Understorey Bird Responses to the Edge-Interior Gradient in an Isolated Tropical Rainforest of Malaysia

The fact that the world is losing its biodiversity due to human activities, particularly around the tropical forest region, has been widely known. One of the biggest threats to biodiversity is the edge effects, especially in isolated and fragmented habitats. The change in understorey bird community composition and habitat variables were studied along an edge-to-interior gradient in a 1248 ha lowland rainforest patch in peninsular Malaysia. Birds and environmental variables such as vegetation structure and litter depth were detected within a 25 m radius of each of 93 sampling points distributed throughout the forest. Species composition differed along the edge-interior gradient at the guild and species level, although only a few species were entirely confined to either edge or interior habitat. Based on bird-habitat associations along the edge-interior gradient, two groups were distinguished. Abundance of the edge-specialist group, mainly foliage-gleaning insectivores and insectivore/ frugivores, was positively correlated with ground cover, light intensity, shrub cover, temperature, and percent of shrub cover between 0.5 and 2 m height. Some of these edge-preferring species also occurred in the matrix surrounding the patch and were extremely abundant, which may create problems for forest species. In contrast, the interior-specialist group, mainly terrestrial insectivores, avoided the forest edge and was positively associated with humidity, canopy cover, number of dead trees, percentage of litter cover, and depth of the litter layer. From a conservation perspective, forest remnants in the lowlands of Peninsular Malaysia that have a deep leaf litter layer, a dense canopy cover, high number of dead trees, and high relative humidity are able to support regionally significant understorey bird species that are sensitive to edge effects. As such these forests have important conservation value.