Habitat insularization alters taxonomic and functional beta diversity of understorey birds in Amazonian forest islands

Abstract

Habitat fragmentation can profoundly alter the composition of biological communities, modifying patterns of beta diversity among areas. These effects are manifested through different components of beta diversity: differences in richness, which reflect variation in species numbers among sites, and species replacement, which indicates changes in species identity. Understanding which of these components is most affected can reveal distinct mechanisms that structure diversity in fragmented landscapes. In the Amazon, the formation of forest islands resulting from the construction of hydroelectric dams creates an archipelagic landscape whose insular patches vary in area and degree of isolation, serving as a natural experimental system to investigate how habitat fragmentation influences these components. We examined the effects of fragmentation on the taxonomic and functional beta diversity of understorey birds, specifically assessing how island area and isolation influence these patterns. Although functional and taxonomic beta diversity follow the same logic, the former is based on differences in ecological guilds, allowing detection of changes in both the number of functional guilds and their composition. The study was conducted at the Balbina Hydroelectric Reservoir (Amazonas, Brazil), where understorey birds were sampled using mist nets on 32 islands and five continuous forest sites between June and December of 2023 and 2024. With a sampling effort of 21 312 net hours, we captured a total of 1634 individuals belonging to 114 species. Both taxonomic and functional beta diversity were higher on islands compared to continuous forests, indicating that insularization increases compositional variation among communities, mainly due to species richness differences. Island area and isolation significantly influenced beta diversity, contributing to greater differences in taxonomic and functional richness among sites, suggesting that selective local extinctions play a central role in community structuring. Our results suggest that, although insularization increases beta diversity, this increase probably reflects underlying species and functional losses, which may have consequences for the maintenance of ecological functions and ecosystem services provided by Amazonian understorey birds.