Response of Avian communities to edges of tropical montane forests: Implications for the future of endemic habitat specialists

Tropical montane landscapes harbor diverse flora and fauna, and many
species there are ecological specialists with narrow elevational
distributions, limited geographic ranges, and small global populations.
Along elevational gradients, environmental conditions and community
composition change dramatically over small spatial scales. As forests are
disturbed and edges formed with modified habitat, natural communities
could be affected differently across elevations by the many physical and
biotic changes at edges. We asked whether forest edges produced altered
patterns of avian species composition along a cloud forest - dry forest
gradient on the Pacific slope of the Tilarán mountains in Monteverde,
Costa Rica. A strong moisture gradient produces cloud forests
near the ridgetops, with a concentration of species endemic to the Costa
Rica – Panama highlands that are habitat specialists. We conducted 552
point counts across 110 locations from 1100 to 1800 m elevation, yielding
6586 detections of 115 species in 10 km2 of montane forest. We analyzed
differences in species composition and single-species abundances between
interior and near-edge forest habitats for species grouped by geographic
range size. Species composition changed markedly from forest edge
to interior in cloud forest habitats, but not in drier forests
downslope. Endemic species, especially in cloud forest, were
detected less frequently in mature forest near edges than in mature forest
interior, and this difference was more pronounced than for cosmopolitan
species. On tropical mountainsides, we can expect
habitat-specialist endemic species to be more sensitive to further habitat
modification. This sensitivity could limit the resilience of tropical bird
communities.