Increasing numbers of species are being transported beyond their natural range boundaries by humans. These non-native species can have severe negative impacts on native biodiversity. In order to guide management of these species it is important to be able predict where non-native species will spread to, and what impact they will have. This thesis aims to improve our understanding in both these areas, using the expansion of non-native birds in the Iberian Peninsula as a study system. The number of non-native passerines in the Iberian Peninsula has increased in the late 20th century, with the common waxbill Estrilda astrild, yellow-crowned bishop Euplectes afer, red avadavat Amandava amandava and black-headed weaver Ploceus melanocephalus all established as breeding species since 1960.
Methods to (1) account for dispersal limitation when modelling the distribution of spreading non-native species and (2) evaluate the likely transferability of native trained species distribution models were developed. The consistency of the species-environment relationship during expansion in the non-native range was also examined. The ability of vacant niches to facilitate the spread of non-native species was tested, and a framework for detecting the early impacts of non-native species was developed.
Species distribution models of the potential distribution of non-native species are improved by incorporating dispersal. Dispersal is an important constraint on the distribution of non-native species, and interacts with environmental suitability to alter the species-environment relationship between the range-margin and the range core, and over time. Despite accounting for dispersal limitation in their evaluation, the performance of native-trained species distribution models was poor when most environmental conditions that were analogous to the species native range were within the species niche.
Non-native birds in the Iberian Peninsula utilised similar resources to native seed-eating birds, but small differences in resource utilisation allowed them to exploit rice fields, where resources were under-exploited by native species. Non-native birds could also interact with native reedbed nesting passerines, and indeed aggression between black-headed weavers and native Acrocephalus warblers has been recorded. However, we did not find evidence for competition between these species at current population densities of black-headed weavers.
Further work on non-native species needs to extend the hybrid dispersal-species distribution models developed here, and also to conduct more assessments of the impacts of non-native species in the early stages of their invasion
