Patterns and predictors of β-diversity in the fragmented Brazilian Atlantic forest: a multiscale analysis of forest specialist and generalist birds

1. Biodiversity maintenance in human-altered landscapes (HALs) depends on the species turnover among localities, but the patterns and determinants of beta-diversity in HALs are poorly known. In fact, declines, increases and neutral shifts in beta-diversity have all been documented, depending on the landscape, ecological group and spatial scale of analysis. 2. We shed some light on this controversy by assessing the patterns and predictors of bird beta-diversity across multiple spatial scales considering forest specialist and habitat generalist bird assemblages. 3. We surveyed birds from 144 point counts in 36 different forest sites across two landscapes with different amount of forest cover in the Brazilian Atlantic forest. We analysed beta-diversity among points, among sites and between landscapes with multiplicative diversity partitioning of Hill numbers. We tested whether beta-diversity among points was related to within-site variations in vegetation structure, and whether beta-diversity among sites was related to site location and/or to differences among sites in vegetation structure and landscape composition (i.e. per cent forest and pasture cover surrounding each site). 4. beta-diversity between landscapes was lower than among sites and among points in both bird assemblages. In forest specialist birds, the landscape with less forest cover showed the highest b-diversity among sites (bird differentiation among sites), but generalist birds showed the opposite pattern. At the local scale, however, the less forested landscape showed the lowest beta-diversity among points (bird homogenization within sites), independently of the bird assemblage. beta-diversity among points was weakly related to vegetation structure, but higher beta-diversity values were recorded among sites that were more isolated from each other, and among sites with higher differences in landscape composition, particularly in the less forested landscape. 5. Our findings indicate that patterns of bird beta-diversity vary across scales and are strongly related to landscape composition. Bird assemblages are shaped by both environmental filtering and dispersal limitation, particularly in less forested landscapes. Conservation and management strategies should therefore prevent deforestation in this biodiversity hotspot.