Potential for rat predation to cause decline of the globally threatened Henderson petrel Pterodroma atrata: evidence from the field, stable isotopes and population modelling

Abstract

Past studies have indicated that Pacific rats Rattus exulans are significant predators of the chicks of surface-breeding seabirds, namely gadfly petrels Pterodroma spp., on Henderson Island, central South Pacific. Further fieldwork in 2003 confirmed the heavy predation of chicks of Murphy's petrel P. ultima by rats. By extension, heavy predation is also likely each year on the endangered Henderson petrel P. atrata, for which Henderson Island is the only confirmed breeding site. To assess how important petrels are in the overall diet of rats, we conducted stable isotope analyses of rats from the shore, where petrels are most concentrated, and from about 1 km inland, where fewer nest. The carbon isotope results suggested that inland rats obtain about 30% of their food from marine sources, while the figure for shore rats was about 40%. We consider factors that may have acted to inflate these proportions. If, as suggested by these results, petrels are not the predominant component of the rats' diet, then rat populations and hence rat predation on petrels may not diminish even if petrel populations decrease further. In the light of probable low annual breeding success, we drew on vital rate information for the cahow P. cahow to model changes in the Henderson petrel population, and found a negative growth rate (λ = 0.9918) under present conditions. Growth rate became positive if annual adult survival rose above 0.95 or breeding success above 0.25, the latter unlikely while rats remain on Henderson. We conclude that the Henderson petrel population will probably continue to decline in the absence of conservation intervention.

The evolution of north‐east Atlantic gadfly petrels using statistical phylogeography

“…Available estimated adult survival rate in other Pterodroma is 0.93 (Simons ; Brooke et al . ), and age of sexual maturity in other Pterodroma is approximately 6 years of age (Simons ; Warham ). Thus, using values of 0.93 and 6 for p and A, respectively, equation (1) gives a generation time T of approximately 20 years.…”

Review article: The catastrophic impact of invasive mammalian predators on birds of the UK Overseas Territories: a review and synthesis

“…On Henderson Island, inferences about historical seabird population sizes have been made based on backward extrapolation of current rates of decline and population sizes to the time at which rats colonized the island; they suggest that millions of pairs of gadfly petrels Pterodroma spp. may have been lost (de Brooke et al. 2010).…”

“…In most cases, data are inadequate to determine whether this is the case, but several lines of evidence suggest that some populations and communities may still be in decline. de Brooke et al. (2010) have shown that where breeding success of long‐lived petrel species is chronically reduced by invasive predators, population declines can continue over many centuries before extirpation finally occurs.…”

Natural and synthetic vocalizations of brown rat pups, Rattus norvegicus, enhance attractiveness of bait boxes in laboratory and field experiments

“…Adopting a generalist diet is one of the key adaptations that has contributed to the success of mice and rats, and that has allowed them to inhabit almost every ecosystem . Hunting even other vertebrates such as birds for prey, rats also endanger seabird colonies on islands, and as invasive species in New Zealand, both brown rats, Rattus norvegicus , and black rats, R. rattus , greatly harm the indigenous fauna . When humans realized the many adverse effects associated with the presence of rats and mice, they designed tactics to trap them, in turn exerting selective pressure on mice and rats to evolve counteradaptations.…”