Non-native species are one of the greatest threats to biodiversity worldwide, due to their direct and indirect effects on native communities. There are two opposing hypotheses to explain how non-native species successfully establish outside their native range. The first posits that non-native species are closely related to local native species through environmental filtering which selects species with similar traits; the second, that non-native species are distantly related to native species in the area in which they establish due to limiting similarity which minimizes competition. We assessed support for these two hypotheses by characterising the functional trait space of bird communities in Italian cities. We surveyed 220 points in breeding and winter periods along an urbanisation gradient in six cities. We assessed the two opposing hypotheses by calculating functional diversity metrics (Functional Dispersion i.e. quantification of the distribution of functional elements in the niche space, and Contribution i.e. contribution of each species to the niche space) for each community. We then modelled these metrics in relation to the presence/absence of non-native parakeet species along the urbanisation gradient. We found that non-native parakeet species were more likely to establish in the vacant functional niche space of urban bird communities, showing marked dissimilarity to native species in terms of niche space. Our results suggest that limiting similarity is the main mechanism promoting invasions at the local scale. Urban environments offer novel opportunities that are exploited by non-native birds, minimising competition with native species. This insight into niche space processes in urban areas, which can act as centres for expansion of non-native birds into other environments, can be used when implementing management strategies to enhance environmental filtering in these areas, thus reducing the chances of further establishment of non-native species.

Introduced alien species have direct and indirect effects on native communities, leading to lower taxonomic diversity and negative impacts on ecosystem functioning. Moreover, other aspects of diversity could be negatively affected, through alteration of functional and phylogenetic diversity of a community. This is particularly evident in habitats where human disturbance may favour alien species, posing an additional stressor on native communities. Following the community resistance hypothesis (higher diversity, higher resistance to invasion), we hypothesized: i) higher taxonomic, functional and phylogenetic diversity (TD, FD and PD respectively) in non-invaded bird communities (i.e. no alien bird species); and, ii) lower alien species impact on all diversity metrics in less human-disturbed areas. We surveyed bird communities in a modified Mediterranean landscape subject to varying levels of human disturbance. We tested whether TD, FD and PD indices were significantly different between non-invaded and invaded bird communities, and assessed the effect of landscape composition and configuration on these indices. We found that non-invaded communities retained higher TD and FD than invaded communities. Alien birds occupied novel parts of the functional space in invaded communities, but that they did not fully compensate for the taxonomic and functional diversity loss caused by the absence of native species. These results were consistent across different habitats, suggesting weak environmental filtering of communities. Generally, both communities were negatively affected by more human-disturbed areas (e.g. agriculture and urban areas) and enhanced by forest areas and by landscape heterogeneity. Our results suggest that the occurrence of alien birds negatively affects TD and FD (but not PD) of bird community assemblages, but that this impact is stronger in human-modified landscapes. Therefore, since the conservation of biodiversity in anthropogenic habitats is a worldwide challenge, researchers should prioritize efforts to assess the effects of alien species on communities inhabiting those habitats.