Ship rat demography and diet following possum control in a mixed podocarp-hardwood forest

Control of one pest species may permit increases in abundance of other pests, thereby reducing the overall net benefit from pest control. We provide evidence that control of introduced possums (Trichosurus vulpecula) may increase ship rat (Rattus rattus) abundance in some New Zealand native forests. Ship rat abundance in a podocarp–hardwood forest was assessed using simple interference indices over 14 years (1990–2004) that included two aerial possum-poisoning operations (1994, 2000). Ship rat demography and rat and possum diet were measured from June 2001 to June 2003 when the rat population was increasing after the 2000 poisoning. Mean ship rat abundance indices increased nearly fivefold after possum control and remained high for up to 6 years after the 1994 poisoning. Rat fecundity was high (50–100% of adult females breeding), even during winter, and young animals dominated the population (73% in age classes 1–3) in 2001–2002 when rat numbers were increasing. During 2002–2003, rat abundance stabilised, without marked winter or spring reductions, the population aged significantly (only 32% in age classes 1–3), and fecundity declined to low levels (4–27% of adult females breeding). Although seeds and fruit dominated the diet of rats driving population recovery after control (74.0% of total diet by dry weight), rat fecundity was instead closely correlated with the proportional consumption of invertebrates (r = 0.91). Juvenile survival was correlated with proportional seed consumption (r = 0.75), while adult survival was correlated with combined seed and invertebrate consumption (r = 0.83). Adult rats ate more seeds and fewer invertebrates than juvenile rats. Seeds and fruit also dominated possum diet (52.2% of total diet). These results are consistent with the hypothesis that increased rat abundance following possum control is a consequence of greater availability of, or reduced competition for, seeds and fruit.