Biotic homogenization: Loss of avian functional richness and habitat specialists in disturbed Andean temperate forests

Abstract

Habitat loss and degradation have the potential to alter the species richness–functional diversity relationship decreasing both species richness and functional richness, and increasing functional biotic homogenization. These anthropogenic disturbances may have strong impacts on species-poor systems with low functional redundancy. We tested the species richness–functional diversity relationship and the potential effects of deforestation and degradation on functional diversity of avian assemblages in a Biodiversity Hotspot, in southern Chile. For 101 sites established across a disturbance gradient, we conducted 505 point-transect surveys to measure avian community structure, 505 vegetation plots to quantify stand-level structure, and measured deforestation at the landscape-level. We used three functional diversity indices for avian assemblages (functional richness, functional evenness, and community-weighted specialization as a measure of functional biotic homogenization). We found a non-saturating relationship between species richness and both functional richness and community-weighted specialization, suggesting low functional redundancy. We also found a non-significant decline of functional evenness indicating that when adding new species to the initial pool, functional redundancy increases slightly. Deforestation led to a decrease in both functional richness and community-weighted specialization, particularly precluding the persistence of specialized species in disturbed forests. Functional evenness did not vary with deforestation suggesting that the regularity of density distribution in filled niche volume may be relatively resilient to disturbance. However, the fact that community-weighted specialization decreased linearly under deforestation serves as evidence of functional biotic homogenization processes (“loser” specialists being replaced by “winner” generalist species) in a Biodiversity Hotspot.