Introduced species are one of the worst threats currently facing global biodiversity. They are known to impact native species and ecosystems via a wide variety of mechanisms, many of which are subtle and difficult to detect. Measuring the impacts of introduced upon native species is important as it allows vulnerable species to be identified and thus resources to be allocated to their protection. Devising and testing methods of quantifying these impacts, the theme of this thesis, is therefore important for conservation as well as being of scientific value. In this thesis I describe three approaches to measuring the impacts of introduced species on native ecosystems. Firstly, through a large scale field experiment using a series of paired fenced 'and unfenced plots designed to measure the effects of excluding introduced hedgehogs from invertebrates on the machair grassland habitat of South Uist. This showed higher than predicted variability among invertebrate populations and the impacts of this high variability are considered here. Secondly, I used stable isotope analysis to look at the diet of South Uist hedgehogs and infer their impact on prey species including invertebrates and the eggs of wading birds. The composition of hedgehog diet was shown to be largely consistent between survey months, size classes and sexes of hedgehogs. Estimates for the biomass of bird eggs a) produced by wading birds and b) consumed by hedgehogs each year were also calculated. Thirdly, I analysed two ecological networks based on real-world ecosystems using a novel mathematical model in order to determine which species were most at risk from the addition .of a series of simulated introduced species based on realworld invasives. These showed that introduced species tend to concentrate their impacts on particular taxa, even in cases where they did not predate those taxa directly. Finally, I discuss the findings and implications of these three approaches in the context of other current research and consider how the effects of invasive species can best be quantified and monitored in order to benefit land managers, conservation scientists and ultimately the ecosystems they work to protect.