Prospects for change in endemic populations along a steep tropical environmental gradient

Endemic species of birds are concentrated near the top of the Tilarán mountains of Costa Rica.  Simple projections of climate change suggest that the local populations of these species are precariously perched in cloudforests likely to change significantly.  Species with ranges limited to Costa Rica and Panamá also have locally narrow distributions on the mountainside, and tend to be numerically rare.  High landscape-level “beta” diversity results from such habitat specialization.  The lower range limits for many highland species are concentrated in a transition zone between cloudforest and rainshadow forest which is poorly protected.  There is good correlation between tree species composition and the species composition of bird communities.  “Climate envelope” projections based on temperature are likely unrealistic because species’ distributions are better predicted by distance from the continental divide and other surrogates for environmental moisture.  Projecting range contractions for montane species, and particularly rare endemics, will require better understanding of the climate system and its responses to anthropogenic warming, including ongoing changes in the cloud bank carrying Caribbean moisture to the Pacific slope of the mountains.  Census data suggest that many highland species are sensitive to climate change produced by edge effects in fragmented forests, and this could be a harbinger of changes to come.  Variability in measured densities over six years suggests that some species are threatened even in extensive primary forest.