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Inputs of marine resources contribute to high consumer densities on small, barren islands in the and Gulf of California, Mexico. Episodes of rainfall associated with El Nino Southern Oscillation (ENSO) events, however, stimulate plant growth and seed production and replenish soil detritus and seed banks. We studied the effects of marine inputs and pulsed ENSO resources on rodent populations on islands before, during and after the strong 1997-1998 ENSO event. Stable isotope analysis was used to determine rodent diets. During dry years, P. maniculatus consumed marine prey near shore and on small islands, and were much more abundant in these areas than farther inland and on larger islands. Captures were particularly concentrated near shore on three islands inhabited by Chaetodipus rudinoris, a granivore that was numerous in inland areas. Following the ENSO, Peromyscus maniculatus increased in abundance by 408%, compared to only 160% for C. rudinoris, and new individuals invaded inland areas. P. maniculatus populations declined sharply the following year, however, whereas C. rudinoris populations remained relatively stable. In response to ENSO conditions, P. maniculatus ate more seeds on islands with C. rudinoris than on islands without C. rudinoris. P. maniculatus also increased less markedly and declined more quickly in abundance where C. rudinoris was present. We speculate that P. maniculatus is capable of rapid numerical response to resource pulses but depends on marine foods during dry years. C. rudinoris is restricted to islands with sufficient seed resources, but maintains stable populations by exploiting low-density seeds and caching seeds from wet years. On islands with both species, C. rudinoris restricts P. maniculatus to near-shore areas via interference, but spillover consumption of seeds by subsidized P. maniculatus may limit the response of C. rudinoris to ENSO resources. Combined with differences in life history and dispersal abilities, these ecological factors help explain the distinct biogeographic distributions of these rodents on Gulf islands.