The destruction of vegetation by introduced rabbits and hares has been documented for many areas in the world. This destruction has often extended to seabirds. Perhaps the most dramatic example occurred on Laysan Island in the Hawaiian archipelago, where rabbits of unknown species were introduced in 1903. According to Warner (1963) it took less than 20 years for the rabbits to remove every green plant but three patches of Sesuvium portulacastrum. The Laysan duck (Anas laysanensis) was brought perilously close to extinction. The rabbits were eliminated in the 1920's, and the population of ducks increased to over 600 by 1963, a figure thought to approximate the pre-disturbance population.
European hares (Lepus europaeus) were introduced on Smith, San Juan, and Long islands, in Washington. On Smith Island, these burrowing animals apparently grazed nearly all the succulent vegetation close to the ground. By 1924, their burrows riddled the bluffs, causing them to cave into the ocean (Couch 1929). Couch found no seabirds nesting on the island, but found numerous tufted puffins (Lunda cirrhata) present on the bluffs, but not nesting. A removal campaign was directed against the hares in 1924 and in a few years they were gone. Smith Island now supports nesting pelagic birds (D. Manuwal, personal communication).
Accounts of hare and rabbit introductions to islands are legion, but not all such introductions have drastically affected seabirds. Manana Island, Hawaii, is such a case. Tomich et al. (1968) believed that introduced rabbits (Oryctolagus cuniculas) were not even indirectly detrimental to the nesting noddies (Anous tolidus) and sooty terns (Sterna fuscata). In some situations, introduced lagomorphs have been credited with benefiting seabirds. Lockley (1942) suggested that rabbits helped to open new breeding colonies of manx shearwaters (Puffinus puffinus) at Skomer and in west Wales in general. In Alaska rabbits were introduced to Middleton Island in 1952 (Rausch 1958) and to Ananiuliak Island at an earlier unrecorded date. Both have developed sustaining populations in the presence of large seabird populations without measurable effect on the birds. On Ananiuliak glaucous-winged gulls (Larus glaucescens) have been observed feeding on rabbits (W. S. Laughlin, personal communication).
Invertebrates have been introduced on three islands in the Aleutians. The black fly (Simulium sp.) reached Adak by 1958, Shemya by 1964, and Amchitka in connection with activities of the Atomic Energy Commission in 1968. Apparently the insects were transported on jet aircraft. The pest appears well established on Adak, but its status on the other two islands is uncertain. Like the mosquito, the female black fly sucks blood from warm-blooded animals, and in the process becomes the vector of a Leucocytozoan blood parasite of birds. In years of black fly abundance at Seney (Michigan) National Wildlife Refuge the blood parasite has been responsible for reproductive failure in Canada geese (Branta canadensis; Sherwood 1968). If black fly problems reach such a scale in the Aleutians, the parasites might prove limiting to pelagic birds as well as to waterfowl. Winds, for which the Aleutian region is famous, constitute a limiting factor for obligate blood-feeding Simuliids and may control the severity of this problem.
Predatory Animals
The list of introduced animals that prey on seabirds is extensive. Often several animals have been introduced to the same island. For example, in 1951 Amchitka Island in the Aleutians supported populations of feral dogs (Canis familiaris) and cats (Felis catus), rats (Rattus norvegicus), and arctic fox. Their presence resulted from three of the usual sources of predator introductions: escape of pets, escape from visiting ships (and aircraft), and commercial introductions. Add introductions to control pests, such as that of the mongoose (Herpestes auropunctatus) to the Hawaiian Islands, and only one source remains—the desire of man to improve on nature. In the Aleutians this impulse has taken the more innocuous form of fish and plant introductions, such as rainbow trout (Salmo gairdneri) on Adak and Shemya, and trees (mostly Sitka spruce, Picea sitkensis) on every military base in the "Chain."
Rats appear to be the most commonly introduced predators on a worldwide scale. Ships furnish the traditional source of their introduction, but one of us (R.D.J.) has observed them disembarking from a military aircraft at Cold Bay on the Alaska Peninsula. These animals probably entered the plane at Adak, which received rats from military ships early in World War II.
Rats may be able to exploit a larger percentage of the seabird species on a given island than other introduced predators because they can enter crevices and burrows in search of the birds and their eggs and young. They also destroy ground-nesters, and cliff-nesters may not be altogether safe from them. Clayton M. White (personal communication) found that Rattus norvegicus had ravaged every one of 16 eyries of the peregrine falcon (Falco peregrinus) that he checked in 1971 at Amchitka Island, Alaska. Only one egg had tooth marks, however. Kenyon (1961) ascribed the disappearance of the song sparrow (Melospiza melodia maxima) and the winter wren (Troglodytes troglodytes kiskensis) from Amchitka to predation by rats.
Many authors have mentioned potential rat damage, but few have quantitatively documented it. Imber (1974), however, provided data concerning the magnitude of rat predation on diving petrels and storm-petrels on some New Zealand islands. He found that rats were taking between 10 and 35% of the chicks of gray-faced petrels (Pterodroma macroptera gouldi) on Whale Island in the parts of the colonies where burrows were dense. On those parts of the island with a very low density of petrel burrows, rats were believed to have killed virtually every chick. Imber revealed that where Polynesian rats (Rattus exulans) occur, diving petrels and storm-petrels are rare or absent, though they are widespread on neighboring islands. Imber concluded from his studies of the ecology of petrels and Polynesian and Norway rats that a petrel colony is endangered if invaded by a species of rat whose maximum weight approaches or exceeds the mean adult weight of the petrel. Harris (1970), who worked with dark-rumped petrels (Pterodroma phacopygia) on Santa Cruz in the Galapagos Islands, indicated that black rats (Rattus rattus) were responsible for the extremely low nesting success of the petrels there.
In British Columbia, Campbell (1968) recorded predation by the Alexandrian rat (R. rattus) on ancient murrelets (Synthliboramphus antiquus) at Langara Island. The extent of damage to the murrelet population is not known.