High critical forest habitat thresholds of native bird communities in Afrotropical agroforestry landscapes

Abstract

Our knowledge on the relationship between tropical forest cover and biotic communities is still limited. Understanding the relationship between forest cover and bird functional guilds may serve as a valuable tool to assess how much forest is necessary to conserve significant portions of typical forest assemblages. We sampled birds (198 species, 6883 encounters) along a full gradient of deforestation across 4000 km2 of forest-dominated landscapes in Southwest Cameroon and applied multivariate adaptive regression splines to model α-, β- and γ-richness of guilds in relation to forest cover. Overall, β- and γ-richness remained constant above 42% forest cover. However, total α-richness as well as all richness partitions of Guinea-Congo biome-restricted, large-bodied arboreal foliage gleaning, tree nesting, and frugivorous species declined when forest cover was below 74%. Moreover, ant-followers and terrestrial insectivores showed their highest diversity at zero deforestation. In contrast, open-land, granivorous, opportunistic insectivorous and widespread species strongly increased below 42% forest cover. High β-diversity at intermediate deforestation conditions indicate that the sharp decline of original forest bird diversity may only be compensated by habitat and foraging generalists, which benefit from high habitat heterogeneity. Our study implies that Afrotropical forest bird diversity decreases non-linearly with forest loss. Critical habitat thresholds estimated by us at above 70% are much higher than those previously reported and highlight the need to integrate substantial proportions of natural vegetation within wildlife friendly farming schemes.