Bird Assemblages in Naturally Fragmented Upland Forests in Pirin National Park, Bulgaria

The effects of habitat fragmentation on biodiversity has become a major focus of conservation biology and landscape ecology, but most of the studies have been restricted to anthropogenic impact on habitats, while little is known about the natural fragmentation. I examined the effects of the natural fragmentation of upland temperate forests on bird assemblages, by comparing community patterns in forest patches and continuous forests, and by analyzing the effects of patch area and isolation on community structure. Bird communities did not differ significantly between studied habitat types, but when the edge and the interior of continuous forests were analyzed separately, patches and the edges of continuous forest supported more species rich, diverse and abundant bird communities than the interior of continuous forest. Patch area was the major predictor for species richness, while isolation was not significant. The response to natural fragmentation was guild- and species-specific, and the key variables determining habitat use by birds were canopy closure and shrub cover. Most of the changes in bird community structure could be accounted for edge effects.