In some places where seabird colonies did not supply a croppable economic resource, the islands were used for alternative crops with at least temporary commodity value (e.g., foxes were introduced in the Aleutian Islands; Bent 1919). Large herbivores were introduced to supply meat for island residents (e.g., Saint Matthew Island; Klein 1959), as well as pigs, cattle, sheep, goats, and rabbits on islands in the North Atlantic and southern oceans (many authors). Increases in many seabird populations over the last 75 years have been generally associated with relief from predation by humans such as the fowlers, eggers, and plume hunters of the 19th century. Such relief may have been partly responsible for the increase of North Atlantic gannets, Sula bassana, and common murres or guillemots, Uria aalge (Fisher and Vevers 1943, 1944; Cramp et al. 1974). On a smaller scale, several population increases along the coast of New England have been recorded following the enactment of protective legislation (Dutcher 1901, 1904; Norton 1921, 1924; Palmer 1949; Drury 1973).
Coulson (1974) argued that in addition to relief from predation, the explosion of the population of kittiwakes (Rissa tridactyla) in this century resulted from access to previously un-occupiable breeding sites. Nesting cliffs and buildings suitable for kittiwake nesting are abundant and now protected from egging or fowling.
Positive Effects
There can be little doubt that human activities have also had marked positive effects in some cases. For example, Fisher (1952) suggested that the North Atlantic fulmar (Fulmarus glacialis) was provided food first by whaling, then by commercial fishing, and that this food allowed the species to increase steadily over the last 3 centuries.
The worldwide increase of gulls (Larus argentatus, L. fuscus, L. dominicanus, L. ridibundus, L. novae-hollandii) has been credited to availability of food from wasteful human garbage disposal (Murray and Carrick 1964; Fordham 1968, 1970; Harris 1964; Harris and Plumb 1965; Kadlec and Drury 1968; Brown 1967; Mills 1973; Vermeer 1963).
It is hard to dismiss the evidence pointing to the impact of human activities on seabird populations during the last 3 centuries. Yet it would be misleading to assume that without man's interference seabird populations would have remained stable. Success in designing programs of protection and population enhancement must allow for the realities—that seabird populations fluctuate inherently, and that secular changes occur regularly in their environment.
Impact of Natural Events
Some population changes appear to result from sudden impacts; other changes are gradual.
Sudden Disasters
Gromme (1927) reported windrows of dead murres in the Unimak Pass and Alaska Peninsula; die-offs of murres in winter storms in the Atlantic and Arctic Oceans were reported by Tuck (1960) and Dement'ev et al. (1968).