Invasive alien trees reduce bird species richness and abundance of mutualistic frugivores and nectarivores; a bird's eye view on a conflict of interest species in riparian habitats

Invasive alien plants have major ecological effects, in particular in riparian habitats. While effects of alien tree invasions on riparian plants are well studied, effects on animals are less well understood. Invasive alien trees can have a positive effect by adding habitat and food sources, or have a negative effect, by replacing native food plants. Here we use birds as indicators to determine the impacts of an invasive Eucalyptus tree species in riparian areas of the Cape Floristic Region (CFR) of South Africa. Birds are an ideal study group because they are mobile, respond quickly to habitat changes and feed at different trophic levels. Fixed-point bird counts were done during winter and spring at near-pristine and Eucalyptus camaldulensis invaded riparian habitats. A total of 1142 birds from 44 species were recorded. Bird assemblages in invaded sites are almost a complete subset (24 species) of those in near-pristine areas (42 species). Invaded areas were missing 18 species and contained a total of 128 fewer individuals. This is due to declines in insectivores, frugivores, granivores, raptors and omnivores and the absence of nectarivores in invaded sites. From a bird's perspective, the prioritisation of E. camaldulensis removal from the CFR's river systems is justified, but whether bird species will return to cleared areas needs to be determined.