The unified neutral theory of biodiversity and biogeography predicts that distance decay in community similarity is based on dispersal distance and ecological drift across homogeneous environments, whereas niche-assembly theory predicts distance decay is associated with environmental conditions and interspecific associations. We used multiple regression on distance matrices to test predictions of neutral theory and niche-assembly theory by investigating the relationship between bird community similarity and geographic distance, landscape conditions, and numbers of diurnal avian predators and brood parasites. We studied summer resident birds at 282 study sites in the Oaks and Prairies region of Texas during 2008. FRAGSTATS was used to measure percent landcover, edge density, mean patch area, and fractal dimension for four cover types (woodland, grassland, agriculture, and developed area) within 1-km-radius extents around each study site based on 2001 National Land Cover Data. For a subset of 75 study sites, we also measured the landscape variables at medium (1.8-km-radius) and large (24.1-km-radius) spatial extents associated with species’ median and maximum dispersal distances.
Results/Conclusions
Our results did not support neutral theory or niche-assembly theory for the community we studied. Geographic distance, landscape conditions measured at multiple spatial extents, and interspecific associations explained little of the variation in community similarity. Neutral theory predicts that distance decay in community similarity occurs across homogeneous environments, and thus we used a relatively homogeneous study area for our analysis. Nevertheless, we did not find support for neutral theory. It is possible that the vagility of the bird species we studied may have swamped the distance effects predicted by neutral theory. Distance decay predictions of niche-assembly theory may apply better where environmental differences across the study area are more pronounced. We predict that additional landscape metrics for more recent conditions measured at multiple spatial extents around each study site will explain more of the variation in bird community composition.
