Investigating trophic impacts of invasive rats on islands

Determining the trophic impacts of introduced animal species has become increasingly important for conservation ecology as their direct and indirect effects on native species and ecosystems can be devastating. I investigated a landscape-level trophic cascade induced by introduced Norway rats (Rattus norvegicus) through comparisons of marine bird densities and marine rocky intertidal community structures on invaded and rat-free islands throughout the entire 1,900 km archipelago of the Aleutian Islands, Alaska. On islands with rats, densities of birds that forage in the intertidal were lower, marine intertidal invertebrates were more abundant, and fleshy algal cover was reduced. My results demonstrate that invasive rats directly reduce bird densities through predation and significantly affect invertebrate and marine algal abundance in the rocky intertidal indirectly via a cross-community trophic cascade, unexpectedly changing the intertidal community structure from an algal to an invertebrate dominated system.;Many introduced animals are omnivores which makes quantifying their diets particularly problematic, especially for nocturnal animals living in remote locations. As a general tool to evaluate the trophic impacts of introduced Norway rats, one of the most common introduced species on islands, I established the parameters for and executed a stable isotope mixing model designed to estimate rat diet on the Aleutian Islands. I established the stable N and C isotope retention times for and discrimination factors between 4 diets and fur, kidney, liver, muscle, serum, and red blood cells of captive rats for best interpretation of the model. I used the model on 3 tissues from rats from the Aleutian Islands to determine temporal variations in diet that coincide with seasonal differences in prey availability. Terrestrial vegetation comprised the bulk of rat diets throughout the time period represented in the study. Rat predation on marine bird chicks and land birds peaked during bird nesting season, whereas predation on marine amphipods peaked during late summer. The model results were corroborated with rat behavioral observations and stomach content analyses. My work demonstrates: (1) the importance of determining accurate stable isotope model parameters from captive animal studies, (2) the utility of concentration dependence stable isotope mixing models for estimating diets of omnivores, and (3) the existence of a landscape-level, cross-community trophic cascade induced by an introduced species.