Impacts of invasive mammals on California Channel Island ecosystems

Marine islands make up about 3% of the earth's land area, but contain 15-20% of plants, birds and reptiles.  They were home to 64% of IUCN-listed Extinct species and contain 43% of current IUCN Critically Endangered species. The primary driver of extinction and endangerment for island species is invasive vertebrates.  Invasive vertebrates can also have widespread indirect impacts on island ecosystems through the population reduction or elimination of native species.  Here I review the impacts of invasive mammals on the the eight California Channel Islands.  These islands have 966 km² of land area and contain over 100 endemic species and subspecies of vertebrates and plants, a level of endemism per unit area nearly twice that of the Galapagos Islands.  At least 15 invasive mammal species have been introduced to the Channel Islands in the last 300 years with three to nine species on each island. In the last 75 years there have been 28 invasive mammal eradications on the nine Channel Islands, and three islands are now completely free of invasive mammals. Before/after comparisons following introductions and eradications, detailed plot-level studies and observational studies of diet and behavior suggest that these invasive species have contributed to the extinction and endangerment of native species by direct predation and grazing as well as indirectly through introduction of disease, habitat destruction and disruption of functional roles.  They have even influenced short-term geological processes such as beach formation and landslides.