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. 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. 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. There are other invasive species on islands that wreak havoc on native bird populations (e.g. brown snakes on Guam), 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. Two comprehensive sources for the current state of island invasions and island invasive eradications are the Threatened Island Biodiversity database (TIB) and the Database of Island Invasive Species Eradications (DIISE).
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. On the Kerguelen Islands in the Southern Ocean, cats kill 1.2 million nesting seabirds annually. One study stated that introduced feral cats have been responsible for the extinction of 10 petrel species in the 20th century alone. 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. 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. 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. 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. 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.
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. 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.
Rabbits The most recent report indicates that rabbits have been introduced on over 800 islands worldwide. 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.
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