Over the past decade, there has been a growing renewed interest in understanding the determinants of beta diversity, the variation in species composition among local communities, as a major component of biodiversity that can influence ecosystem functioning. One well-recognized determinant of beta diversity is dispersal of individuals among local communities. In the current literature, dispersal is almost invariably thought to homogenize communities, decreasing beta diversity. However, empirical tests by field experiments remain limited. We manipulated dispersal of nectar-inhabiting bacteria and yeasts via flower-visiting animals to examine how dispersal influenced microbial beta diversity among flowers and its functional consequences for nectar chemistry.
Contrary to the prevailing view that dispersal lowers beta diversity, we found beta diversity was highest when dispersal was least limited. Null-model analyses, coupled with supplementary analyses of alpha, beta, and gamma diversity as a function of dispersal manipulation and total abundance, suggested that this enhancement of beta diversity was driven in part by priority effects, rather than merely reflecting statistical inevitability or increased successional variability. Dispersal is highly stochastic, generating variability in species arrival history and consequently the potential for community divergence via priority effects, in these and likely many other microbial, plant, and animal communities. Yet most previous experiments eliminated this possibility. We suggest that the positive effects of dispersal on beta diversity, like the one we report here, may have been greatly underappreciated.