Niche and neutral factors affecting bird community assembly at multiple spatial scales.
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Pinney, Tracy A. 1975-
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Neutral theory suggests communities are assembled based on demographic stochasticity and dispersal distance, while niche theory suggests that communities are assembled based on habitat requirements and interspecific interactions. The relative importance of niche and neutral processes in community assembly has received much attention. I hypothesized that niche processes were more important than neutral processes in structuring bird communities in central Texas. I sampled birds at 262 – 323 study sites during the summer breeding season in 2008, 2009, and 2010. To account for niche processes, environmental variables were measured at three spatial extents around each site using remotely-sensed landcover data at two spatial resolutions (10 meter and 30 meter resolution landcover maps). I also included niche variables representing interspecific interactions (avian nest predator abundance and the abundance of a brood parasite). Spatial eigenvectors were used to account for dispersal-based neutral processes. I used multiple regression and variance partitioning to determine the factors most important for bird assemblages in three different guild groupings (habitat, migratory, and trophic). Both niche and neutral variables were important for most guilds in all years. For most guilds, niche variables accounted for more of the variation in community composition than did spatial variables, suggesting that niche processes were more important than were neutral processes in structuring these assemblages. However, for a few guilds (permanent residents, short-distance migrants, and herbivores) in some years, spatial variables accounted for more of the variation in community composition than did niche variables, suggesting that neutral processes may play a dominant role in structuring some bird assemblages. Some of these results varied from year to year, suggesting the relative importance of niche and neutral processes may fluctuate temporally in bird communities. I also investigated the importance of environmental variables measured at multiple spatial extents on bird community composition. Overall, environmental variables measured at a small spatial extent around each study site explained more of the variation in community composition than did environmental variables measured at a medium and large spatial extent. Understanding the factors that are important for bird community assembly has important conservation implications and can inform management decisions for guild-level assemblages.