The lizard fauna of Guam's fringing islets: island biogeography, phylogenetic history, and conservation implications

We sampled the lizard fauna of twenty-two small islets fringing the Pacific island of Guam and used these data to shed light on the processes responsi ble for present-day diversity. Habitat diversity, measured by islet area an d vegetation complexity, was significantly correlated with the number of sp ecies found on an islet. However, islet distance and elevation were not sig nificant predictors of diversity. Distribution patterns were slightly diffe rent for the two major families in our sample, Scincidae and Gekkonidae: sk inks needed larger islets to maintain a population than did geckos. Presenc e/absence patterns were highly and significantly nested, and population den sity was correlated with the number of islets on which a species was found. An area cladogram was poorly supported and showed no faunal similarity bet ween nearby islands. These patterns indicate that extinctions on most islet s were due mostly to non-catastrophic, long-acting biological causes. The p resence on the islets of species extirpated on Guam and the lack of signifi cant nestedness on islands with greater maximum elevation highlight the imp act that predators (primarily brown treesnakes) can have. Our findings also show that small reserves will not suffice to protect endangered lizard fau nas, and that the islets may serve as a short-term repository of such speci es until snake-free areas can be established on Guam.