Bathing birds bias beta-diversity: frequent dispersal by gulls homogenizes fauna in a rock-pool metacommunity.

Metacommunity theory generally predicts that regional dispersal of organisms among local habitat patches should influence spatial patterns of species diversity. In particular, increased dispersal rates are generally expected to increase local (alpha) diversity, yet homogenize local communities across the region (decreasing beta-diversity), resulting in no change in regional (gamma) diversity. Although the effect of dispersal on alpha-diversity has garnered much experimental attention, the influence of dispersal rates on diversity at larger spatial scales (beta and gamma) is poorly understood. Furthermore, these theoretical predictions are not well tested in the field, where other environmental factors (e.g., habitat size, resource density) likely also influence species diversity. Here, we used a system of freshwater rock pools on Appledore Island, Maine, USA, to test the effects of dispersal rate on species diversity in metacommunities. The pools exist in clusters (metacommunities) that experience different levels of dispersal imposed by gulls (Larus spp.), which we show to be frequent passive dispersers of rock-pool invertebrates. Although previous research has suggested that waterbirds may disperse aquatic invertebrates, our study is the first to quantify the rate at which such dispersal occurs and determine its effects on species diversity. In accordance with theory, we found that metacommunities experiencing higher dispersal rates had significantly more homogeneous local communities (reduced beta-diversity) and that gamma-diversity was not influenced by dispersal rate. Contrary to theoretical predictions, however, alpha-diversity in the rock pools was not significantly influenced by dispersal. Rather, local diversity was significantly positively related to local habitat size, and both alpha- and gamma-diversity were influenced by the physicochemical environment of the pools. These results provide an important field test of metacommunity theory, highlighting how local and regional factors interact to drive patterns of species diversity in metacommunities, and demonstrate that waterbirds are indeed important dispersal vectors for aquatic invertebrates.