Estimating impacts of invasive rats using analyses of stable isotopes from laboratory and field studies

The majority of studies examining the impacts of invasive species attempt to demonstrate direct effects of invader predation and herbivory on native species within an invaded ecosystem. However, estimating the foraging ecology of invaders is frequently problematic due to difficulties in obtaining accurate data demonstrating their feeding habits over time. To best estimate the foraging ecology and thus impacts of invasive Norway rats (Rattus norvegicus) in the Aleutian Islands, we analyzed behavior, stomach contents, and the δ¹⁵N and δ¹³C values of several tissues from wild rats and their potential diet items. In addition, we conducted a year-long laboratory study to determine fractionation values between diet and rat tissues held on varying diets, and the half-lives and thus turnover times of nitrogen and carbon isotopes in six tissues from captive rats held on C3 plant-based diets and switched to fish and C4 plant-based diets. Carbon isotope turnover was most rapid in liver tissue, followed by plasma, kidney, red blood cells, muscle, and fur. The δ¹⁵N values were significantly higher for female rats, so nitrogen isotope turnover was analyzed separately by sex. In female rats, plasma exhibited the fastest nitrogen isotope turnover, followed by liver, kidney, red blood cells, muscle, and fur. In male rats, the order was the same except muscle had a faster δ¹⁵N turnover than red blood cells. Finally, we applied the isotope turnover and fractionation factor data from our laboratory experiments to most accurately build 4-source, concentration dependent stable isotope mixing models with our wild rat isotope data. This allowed us to best estimate the foraging ecology and potential impacts of invasive rats over time in the Aleutian Islands.