Interactions between petrels, rats and rabbits on Whale Island, and effects of rat and rabbit eradication

Norway rats (Rattus norvegicus) were present on Whale Island (Moutohora), Bay of Plenty, New Zealand between about 1920 and 1987. During 1969-1971 they reduced by less than 10-35 % the breeding success of grey-faced petrels (Pterodroma macroptera gouldi), by eating unattended eggs and killing young or weak chicks. Rabbits (Oryctolagus cuniculus), introduced to Moutohora (240 ha surface area) in about 1968, multiplied rapidly to reach a density of up to 375 individuals/ha by early 1973. Their young and corpses were also eaten by rats, and this additional food, available when petrels were absent or less vulnerable (February-June), apparently increased the rat population. During 1972 to 1977, production of fledged young by the petrels was negligible. Applications of anticoagulant baits to kill rats in 1978, 1980 and 1982 resulted in large numbers of young petrels fledging only in those years; there was no carry-over effect in following years. Island-wide laying by hand of brodifacoum baits (Talon 50WB) in 1985 decimated the rat population. Further poisoning, associated with a tandem operation to kill rabbits, led to the eradication of both mammals by late 1987. Breeding success of the petrels from 1985 to 1988 (no data for 1989), and from 1990 to 1994, was consistent and increasing. This study shows that the effect of rats as predators of petrels may be modified by external factors governing the abundance and annual cycle of the predator.