Predation by alien species on islands can severely damage biodiversity (Doherty et al. 2016). Many predatory alien bird species have been introduced to islands (Evans et al. 2016): an unintended consequence of these introductions is the predation of threatened birds (e.g. black drongo (Dicrurus macrocercus) predation of the Rota white-eye (Zosterops rotensis) (U.S. Fish and Wildlife Service 2007)).
Information on the environmental impacts of alien birds is unavailable for 187 of the 247 regions of the world that they occupy: many of these regions are islands (Evans & Blackburn 2020). Alien species research tends to focus on species that are perceived to have the most severe impacts (Pyšek et al. 2008). However, the availability of data on the impacts of alien birds across the regions that they occupy is not associated with the severity of these impacts: rather it is positively associated with human development, alien bird species richness and alien bird residence time (Evans & Blackburn 2020). Therefore, we cannot assume that regions lacking impact data do so because the impacts sustained within them are less severe: damaging predation impacts may be going unnoticed on islands.
Numerous studies have documented alien birds preying on birds on islands, but our understanding of the number and type of species affected has been limited by the lack of a global review of these impacts. Here I identify the alien bird species with recorded predation impacts to birds, the locations where these impacts occur, and the number and type of bird species affected. I then identify the small islands around the world that are occupied by these alien birds, and by taking into account their life-history traits and predation history, I also identify the near-threatened and threatened bird species on these islands that they may be preying on.
Results/Conclusions
Alien bird predation is primarily a concern for threatened bird conservation on small islands: almost all impacts to near-threatened and threatened birds have been recorded on islands, and their median size is 128km2. Alien bird predation is also a significant unknown threat to avian biodiversity: there are 152 small islands around the world that are occupied by alien birds that prey on birds, but information on their impacts is unavailable for the vast majority of these islands. On them, where the impacts of alien species can be severe, alien birds may be preying on approximately 5% of the world’s near-threatened and threatened bird species.