The effect of rats on island biodiversity: field experiments, stable isotopes and simulations.

Invasive species have been identified by the IUCN as the second biggest threat to global biodiversity and, arguably, the greatest threat to small islands. However, the precise mechanisms by which invasive species impact native biodiversity are often poorly characterised. For example, invasive omnivorous rats are likely to be feeding at several trophic levels, but their impacts on anything other than high-profile vertebrates (usually seabirds) are seldom recorded. As a nation of islands, the UK has much to lose from the threat of invasive species and, potentially, much to gain from devising effective and innovative ways of controlling them. This project will investigate the impacts of invasive rats on food webs by combining three current strands of scientific investigation / food webs, invasive species and stable isotope analysis / via a dynamic new collaboration between experts at three UK institutions, the Universities of Bristol and Exeter and the Central Science Laboratory (CSL). The study will combine field work at two sites; Flat Holm in the Bristol Channel and islets in the Sound of Harris, Outer Hebrides. The first part of the work involves producing an island-wide food web for Flat Holm, identifying as many species as possible. The second part of the fieldwork is an experiment using a replicated design to investigate the effects of rat eradication using five pairs of islets in the Sound of Harris. Rats will be eradicated from one of each pair using well-established methodology, allowing comparisons to be made between islands with and without rats. Training in rat eradication techniques which will be provided by Roy at CSL in York. Rat eradication from islands has become a widely accepted conservation management tool, used safely and effectively in many parts of the world, including a number of UK islands. Food webs will be constructed for each island, though on a simpler scale than for Flat Holm, and samples will be taken for stable isotope analysis. Samples of rat tissue, along with samples of their likely foodstuffs and competitors, will be analysed. By comparing the isotopic signatures of rat tissue with those of the potential prey species we can identify the trophic level(s) at which rats are feeding, casting clearer light on how rats are impacting island ecosystems. By analysing signatures from species competing with rats (e.g. rabbits or mice), using samples taken both before and after the rat eradication, we will be able to identify shifts in the competitors niche breadth and mean trophic level following rat eradication. Finally, the results from the fieldwork will be used to simulate the effects of rat removal in silico. By combining the data from the Flat Holm and Sound of Harris food webs, as well as relevant data from the literature, we will be able to develop a model predicting the effects of rat eradication at the community level. This will be of practical use in predicting which islands will achieve the greatest benefits from rat eradication, allowing limited conservation funding to be allocated most effectively. The project combines three established strands of investigation into an innovative and achievable course of training with great practical application. The student will benefit from expert supervision, centred at the University of Bristol, conveniently close to Flat Holm island. The Scottish fieldwork will take place under the supervision of CSL, and will benefit from the network of contacts and expertise already established by the ongoing Hebridean Mink Project. The stable isotope analysis will take place at the Cornwall campus of the University of Exeter, under the supervision of one of the UK pioneers of the technique. The results will be of interest to conservation managers with responsibility for UK islands, and to the invasive species community, both within the wider UK and further afield.