Interactive effects of environmental filtering predict beta-diversity patterns in a subtropical forest metacommunity

The metacommunity framework offers a possibility to better understand how ecological processes influence patterns of species diversity along environmental gradients. The species-sorting perspective predicts that community composition varies in response to differences in environmental conditions among habitat patches. Our study focused on this perspective, aiming to understand how environmental filtering processes interact directly and indirectly on diversity patterns in an area of 95,000 km2 (data from 432 forest plots). We employed structural equation modeling (PLS path modeling) to disentangle the interactive effects of topography, climate, water-energy balance, and geometry of forest patches upon the alpha and beta diversity of a subtropical forest metacommunity in southern Brazil. Factors related to environmental filtering showed substantial effects upon tree alpha and beta diversity. The total amount of variation in beta diversity explained by environmental filtering was high (64%) and was even more when together with alpha diversity (73%), corroborating the prediction of species-sorting model at the metacommunity level. Climatic extremes, water-energy balance and alpha diversity were the key determinants of beta diversity and patch size and water-energy balance the key determinants of alpha diversity in the South Brazilian Atlantic forests. Partial mantel test showed that environmental effects occurred largely independent of spatial effects, reinforcing the tested prediction. Our study provides strong empirical support for the prediction that beta diversity primarily reflects deterministic factors associated with species niches and their responses to environmental conditions in the studied spatial scale.