The indirect effects of habitat disturbance on the bird communities in a tropical African forest

Tropical forests are increasingly threatened by anthropogenic activities often resulting in habitat and biodiversity loss. To effectively manage and protect these areas, it is important to have an understanding of the factors affecting their biodiversity. Previous research has shown that birds in tropical regions are severely affected by human-induced habitat conversion and disturbance. The effects, however, are often area and guild-specific and the underlying mechanisms are frequently unclear. In this study, we disentangle and quantify the direct and indirect effects of human population density, distance to forest edge, habitat disturbance, and vegetation structure and composition on the total abundance and species richness of birds in Kakamega Forest, Kenya. Specifically, we use structural equation modeling to develop and test path models, which reflect the potential causal relationships between the bird assemblages and the chosen explanatory factors. Relationships were tested on the overall bird community and on five different guilds, classified according to birds' forest specialization and feeding preferences (i.e. forest specialists, generalists and visitors, and frugivores and insectivores). Results showed that habitat disturbance, caused by logging, had a weak positive direct effect on the bird communities, but also had a strong detrimental indirect effect, particularly on the total abundance and species richness of forests specialists and insectivores. The negative effect was mediated through changes in the forest's vegetation structure and composition. Shorter distances to the forest edge also had a negative effect, on all bird categories except on forest visitors, which also benefited from higher levels of disturbance. Our study shows that although in some cases habitat disturbance may have no strong direct negative effects it can still negatively influence bird communities in an indirect way. In the case of Kakamega Forest, we suggest that to conserve bird communities it is important to maintain the forest's compositional and structural diversity by reducing human-induced habitat disturbance, such as illegal logging activities.