Bird community ecology and composition in afrotemperate forests of the Drakensberg Mountains, South Africa.

Abstract

Recent research has emphasized the importance of understanding the consequences of species loss, not just for biodiversity per se, but also for ecosystem resilience and functioning. Firstly, a baseline analysis of the effects of a naturally patchy landscape on avian community composition and resilience in a high altitude Afrotemperate forest landscape in South Africa is presented. Bird data from a point count survey of 706 counts at 31 forest sites was used to test whether taxonomic species diversity, functional species richness and density of birds provide insight into community resilience in this historically patchy ecosystem. Bird species richness of forest patches ranged from 17 to 38, with a total species richness of 50. Density was slightly but negatively related to both area and species richness. That density compensation is occurring in these Afrotemperate forests suggests a level of resilience in this system. Following on from this, cumulative species-area and function-area graphs were derived to quantify the loss of forest area or taxonomic bird species richness that this landscape may potentially sustain before ecosystem functioning is negatively affected. The concept that species’ patterns of distribution, abundance and coexistence are the result of local ecological processes has recently been challenged by evidence that regionalscale processes are equally instrumental in shaping local community composition. The historically and naturally fragmented Afrotemperate forests of the uKhahlamba– Drakensberg Park, South Africa, offer an interesting test case. In this study the relative effects of local and regional-scale processes on species assemblages in a naturally patchy forest system were investigated. By employing species-area and species-isolation relationships, and nested subset analyses, we showed that isolation (regional-scale process) had a greater effect on bird species richness and composition than area (local-scale process), though the species-area relationship was significant. Using generalized linear models and an information-theoretic approach to model selection, patch area, the size of the regional species pool as well as the distance to the nearest Eastern Mistbelt forest were all influential in determining local bird species richness in these montane forests. Thus, localities are regionally enriched within the constraints on species occupancy provided by the available habitat.