Marine resources subsidize insular rodent populations in the Gulf of California, Mexico

Inputs of energy and nutrients from one ecosystem may subsidize consumers in adjacent ones, with significant consequences for local communities and food webs. We used stable isotope and faecal pellet analysis to quantify use of ocean-derived resources by small mammals on islands in the Gulf of California, Mexico. Rodents were live-trapped on grids originating near shore and extending 125-200 m inland to evaluate the extent to which rodents transport marine nutrients inland, and to determine whether marine foods subsidize island populations, permitting higher densities than would be possible based on terrestrial resources alone. Both faeces and stable carbon and nitrogen isotopes revealed that omnivorous mice (Peromyscus maniculatus) consume ocean-derived prey, including littoral and supralittoral invertebrates, and that their diets differed markedly from those of granivorous rodents (Chaetodipus rudinoris). On a small, seabird roosting island, marine prey were important in the diet of mice regardless of their proximity to shore, underscoring the pervasive influence of the ocean on small islands with relatively large coastline area. On a large island, however, consumption of marine foods declined sharply > or =50 m from shore, which suggests that mice are poor conduits of inland movement of energy and nutrients from the sea. Marine resources seemed to act as subsidies for omnivorous rodents: more P. maniculatus were captured near shore than farther inland and there was an inverse relationship between island area and rodent abundance, suggesting that small islands with large amounts of marine inputs support the highest population densities. Patterns of local and island-wide abundance of P. maniculatus are likely the result of several interacting factors, including frustrated dispersal, competition with C. rudinoris, and the absence of predators. We speculate, however, that the availability of marine resources allows P. maniculatus to reach high densities and to persist on small islands in the Gulf despite low and unpredictable terrestrial productivity. Spatial trophic subsidies thus provide a possible mechanistic explanation for the widely reported inverse relationship between population density and island or habitat area.