Alien bird species decrease the diversity of bird communities across
human-disturbed landscapes
Abstract
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.