Flightless animals have no means of immigration, hence little probability of colonizing islands. In these circumstances marine birds evolve populations in relatively simple ecosystems (Carlquist 1965; MacArthur and Wilson 1967), though the degree of simplicity depends on several variables, including the island's size and its distance from a source of immigrants. These systems have achieved ecological homeostasis through reciprocal adaptation over an extended period. Experience has shown that introductions to such systems result in severe perturbations (Odum 1971:221).
The introductions can be categorized as being either intentional or accidental events. Effects of such introductions have varied widely, depending on the type of animal introduced, the types of birds present and the habitat they occupy, the size and shape of the island, the type of nesting area used by the birds, and the status of their populations before the introduction. An example drawn from our Aleutian experience with gallinaceous birds illustrates the interaction of these variables. The dark phase of the arctic fox (Alopex lagopus) was introduced to Adak and Amchitka islands, both of which had native populations of the rock ptarmigan, Lagopus mutus (Gabrielson and Lincoln 1959). Foxes were released on Adak in 1924, and on Amchitka in 1921. Adak has an area of 751 km2 and Amchitka 350 km2. Adak is irregular in shape with extensive precipitous shorelines, relatively few beaches, and a large, central mountainous hinterland which foxes rarely penetrated. Amchitka, on the other hand, presents a zone of marine planation on its eastern two thirds, low mountains on the rest, shelving beaches around most of the island, and a long, linear, narrow shape that foxes explored completely. By 1949 ptarmigan were difficult to find on Amchitka, and then only in the highest, steepest section of the mountains. They were extirpated from the low, eastern two thirds of the island. The foxes flourished on Amchitka, but did much less well on Adak, where the ptarmigan population fluctuated in a normal cyclic manner, apparently uninfluenced by the foxes. Then the foxes were eradicated on Amchitka in the 1950's, and by 1962 the ptarmigan had spread over the whole of the island and become one of the most conspicuous avian features of the landscape.
Animal Introductions
Non-predatory Animals
Man has taken ungulates with him to many islands. Although numerous records of livestock introductions are available, few provide information relating to the effects of these animals on the habitat and their fauna unless the impact has been severe.
A most noteworthy example of destruction by ungulates occurred on Guadalupe Island off the coast of Baja California. Domestic goats (Capra hircus) were introduced in the unrecorded past with the result that little of the once abundant vegetation remains. In its place introduced species capable of withstanding heavy grazing are abundant over most of the island. Several endemic avian species are now considered extinct, including the Guadalupe storm-petrel, Oceanodroma macrodactyla (Howell and Cade 1954; Jehl 1972).
Sheep (Ovis aries) have been introduced to seabird nesting islands with varying results. In Bass Strait, Australia, Norman (1970) studied the effects of introduced sheep on vegetation and birds. He cited various papers attributing destruction of colonies of shearwaters (Puffinus sp.) to the activities of sheep, primarily their treading on the burrows. He found, however, that on Big Green Island and Phillip Island, sheep were not responsible for declines in shearwater breeding success, nor did they prevent expansion of colonies.
Other authors have reported damage to seabird nesting areas by sheep. One such example in the eastern North Pacific region concerns Protection Island, Washington. According to Richardson (1961), 100 to 300 sheep grazed freely on the island since 1958. He reported damage by grazing and frequent trampling of nesting areas of rhinoceros auklets (Cerorhinca monocerata). Landslides were initiated by these activities, rendering the slopes unusable by auklets. Of the burrows in his study area, 46% were buried by slides. He did not determine mortality.
Other avian consequences may flow from sheep introductions. Husbandry of these ungulates has been practiced with varying success for many years in the Aleutian Islands, most notably on Umnak and Unalaska islands, both of which have large native populations of bald eagles, Haliaeetus leucocephalus (Gabrielson and Lincoln 1959). Before the introduction of sheep, these raptors were oriented to the sea, hunting fish and seabirds. Sheep presented a new resource and presently the industry found itself confronted by a formidable predator, and demanded that eagles be destroyed (letter to William Egan, Governor of Alaska, from James S. Bynum, Secretary-treasurer, Umnak Company, Inc.).
Other ungulates introduced on Alaska islands include cattle (Bos taurus) on Chernofski and Chernabura islands; caribou (Rangifer tarandus) on Adak; reindeer on St. Matthew, Nunivak, Atka, Umnak, St. Paul, St. Lawrence, Hagemeister, and Kodiak as well as many interior locations; deer (Odocoileus hemionus) on Kodiak and Afognak; elk (Cervus canadensis) on Afognak; and musk oxen (Ovibos moschatus) on Nunivak. All these animals have maintained populations on islands for a time, and some appear likely to do so into the distant future. Specific effects on seabirds is generally not known, but trampling of grassy slopes such as that reported for sheep develops in some cases. Bailey et al. (1933) speculated that nests of the snow goose (Anser caerulescens) were destroyed by reindeer or their herdsmen in the Point Barrow area.