition to knowledge of local environmental conditions and species interactions. The results of recent research support this assertion (Heino 2005, Muneepeerakul et al. 2007, 2008a, b, Heino and Mykra 2008, Hitt and Angermeier 2008, Brown and Swan 2010, Clarke et al. 2010). Within this framework, a rapidly emerging focal point for research is the role of spatial configuration of riverine networks in dictating properties of communities. Configuration of riverine networks affects occupancy patterns of some organisms (Grant et al. 2009), changes the relative balance of local and regional forces in structuring stream invertebrate communities (Brown and Swan 2010), and affects the performance of bioassessment metrics (Hitt and Angermeier 2008). Multiscale, multispecies approaches to investigating the influence of dendritic network structure on riverine communities are the topic of this special series of articles. The contributions in this special series are bound together by a recognition of the need for simultaneous consideration of multiple spatial scales and a focus on the effects of spatial structure of riverine systems when attempting to explain the structure and func1 E-mail addresses: chris.swan@umbc.edu 2 bbrown3@umbc.edu J. N. Am. Benthol. Soc., 2011, 30(1):232–234 ’ 2011 by The North American Benthological Society DOI: 10.1899/10-150.1 Published online: 18 January 2011