Norway Rats as Predators of Burrow-Nesting Seabirds: Insights from Stable Isotope Analyses

Introduced species to oceanic islands can cause tremendous declines and extinctions of native avifauna. On Langara Island, British Columbia, Canada, the burrow-nesting ancient murrelet (Synthliboramphus antiquus) has declined from an estimated original population of 200,000 pairs to 14,600 pairs in 1993. Previously, causes of this decline were unknown, but the introduction of Norway rats (Rattus norvegicus) has been implicated as a major factor and has resulted in a recent rat eradication program. A major obstacle in the investigation of the effects of rats on seabirds, here and elsewhere, has been the inability to accurately assess the importance of seabirds to the diets of rats that also consume a variety of plants and invertebrates. We used stable-carbon (δ13C), nitrogen (δ15N), and sulphur$(\delta {}^{34}\text{S})$isotope analyses of muscle and liver tissues of rats and prey organisms from 3 regions of Langara Island to evaluate evidence for marine foods, including seabirds, in the diets of rats. Rats were segregated into 3 isotopic groups corresponding to upland, littoral, and seabird nesting areas on the island. We interpret these groups to represent individuals consuming predominantly C-3 terrestrial foods (x̄ δ15N = 5.4‰; x̄ δ13C = -24.9‰; n = 24), intertidal invertebrates (x̄ δ15N = 8.9‰; x̄ δ13C = -14.3‰; n = 21), and ancient murrelet adults, chicks, or eggs (x̄ δ15N = 13.2‰; x̄ δ13C = -17.6‰; n = 8). We found strong correlations between liver and muscle isotope values for both13C and15N, suggesting that dietary preferences within individuals remained relatively constant. Stable-sulphur isotope values of rat liver were less useful in segregating marine and terrestrial dietary inputs, possibly because sources of sulphur to the terrestrial food web were of marine origin (pooled x̄ δ34S value = 17.8‰; n = 20). Our measurement of liver and muscle tissues gave dietary estimates based on relatively short- and long-term integrations and revealed the 3 groups of rats remained isotopically segregated at least over the 2-month period of ancient murrelet breeding on Langara Island. Our results have important ramifications for dietary investigations of introduced fauna and their effects on native seabirds on oceanic islands.