Novel ecosystems support substantial avian assemblages: the case of invasive alien A cacia thickets

Aim Altered habitats may form entirely novel ecosystems that support new combinations of species. How indigenous species use invaded areas is, however, not well understood. Here, we investigate the value of A ustralian A cacia thickets as novel ecosystems in the W estern C ape of S outh A frica by surveying bird assemblages within them. Location Western C ape P rovince of S outh A frica. Methods Birds were surveyed quantitatively in a variety of A cacia thickets in the south‐western W estern C ape in three seasons to examine species richness, abundance and functional diversity. We also examined the extent to which avian diversity was related to differences in patch‐level vegetation structure. Results Significant variation was observed in assemblage richness, density and biomass across sites. Diversity increased with productivity, but declined with stem density and canopy cover. On average, A cacia thicket patches were used by c . 20 species (with a regional richness of 76 species), had a mean density of 7.78 birds ha −1 and a mean biomass of 0.224 kg ha −1 . The most abundant feeding guilds were the mixed feeders and insectivores. Main conclusion A cacia thickets in the W estern C ape support a large subset of the region's birds with the most abundant species being small mixed feeders. Compared with other habitat types, A cacia thickets support avian assemblages with species richness and density similar to some natural sites in the region, but lacking typical nectarivores. Extrapolation to the area transformed by invasive acacias in the C ape F loristic R egion suggests that these novel ecosystems support c . 22 million individual birds or 621 tonnes of avian biomass.