Testing the links between bird diversity, alien species and disturbance within a human-modified landscape

Introduced alien species are associated with lower taxonomic, functional and phylogenetic diversity of native communities and negative impacts on ecosystem functioning. 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 promotes higher resistance to invasion), we predicted: 1) higher taxonomic, functional and phylogenetic diversity (TD, FD and PD respectively) in non-invaded bird communities (i.e. no alien bird species); and 2) higher diversity and resistance to invasion 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 were significantly different between non-invaded and invaded bird communities, and assessed the effect of land classes (forest, agriculture, urban), landscape composition and heterogeneity on these metrics. We found that non-invaded communities retained higher TD and FD, but not PD, than invaded communities. Alien birds occupied marginal niches in invaded communities, and did not fully compensate for the taxonomic and functional diversity loss caused by the absence of native species. These results were consistent across different land classes, suggesting weak environmental filtering of communities. Generally, less human-modified and more heterogeneous areas supported higher TD regardless of the presence of alien species. FD and PD of invaded communities decreased with increases in human-modified areas, whereas non-invaded communities were not affected. Our results suggest that even within a human-modified landscape, invaded community diversity is more affected by, and thus has a lower resilience to, disturbance. Restoring and protecting natural habitats within human-modified landscapes is likely to increase the resilience of native species.

Automated summary

Introduced alien species have negative impacts on the diversity of native communities and ecosystem functioning. Non-invaded bird communities showed higher taxonomic and functional diversity compared to invaded communities. Human-disturbed areas had lower diversity and higher invasion by alien species. Overall, less human-modified and more heterogeneous areas supported higher bird diversity.