Isotopic niche mirrors trophic niche in a vertebrate island invader
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Caution for the indiscriminate conversion of the isotopic niche into ecologic niche was recently advised. We tested the utility of the isotopic niche to answer ecological questions on oceanic islands. We compared the isotopic niches of black rats (Rattus rattus) on two islands in the Gulf of California, Mexico: Farrallón de San Ignacio (FSI) and San Pedro Mártir (SPM). Both islands maintained several species of marine birds, but FSI is devoid of terrestrial vegetation and SPM has several species of terrestrial plants. We tested the hypothesis that rats on FSI have a narrower trophic niche due to its lower diversity of food items. We predicted a smaller variance in δ13C and δ15N values of rat muscle on FSI, and a lower use of marine birds as food on SPM. We also examined stomach contents of rats on both islands to validate the isotopic information. Variances in δ13C and δ15N values of black rats were lower on FSI, and the contribution of marine birds to the diet of rats was smaller on SPM. Stomachs in most rats collected on FSI contained only one or two types of food items, mostly marine birds and terrestrial invertebrates. In contrast, stomachs with only one type of food item were rare on SPM, and in most cases they contained three or more food types. Our findings showed that isotopic variance is a good approximation for trophic niche when comparing populations with access to an assemblage of preys with contrasting biological and isotopic diversity.
KeywordsBlack rats Feeding habits Island ecology Rattus rattus SIAR
We thank Carolina Valdespino, Vinicio Sosa, Alberto González and Enriqueta Velarde for their advice and help in the development of this study, Alfonso Aguirre and Araceli Samaniego from Conservación de Islas for their wonderful support, Ricardo González and Jorge Villavicencio for their assistance in the field work, and Donald L.Phillips and Richard Inger for their comments on the manuscript. The Consejo Nacional de Ciencia y Tecnología supported our work with a research grant to L.G.H.M. (#43343), and a scholarship to M.A.R. Sampling and landing permits were granted by the Wildlife General Direction (DGVS) of the Environment Ministry (SEMARNAT), and the Interior Ministry (SEGOB), respectively. We finally thank to the regional offices of the Islas del Golfo de California Natural Protected Area from the Comisión Nacional de Áreas Naturales Protegidas in Sonora and Sinaloa for allowing us the access to the islands.
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