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Management of invasive alien species (IAS) is notoriously difficult, with many examples of expensive, failed attempts. While there is growing consensus that prevention is better than cure, successful prevention requires user-friendly, effective methods to quantify the ecological impacts of established, emerging and future IAS. In this thesis I develop one such method: the Relative Impact Potential (RIP) metric. Based on the effective Comparative Functional Response method, which highlights damaging IAS by comparing their greater per capita maximum feeding rates relative to trophically analogous natives, it enhances prediction by accounting for proxies of numerical response, such as abundance. It therefore captures the impacts of abundant invaders with low per capita effects alongside low numbers of invaders with high per capita effects, while still maintaining the benefits of the CFR method, such as the ability to incorporate a wealth of biotic and abiotic context dependencies and ease of interpretation. In Chapter 1, I outline the history of the metric and develop it further, outlining a roadmap for future use. Some of these aspects I go on to implement across the different data chapters. In Chapter 2, I use RIP to experimentally assess how sea freshening may affect the impacts of the emerging amphipod IAS, Pontogammarus maeoticus, relative to an established IAS, Gammarus tigrinus, and a trophically analogous native, Gammarus salinus, in the Baltic Sea. I show that this often-overlooked aspect of climate change enhanced the relative impacts of both IAS. In the third chapter, I continue with the abiotic context of Baltic Sea freshening, but here assess the consequences of decreasing salinity on the ecological impact exerted by a native keystone predatory starfish, Asterias rubens, at the limits of its range. Using three experimental salinities, I combine FRs with larval recruitment estimates and show that attack and maximum feeding rates of the starfish are reduced by decreasing salinities, with no instances of predation found at the low salinity. With similar patterns for larval recruitment, the overall Impact Potential of this keystone predator species was lessened by decreased salinity, with potential implications for Baltic community structure. Chapter 4 assesses the role of oxygen saturation on the impact of the predatory invasive fish, the Ponto-Caspian round goby (Neogobius melanostomus), relative to a native endangered analogue, the bullhead (Cottus gobio). Across three oxygen saturation levels, I assess impact using FR, RIP and a new metric called the Relative Total Impact Potential that assesses combined IAS and native impact upon a system across invasion stages. At low saturation, the IAS was shown to have a significantly higher feeding rate relative to the native, RIP revealed that low oxygen exacerbated the high relative impact of the invader, and RTIP was consistently higher at low oxygen. In Chapter 5, I assess the impacts and risks of four commonly traded pet turtles - Trachemys scripta scripta, T.s. troostii, Sternotherus odoratus, Kinosternon subrubrum - using Relative Invasion Risk, an adapted version of RIP that incorporates propagule pressure. T. s. scripta was found to have high impact, corroborating its now banned status in the EU, but still a high risk. In Chapter 6, I assess the potential impacts of two readily available fish species in the pet trade: Tanichthys albonubes and Carassius auratus. Again, I assess impact and risk, but this time incorporate "dispersal enhancing" behaviour measures. C. auratus, the species with the greater invasion history, is shown to be high impact and high risk. While the experiments performed and conclusions drawn vary across the different chapters of this thesis, overall this work highlights the efficacy and flexibility of the RIP family of metrics in answering a wealth of ecological questions, across numerous abiotic contexts and taxa. Finally, in Chapter 7, I call for its enhanced use in IAS risk assessments.

The life history, distribution, abundance and diet of ferrets, feral cats and harriers was examined in a live-trapping study to determine these predators' impact on rabbit populations in the highly rabbit prone semi-arid high country of New Zealand's South Island, and to assess their role as regulators of the rabbit population. In aspects of their life history (including weight, sex ratio, survival, dispersal and breeding season) cats and ferrets were essentially similar to those found in other areas of New Zealand. The distribution of all three species was closely linked to rabbit abundance. Harrier and ferret population fluctuations lagged 1-2 months behind rabbit population fluctuations. Cats lagged rabbits by 3-4 months. These seasonal changes were due to recruitment to the predator populations in summer and autumn, and mortality in winter and spring. Multivariate regression models were built to predict predator numbers. Thirty-two per cent of the variation in ferret numbers was explained by season, rabbit density and altitude effects. Cat variation (38%) was explained by a model incorporating seasonal effects, sward and hare abundance. Seasonal effects and rabbit abundance explained 17% of the variation in harrier numbers. Ferret and harrier abundance were both significantly positively correlated with rabbit abundance, in a manner suggestive of a Type II numerical response. Ferret diet was dominated by rabbit, with much smaller contributions from lizards, sheep carrion, birds, invertebrates, mice and hedgehogs. When young rabbits were available they were eaten in high proportions. The amount of rabbit in the diet varied seasonally, with a decrease in April and June compensated for by increased predation of lizards and other alternative prey types. The contribution of rabbit to ferret diet (91 % by weight) was similar across all rabbit densities measured. Trapping results and ferret diet information were combined with values from the literature, and some "best guesses", to obtain an approximate model of the total predator response to changing rabbit density. The result appeared to be Type II (inversely density dependent), indicating that while predators may limit rabbits in the study area, they do not appear to regulate them. However, the only way to conclusively prove that predators regulate their prey is with experiments in which predator and prey populations are manipulated.