Bird species richness peaks at intermediate levels of farmland in wooded savannas and dry forests, but not in rainforests

Abstract

Despite widespread concern over farmland-driven biodiversity loss in tropical landscapes, the possibly non-linear response of species richness to farmland and the possible dependency of this response on biome remain poorly understood. Drawing on nationwide data about the occurrence of terrestrial bird species at the landscape level in Mozambique, we analysed the effects of farmland extent on species richness, globally threatened, large-range and open-habitat species. In the analysis of species richness, we allowed for non-linear effects and difference in patterns between the rainforest and the wooded savanna–dry forest biomes. According to the intermediate disturbance hypothesis, we found that species diversity peaks at an intermediate level of farmland in wooded savannas and dry forests. However, it monotonically declines with farmland in rainforests, ironically the biome used by Connell to illustrate this hypothesis. This difference across biomes reflects higher resilience of the biota of drier biomes to farmland expansion, possibly related to the role of fire and herbivory in their regulation. Our results confirm that trends in species richness do hide the decline of threatened species and the increase in widespread generalists with farmland extent, contributing to biotic homogenization. Richness alone is thus a poor early-warning indicator of biodiversity loss particularly in wooded savannas and dry forests where species richness initially rises with farmland. Our results emphasize that effective conservation in tropical landscapes requires biome-specific solutions and better indicators of biodiversity loss to guide policies aimed at protecting species richness in rainforests and avoiding biotic homogenization in wooded savannas and dry forests.