Biotic homogenization and differentiation effects of fragmentation vary with spatial scale for multiple levels of understory bird diversity in northwest Ecuador

Biotic homogenization and differentiation are two possible outcomes of tropical forest fragmentation with ramifications for biodiversity conservation. The extent of either outcome may vary depending on spatial scale and heterogeneity of climate and habitats, but these relationships remain poorly understood for taxonomic, functional, and phylogenetic metrics of diversity. Our goal was to determine the extent to which fragmentation is associated with biotic homogenization and/or differentiation across multiple levels of diversity at landscape versus regional scales. We examined diversity and similarity of understory bird communities in northwestern Ecuador, an ecoregion characterized by climatic variation, habitat loss and fragmentation, and high levels of biodiversity and endemism. At the landscape level, fragmentation was associated with biotic differentiation of understory bird communities between formerly contiguous intact and fragmented humid forests. At the regional scale, we found taxonomic homogenization across climate zones and forest types: the bird community in the fragmented humid landscape was as similar to the nearby intact humid forest community (mean distance = 3.1 km) as it was to that of drier, seasonal intact forest 60 km away. Patterns of landscape-level differentiation and regional homogenization were present but substantially less pronounced for functional and phylogenetic diversity, indicating functional and phylogenetic redundancy despite turnover and abundance shifts among dominant species. Our results suggest fragmentation of tropical forest (associated with homogenization between humid and drier/seasonal forest) may have similar impacts on communities as climate drying, and while many species may be lost, fragmented communities may retain some functional and evolutionary resilience against environmental stressors.