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:
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) 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.