Edge Influence on Avian Assemblages in an Isolated Tropical Coastal Forest Reserve, Tanzania

Abstract

Anthropogenic pressure on isolated protected areas without buffer zones is more intense at boundaries and results in an increase in edge effects. These edge effects influence the distribution and composition of various organisms. In this study, we investigated changes in bird community composition and habitat variables along an edge-to-interior gradient in an isolated coastal nature forest reserve near Dar es Salaam. Birds and habitat variables were recorded at 147 sampling points across 49 systematically placed parallel transects in three habitat treatments (edge, near-edge and interior) to determine the response of the avian community to edge effects. In total, we detected 3249 individuals representing 136 species belonging to 46 bird families, with notable variation along the edge–interior gradient. Non-forest species and forest visitors dominated the bird community, accounting for 47% and 24% of detections, respectively. Although there was a variation in avian community along the gradient, the 200 and 400 m communities were not detectably different. The edge had less ground cover compared to areas beyond 200 m from the forest edge, but no edge–interior gradient changes were observed for vertical habitat structure. A Principal Component Analysis (PCA) explained around 63.3% of the total variance across two axes, and the PERMANOVA results indicated a statistically significant difference among the habitats (F = 2.8386, p = 0.022, R² = 0.03793) but the low R² implied that the effect size was small. Although forest bird species were positively associated with vertical habitat structure, low canopy height throughout the study area and the penetration of non-forest bird species into interior habitats served as an indicator of past anthropogenic disturbances. Thus, conservation efforts should be strengthened to allow vegetation recovery, particularly given the reserve's importance for forest species, including those endemic to the coastal forest belt of Eastern Africa.