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.