To assess potential impacts of native and invasive terrestrial mammals on near-shore marine ecosystems on islands within the San Juan archipelago, Washington, we surveyed for the presence or non-detection of predatory mammals on a subset of 14 uninhabited islands: 10 that are part of the San Juan Islands National Wildlife Refuge, 3 that are managed by the Bureau of Land Management (BLM), and 1 that is owned by The Nature Conservancy (TNC). We were unable to detect invasive terrestrial mammals on any of the 14 islands. We found native mice (Peromyscus maniculatus) and voles (Microtus townsendi) on one island each, and native raccoons (Procyon lotor pacifica) on three islands. We speculate that the nearly ubiquitous presence of native river otters (Lontra canadensis) along with native predatory birds may be preventing immigrant invasive mammals (primarily house mice [Mus musculus] and rats [Rattus spp.]) from gaining a foot-hold on the islands. Inadequate habitat and/or island size and distance from larger islands and their source populations along with insufficient trap nights may also have contributed to the non-detection of small mammals on the islands we surveyed. For the best chance at keeping these islands free of invaders, we recommend future surveys using continuous measurement methods (e.g., track plates and monitoring blocks) for early detection of future invasions.
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