FOSSIL BIRDS, REPTILES, AND MAMMALS FROM ISLA FLOREANA, GALAPAGOS ARCHIPELAGO.
AuthorSTEADMAN, DAVID WILLIAM.
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PublisherThe University of Arizona.
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AbstractThis study surveys late Holocene vertebrate fossils from Isla Floreana, Galapagos Islands. 20,000 fossils from four lava tubes near Post Office Bay are loosely associated with four radiocarbon dates of 2400 years BP or younger. Most fossils originated as regurgitated pellets of barn owls (Tyto punctatissima). They include six species now extinct on Floreana: Geochelone elephantopus, Alsophis biserialis, Tyto puntatissima, Mimus trifasciatus, Geospiza nebulosa, and Geospiza magnirostris. These species are, respectively, 1st, 7th, 16th, 6th, 15th, and 2nd in abundance among those recorded as fossils, making up 57% of individuals in the fauna. Thus extinction probably has changed the composition of Floreana's fauna even more than suggested by the number of extinct species alone. The evidence is circumstantial, but I believe that all extinction on Floreana is related to human impact, such as predation, habitat alteration, and introduction of alien animals (rats, mice, cats, dogs, pigs, goats, cattle, and donkeys). Direct human predation was probably the main cause of extinction only for Geochelone elephantopus. Extinction of Tyto punctatissima was probably due to loss of preferred prey species. Extinction of Mimus trifasciatus and Geospiza magnirostris may have accompanied destruction of Opuntia cactus. Extinction of Geospiza nebulosa may be related to habitat changes in the highlands. All extinction on Floreana probably occurred in historic times; whether this is true elsewhere in the Galapagos awaits more research. The lack of fossils of Coccyzus melacoryphus and Dendroica pectechia is further evidence that these birds colonized the Galapagos very recently. Fossils enable us to reconstruct natural, pre-human faunas more completely than previously possible. Modern biogeographical studies usually do not consider how natural the faunas are; they would benefit by considering changes wrought by human impact.