Impacts of an invasive willow (Salix x rubens) on riparian bird assemblages in south-eastern Australia

We explored how a woody plant invader affected riparian bird assemblages. We surveyed 15 200-m-long transects in riparian zones in a much-changed landscape of eastern Victoria, Australia. Abundance, species-richness, foraging-guild richness and composition of birds were compared in transects in three habitat types: (i) riparian zones dominated by the invasive willow Salix rubens; (ii) riparian zones lined with native woody species; and (iii) riparian zones cleared of almost all woody vegetation. We also measured abundance and richness of arthropods and habitat structure to explore further the effects of food resources and habitat on the avifauna. We observed 67 bird species from 14 foraging guilds. Native riparian transects had more birds, bird species and foraging guilds than willow-invaded or cleared transects. Habitat complexity increased from cleared to willow-invaded to native riparian transects, as did abundance of native and woodland-dependent birds. Native shrub and trees species had more foliage and branch-associated arthropods than did willows, consistent with a greater abundance and variety of foraging guilds of birds dependent on this resource. Willow spread into cleared areas is unlikely to facilitate greatly native bird abundance and diversity even though habitat complexity is increased. Willow invasion into the native riparian zone, by decreasing food resources and altering habitat, is likely to reduce native bird biodiversity and further disrupt connectivity of the riparian zone.