Time Lag between Deforestation and Bird Extinction in Tropical Forest Fragments

Tropical forests are becoming increasingly fragmented, threatening the survival of the species that depend on them. Small, isolated forest fragments will lose some of their original species. What is uncertain is how long this process of faunal relaxation will take. We compiled data on birds in five tropical forest frag- ments in Kakamega Forest, Kenya, of known date of isolation. We then predicted the original and eventual species richness of these fragments and, from this difference, the eventual species losses. Expressing the losses to date as a fraction of eventual losses suggests that faunal relaxation approximates an exponential decay with a half-life of approximately 50 years for fragments of roughly 1000 ha. In other words, in the first 50 years after isolation, tropical forest fragments of this size suffer half of the total number of extinctions that they are likely to experience. This result sets the time scale over which humanity must take conservation ac- tion in fragmented tropical forests, may aid efforts to set priorities, and indicates how high the future global extinction rate will be.

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