Avian Extinction on Barro Colorado Island, Panama: A Reassessment

Abstract

Barro Colorado Island (BCI) was formed early in the 20th century when the Chagres River was dammed to form part of the Panama Canal. BCI is useful for studying extinction/colonization patterns on a recently isolated, continental island. Previous avifauna studies were based on lists of birds on BCI only; a new study was made by sampling birds in the forest undergrowth both on BCI and on the adjacent mainland to determine how the areas differ, both in total bird counts and in more strictly defined forest birds, and what the ecological and/or evolutionary reasons for those differences are. The present undergrowth avifauna of BCI has reduced species richness and total abundances. The guild structure of the undergrowth avifauna differs from that on the mainland. The extinction estimate for strictly forest birds is 3 times greater than previous estimates. Species which do not occur on BCI are not a random subset of species from the presumed source fauna; species associated with lower levels of forest seem especially subject to extinction. Even relatively abundant species of the undergrowth are lost. Forest species associated with foothill areas are also prone to extinction on BCI. Seven ecological/evolutionary factors that may be responsible for the depressed species richness on BCI are discussed: biogeographic access, the founder effect, competition, predation, successional habitat changes, island size and environmental heterogeneity and the habitat mosaic (including rainfall and seasonality).