Changes in abundances of forest understorey birds on Africa's highest mountain suggest subtle effects of climate change

Aim Understanding how species respond to climate and land use change is of prime importance for biodiversity conservation. However, studies using historical, empirical data to understand species responses to climate change and anthropogenic habitat disturbance remain scarce, especially for tropical mountain ecosystems. Here, we compare historical to recent mist net data to study changes in abundance of forest understorey birds along two elevational gradients. Location Mount Kilimanjaro, Tanzania. Methods We investigate patterns of understorey bird abundance along elevational and disturbance gradients on two slopes of Mt. Kilimanjaro between 1991 and 2011. To understand species responses to climate change and disturbance, we use a trait-based approach. Specifically, we test whether and how species' traits (temperature, habitat and dietary preferences) affect species responses to environmental change. Results We found that over the 20-year time interval, mean minimum temperature increased across all sites along the elevational gradient, while levels of habitat disturbance remained similar. Mean abundance of understorey bird species increased significantly, especially at higher elevations and on the southern slope. Temporal increases were not related to species' temperature preferences and were detected for habitat generalists as well as herbivorous and omnivorous species, but not for forest specialists and insectivores. Main conclusions Our results suggest subtle effects of climate change on bird abundances which would not have been detectable if we had examined elevational range shifts or changes in species richness. Both warm- and cold-adapted species appear to have benefited from increases in minimum temperatures at high elevations. This was not the case for forest specialists and insectivores which remained stable between sampling periods. This demonstrates that different groups of birds vary in their response to ongoing climatic changes. Our results underline the importance of tropical mountain ecosystems not only as global biodiversity hotspots, but also as safe havens for biodiversity in the face of global climate change.