Three-quarters of all threatened bird species occurring on oceanic islands are at risk from introduced species. Invasive predators, especially rats and cats, represent the greatest threat, but the impacts of habitat modification by herbivores and reduced fitness resulting from introduced micro-organisms are also significant. There are many examples of where these threats, alone or combined, have caused extremely rapid declines and even extinctions. A total of 390 islands worldwide support populations of one or more Critically Endangered or Endangered species and one or more vertebrate invasive alien species that threatens them.
In total, 446 threatened birds (34% of the total) are threatened by invasive alien species (analysis of data held in BirdLife’s World Bird Database 2013). The problem is especially acute on islands, particularly small ones (75% of threatened species on oceanic islands are threatened by invasives), where long isolation has led to the evolution of species that often lack adequate defences against introduced species. The majority of species at risk from invasives are affected by introduced predators; rats and cats are far and away the most severe threat impacting 236 and 199 threatened species respectively. However, many are subject to multiple impacts from a range of non-native species. One such example is Galápagos Petrel Pterodroma phaeopygia (Critically Endangered) which has undergone an extremely rapid decline since the early 1980s owing to a variety of threats, including predation by introduced rats, cats and dogs, and the destruction of breeding habitat by introduced goats and cattle (BirdLife International 2008). The Palila Loxioides bailleui is threatened by the same species of predators and herbivores introduced within its range on the Hawaiian Islands but, in addition, it faces the combined non-native threats of avian pox and avian malaria carried by introduced mosquitoes (BirdLife International 2012). A total of 390 islands worldwide support populations of one or more Critically Endangered or Endangered species and one or more vertebrate invasive alien species that threatens them. These islands represent priorities for eradication or control of the invasive aliens in order to mitigate the threat they pose.
Related Case Studies in other sections
Compiled 2004, updated 2008 and 2013
BirdLife International (2013) Small island birds are most at risk from invasive alien species . Presented as part of the BirdLife State of the world's birds website. Available from: http://birdlife.org/datazone/sowb/casestudy/128. Checked: 19/04/2015
|Key message: Invasive species are a particular threat on islands|