Endemism and local rarity in birds of neotropical montane rainforest

Species' rarity is expressed and measured at a variety of spatial scales, and rarity at different scales could be correlated. Considerable empirical evidence has supported a positive correlation between spatial distribution and local numerical abundance of species, which could create a syndrome of rarity for species limited in geographic distribution and occurring at low densities even in their primary habitats. We used auditory-count data and range information for birds in the Tilarán mountains of Costa Rica to examine the abundance-distribution relationship in cloud forest species and to determine if endemic species (limited to Costa Rican-Panamanian highlands or Central America) are locally rare or restricted in ecological distribution. We found a positive correlation between abundance and distribution at the smallest spatial scale: species occupying few sites in the study area had low abundances where they occur. At larger scales, geographical distribution measured categorically was correlated with local ecological amplitude (zones occupied). Local abundance, however, was not correlated across all species with estimates of range size (km2), although most endemics exhibited below-median abundance. Within families with endemics and cosmopolitan species, 72% of narrow endemics were among the rarest species. For most narrow endemics, we rejected the possibility that they are sufficiently specialized within their habitats to reach relatively high densities. A conclusive test of the abundance-distribution relationship at large spatial scales and evaluation of global population size for endemics will require a comprehensive analysis using abundance data from throughout species' ranges. Such analyses for tropical species will be a long time coming, but it is meanwhile prudent to recognize that many endemics are threatened at several spatial scales, including the risks of small local populations.