Norway rats as predators of burrow-nesting seabirds: Insights from stable isotope analyses

Introduced species to oceanic islands can cause tremendous declines and ext inctions of native avifauna. On Langara Island, British Columbia, Canada, t he 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 an d 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 d iets of rats that also consume a variety of plants and invertebrates. We us ed stable-carbon (delta(13)C), nitrogen (delta(15)N), and sulphur (delta(34 )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, in cluding seabirds, in the diets of rats. Rats were segregated into 3 isotopi c groups corresponding to upland, littoral, and seabird nesting areas on th e island. We interpret these groups to represent individuals consuming pred ominantly C-3 terrestrial foods ((x) over bar delta(15)N = 5.4 parts per th ousand; (x) over bar delta(13)C = -24.9 parts per thousand; n = 24), intert idal invertebrates ((x) over bar = delta(15)N = 8.9 parts per thousand; (x) over bar delta(13)C = -14.3 parts per thousand; n = 21), and ancient murre let adults, chicks, or eggs ((x) over bar egg delta(15)N = 13.2 parts per t housand; (x) over bar C-13 = -17.6 parts per thousand; n = 8). We found str ong correlations between liver and muscle isotope values for both C-13 and N-15, suggesting that dietary preferences within individuals remained relat ively constant. Stable-sulphur isotope Values of rat liver were less useful in segregating marine and terrestrial dietary inputs, possibly because sou rces of sulphur to the terrestrial food web were of marine origin (pooled ( x) over bar delta(34)S value = 17.8 parts per thousand; n = 20). Our measur ement of liver and muscle tissues gave dietary estimates based on relativel y short- and long-term integrations and revealed the 3 groups of rats remai ned isotopically segregated at least over the 2-month period of ancient mur relet breeding on Langara Island. Our results have important ramifications for dietary investigations of introduced fauna and their effects on native seabirds on oceanic islands.