Seabirds include some of the most threatened taxa anywhere in the world. For example, of extant albatross species, 82% are listed as threatened, endangered, or critically endangered by the International Union for Conservation of Nature.Save the Albatross Campaign. 9 February 2011. Species at Risk. Save the Albatross; a global campaign by BirdLife International. Retrieved 25 February 2011 <http://www.rspb.org.uk/supporting/campaigns/albatross/about/species/> The two leading threats to seabirds are accidental bycatch by commercial fishing operations and introduced mammals on their breeding islands. Mammals are typically brought to remote islands by humans either accidentally as stowaways on ships, or deliberately for hunting, ranching, or biological control of previously introduced species.Safina, C. 2008. The state of our seabirds, p. 1-24. In J. Hardesty, S. Holmer, M. J. Parr, D. Pashley, G. Radko, G. Shire, and G. E. Wallace [Eds.], Bird Conservation, the magazine of the American Bird Conservancy. The Plains, VA.Harris, D. B. 2009. Review of negative effects of introduced rodents on small mammals on islands. Biological Invasions 11:1611-1630.Hilton, G. M. and R. J. Cuthbert. 2010. The catastrophic impact of invasive mammalian predators on birds of the UK Overseas Territories: a review and synthesis. Ibis 152:443-458. Introduced mammals have a multitude of negative effects on seabirds including direct and indirect effects. Direct effects include predation and disruption of breeding activities, and indirect effects include habitat transformation due to overgrazing and major shifts in nutrient cycling due to a halting of nutrient subsidies from seabird excrement.Campbell, K. and C. J. Donlan. 2005. Feral goat eradications on islands. Conservation Biology 19:1362-1374.Croll, D. A., J. L. Maron, J. A. Estes, E. M. Danner, and G. V. Byrd. 2005. Introduced predators transform subarctic islands from grassland to tundra. Science 307:1959-1961. There are other invasive species on islands that wreak havoc on native bird populations (e.g. brown snakes on GuamRodda, G. H., T. H. Fritts and P. J. Conry. 1992. Origin and Population Growth of the Brown Tree Snake, Boiga irregularis, on Guam. Pacific Science 46(1):46-57.), but mammals are by far the most commonly introduced species to islands and the most detrimental to breeding seabirds. Despite efforts to remove introduced mammals from these remote islands, invasive mammals are still present on roughly 80% of islands worldwide.Aguirre-Munoz, A., D. A. Croll, C. J. Donlan, R. W. Henry, M. A. Hermosillo, G. R. Howald, B. S. Keitt, L. Luna-Mendoza, M. Rodriguez-Malagon, L. M. Salas-Flores, A. Samaniego-Herrera, J. A. Sanchez-Pacheco, J. Sheppard, B. R. Tershy, J. Toro-Benito, S. Wolf, and B. Wood. 2008. High-impact conservation: Invasive mammal eradications from the islands of western Mexico. Ambio 37:101-107.

Two comprehensive sources for the current state of island invasions and island invasive eradications are the Threatened Island Biodiversity database (TIB)Retrieved 9 January 2018 <http://tib.islandconservation.org/> and the Database of Island Invasive Species Eradications (DIISE).Retrieved 9 January 2018 <http://diise.islandconservation.org/>

Species commonly introduced

Cats

Feral cats (Felis catus) are one of the most commonly introduced mammals and one of the most effective predators on ground-nesting seabirds..Nogales, M., A. Martin, B. R. Tershy, C. J. Donlan, D. Witch, N. Puerta, B. Wood, and J. Alonso. 2004. A review of feral cat eradication on islands. Conservation Biology 18:310-319. On the Kerguelen Islands in the Southern Ocean, cats kill 1.2 million nesting seabirds annually.Pascal, M. 1980. Structure and dynamics of the feral cat population of the Kerguelen Islands. Mammalia 44:161–182.Keitt, B. S., C. Wilcox, B. R. Tershy, D. A. Croll, and C. J. Donlan. 2002. The effect of feral cats on the population viability of black-vented shearwaters (Puffinus opisthomelas) on Natividad Island, Mexico. Animal Conservation 5:217-223. One study stated that introduced feral cats have been responsible for the extinction of 10 petrel species in the 20th century alone.Derenne, P. and J. L. Mougin. 1976. Ecological data on mammals introduced to Hog Island, Crozet Archipelago. Mammalia 40:21–52. Feral cats are such effective predators on ground-nesting seabirds because seabirds did not evolve with terrestrial predators and therefore did not develop defenses against them. In many cases, seabirds do not even recognize feral cats as a threat. It has also been demonstrated that feral cats on islands preferentially eat seabirds, but during the nonbreeding season cats switch their prey base to insects, rats, and mice for subsistence.

Rats Rats are by far the most common introduced mammals on seabird islands. Currently, approximately 80% of islands worldwide have populations of introduced rats. The most common introduced species is the Norway rat (Rattus norvegicus), but black rats (R. rattus, also known as “ship rat”) and Polynesian rats (R. exulans) have found their way to offshore islands and have subsequently decimated populations of breeding seabirds.Jones, H. P., B. R. Tershy, E. S. Zavaleta, D. A. Croll, B. S. Keitt, M. E. Finkelstein, and G. R. Howald. 2008. Severity of the effects of invasive rats on seabirds: A global review. Conservation Biology 22:16-26. Norway rats are thought to be the most destructive due to their high rates of population growth and extreme ecological plasticity that allows them to live in a wide variety of habitats and climates. Introduced rats have been shown to negatively impact 75 species of seabirds in 10 different families; smaller burrow-nesting seabirds such as storm petrels and shearwaters are the most negatively affected, while larger, surface-nesting birds such as gulls seem to be the least impacted due to their ability to defend their eggs and chicks.

Mice Mice are another commonly introduced mammal to islands. The species usually introduced is the cosmopolitan house mouse (Mus musculus). Like rats, house mice have a high intrinsic growth rate and are exceptionally adaptable to a wide range of ecological conditions and therefore can persist on a majority of the world's islands. On the mainland, mice eat mainly grains and insects; however, on tiny, remote islands these food items are frequently unavailable, forcing the mice to adapt and prey on seabird eggs and chicks.

The most spectacular example of this is on Gough Island, a small island in the middle of the south Atlantic. Mice made it to Gough Island in the 19th century and began shifting their prey base to mainly seabird chicks.Cuthbert, R. and G. Hilton. 2004. Introduced house mice Mus musculus: a significant predator of threatened and endemic birds on Gough Island, South Atlantic Ocean? Biological Conservation 117:483-489., BirdLife International 2009. One of the largest and most endangered species of seabird in the world, the Tristan albatross (Diomedea dabbenena) nests mainly on this island and do so because of its historical lack of terrestrial predators. The mice introduced to Gough Island have evolved to eat exclusively albatross chicks and grow to twice the size of a normal house mouse on average as a result.Wanless, R. M., A. Angel, R. J. Cuthbert, G. M. Hilton, and P. G. Ryan. 2007. Can predation by invasive mice drive seabird extinctions? Biology Letters 3:241-244. Even though the albatross chicks are massive compared to the mice, the chicks do not recognize mice as a threat and allow the mice to eat them alive while they sit at their nests.

Foxes Where introduced, Arctic (Alopex lagopus) and red foxes (Vulpes vulpes) are extremely adept predators on ground-nesting seabirds.Bailey, E. P. 1993. Introduction of foxes to Alaskan Islands - history, effects on avifauna, and eradication. U.S. Fish and Wildlife Service Resource Publication (1):1-52. Foxes are able to depredate both adult and young seabirds because these seabirds have never had to develop a defense against these predators. Foxes have been present on the Aleutian Island chain in Alaska since the 1700s and have drastically reduced seabird populations and even extirpated species from certain islands. Patagonian fox (Lycalopex griseus) has also been introduced to the Falkland Islands, home to some of the largest seabird colonies in the world.Hall, J. R., Woods, R. D., De Brooke, M. L. & Hilton, G. M. 2002. Factors affecting the distribution of landbirds on the Falkland Islands. Bird Conservation International 12:151–167.

Raccoons Raccoons (Procyon lotor) are omnivorous mammals that can subsist on a diet of seabird eggs and chicks where they are introduced. Raccoons decrease seabird breeding success; for example, in 2004 raccoons were introduced to gull breeding colonies in the Isles of Shoals, Maine.Ellis, J. C., J. M. Farina, and J. D. Witman. 2006. Nutrient transfer from sea to land: the case of gulls and cormorants in the Gulf of Maine. Journal of Animal Ecology 75:565-574. Islands without raccoons were 17 times more likely to hatch and fledge chicks than islands with introduced raccoons present. In another example, raccoons introduced to the Scott Islands, British Columbia, in the 1930s devastated what was the largest aggregation of breeding seabirds in the eastern Pacific south of Alaska.Hipfner, J. M., M. J. F. Lemon, and M. S. Rodway. 2010. Introduced mammals, vegetation changes and seabird conservation on the Scott Islands, British Columbia, Canada. Bird Conservation International 20:295-305.

Rabbits The most recent report indicates that rabbits have been introduced on over 800 islands worldwide.Flux, J. E. C. and P. J. Fullager. 1983. World distribution of the rabbit Oryctolagus cuniculus. Acta Zoologica Fennici 174:75-77. However, this report is nearly 20 years old and more introductions have certainly happened since that time. Through grazing, rabbits alter the vegetative environment of islands that seabirds depend on for breeding activities.Gillham, M.E. 1963. Some interactions of plants, rabbits and seabirds on South African islands. Journal of Ecology 51:275-294.

Livestock There is a long history of introducing livestock to offshore islands in an attempt to incite ranching and establish permanent human residence on these islands. The negative effects of grazing animals on island ecosystems is mainly through change in the vegetation cover, but these animals also have direct effects on breeding populations of seabirds through the trampling of nests and chicks.Micol, T. and P. Jouventin. 1995. Restoration of Amsterdam Island, south Indian-Ocean, following removal of feral cattle. Biological Conservation 73:199-206. There is even an instance of pigs digging up burrows and consuming endangered Townsend's shearwaters (Puffinus auricularis) in the Revillagigedo Islands off the western coast of Mexico.Martinez-Gomez, J. E. and J. K. Jacobsen. 2004. The conservation status of Townsend's shearwater Puffinus auricularis auricularis. Biological Conservation 116:35-47.

Methods of introduction and establishment Most seabirds breed on islands that are hundreds or even thousands of kilometers from the mainland; therefore, the most common way mammals get to these islands is by anthropogenic means.Myers, J. H., D. Simberloff, A. M. Kuris, and J. R. Carey. 2000. Eradication revisited: dealing with exotic species. Trends in Ecology & Evolution 15:316-320. People have brought mammals to islands both intentionally and accidentally. Regardless of their method of introduction, once invasive mammals establish populations on these islands, their populations grow rapidly due to the quick and effective filling of empty niches as well as the seemingly endless supply of easy-to-capture food in the form of unsuspecting seabirds, mainly eggs and chicks.

Accidental