How important are seabirds in the diet of black rats on islands with a superpredator?

Abstract

This study assessed the impact of introduced black rats (Rattus rattus) on Cory’s shearwater (Calonectrisdiomedea borealis) in a multi-invaded insular ecosystem where rats are mesopredators. We hypothesizedthat black rats should have little impact on Cory’s shearwaters in the presence of cats as superpredators.Stomach contents and stable isotope analysis (SIA) in tissues of black rats were analyzed to assess thetrophic ecology and the importance of Cory’s shearwater in their diet. We also studied the isotopic sig-nature in tissues of house mouse (Mus domesticus) to confirm previous data showing no predation of thisspecies on Cory’s shearwaters. For both rodent species, temporal variation in diet composition in responseto the availability of seabird prey was evaluated, and short- and long-term consistency in diet was testedusing different tissues from the same individual. For black rats a Bayesian isotope mixing model (SIAR)was applied to determine the relative contribution of each prey to the individual diet. SIA of mouse tis-sues varied between the Cory’s shearwater breeding and non-breeding periods. However, no significantdifferences were found in diet and SIA for black rats. In contrast, individuals of both species showed astrong consistency in diet which apparently benefited their body condition index. Although black ratssupplement their diet with Cory’s shearwater eggs and chicks (8.3% in stomach contents and 10.6% in theSIAR model), their current impact on the Cory’s shearwater population appears to be small, probably dueto several factors including the small size of the rat population and a high level of rat predation by cats.