Stable isotopes reveal evidence of predation by ship rats on seabirds on the Shiant Islands, Scotland

1 Introduced predators are a major threat to native island populations, yet direct evidence of predation is often lacking, especially when it is difficult to detect by traditional dietary methods. 2 Historical declines of nesting seabirds on the Shiant Islands, Outer Hebrides, roughly coincided with the accidental introduction of ship rats Rattus rattus in c . 1900. Rats have been implicated in declines of seabirds, but the Shiant population is one of two remaining naturalized R. rattus populations in Britain, prompting calls for their protection. 3 Live‐trapping studies with stable isotopes and gut content analysis were used to investigate whether ship rats prey on Shiant Islands seabirds. Another aim of this study was to determine whether marine‐derived foods subsidize rat populations, permitting higher densities, greater productivity and larger body size than expected from terrestrial resources alone. 4 Comparisons of stable carbon and nitrogen isotopic signatures of rat tissues with those of seabirds, marine invertebrates, marine algae and land‐based foods revealed that seabirds and other marine prey were the primary source of protein for rats living in colonies or near the shore. These results were corroborated by gut content analysis, and suggest a greater role for active predation of seabirds by rats than has previously been apparent at this locality. 5 Seabird colonies and especially coastal areas supported higher numbers of rats than more inland habitats. Coastal and colony‐dwelling rats were more active reproductively and were larger than those living inland. 6 Although rats are capable of surviving solely on terrestrial foods, their ability to use marine prey may buffer populations during lean times, i.e. outside the seabird nesting season, and may in part explain their success and status as pests on islands world‐wide. Overall, this work reveals the value of stable isotopes in identifying predation by exotic species, but also underscores potential uncertainties inherent in all diet‐based methods in distinguishing predation from scavenging.