| Summary: | Exotic species can have dramatic negative impacts on native wildlife and, in particular, predation by exotics is a leading cause of native prey declines globally. Human commensals are a distinct type of exotic species that directly benefit by associating with urban habitats, facilitating some to spillover into urban bushland remnants. Despite being a global phenomenon, the ecological impacts of commensal species in natural areas have been largely overlooked. This thesis quantifies the predatory impacts of an archetypal commensal rodent, the black rat (Rattus rattus), in two contrasting environments: urban remnants in Australia, and sensitive alpine grasslands in New Zealand. In Sydney, I undertook large-scale predator removal experiments to quantify wildlife responses to black rat removal. I found that black rats were an additive source of predation for arboreal nesting birds, but had no additive predatory effect on skinks, and positive effects on invertebrates. I suggest that skinks may respond appropriately to exotic black rats because of their shared evolutionary history with native (Rattus) predators, and that black rats have adopted an apex predator role with positive consequences to lower trophic invertebrates. In New Zealand, I used a functional response framework to quantify foraging rates of exotic rodents on invertebrate and fruit prey in an alpine ecosystem. I found that both rats and mice were extremely effective at locating invertebrate prey compared to fruit prey, but that each rodent species was affected differently by climate. I also found individual variation in predatory impacts, which was related to individual temperament. My work demonstrates that black rats can have complex impacts with trophic consequences that vary among different prey species and black rat individuals. Strategies developed to manage the impacts of black rats should endeavour to account for evolutionary history, indirect effects and individual variation in foraging.
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