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Invasive alien species are spreading

David Mudge

Invasive species of animals, plants and disease-causing micro-organisms have already caused numerous extinctions, and remain a particular threat to birds on oceanic islands. Certain diseases appear to be spreading to previously unaffected bird populations, some of which are already threatened by other factors. Global travel, worldwide trade and a changing climate are encouraging the further spread of invasives.

Key messages and case studies

Invasive species are the main cause of recent bird extinctions
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Humans have been transporting animals and plants from one part of the world to another for thousands of years, sometimes deliberately as in the case of livestock, and  companion animals such as cats and dogs and sometimes accidentally (for example rats escaping from boats). Often introduced species fail to establish themselves in the wild in new locations. However, a proportion thrives and spread. Such 'invasive alien species' may have catastrophic impacts on local fauna and flora. Invasive species can affect native ones by eating them, competing with them, hybridising with them, disrupting or destroying their habitat or, in the case of pathogens or parasites, by sickening or killing them. Over the last five hundred years, invasive alien species have been partly or wholly responsible for the extinction of at least 65 bird species, making this the most common contributory factor in recent losses to the world’s avifauna (Invasive alien species have been implicated in nearly half of recent bird extinctions ).

Invasive species are a particular threat on islands
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Overall, invasive species affect around half of currently threatened bird species. Island species are particularly susceptible because of their isolated evolutionary history, with three-quarters of threatened birds on oceanic islands affected by invasive species (Small island birds are most at risk from invasive alien species ). Rats and cats have had far and away the greatest effect, threatening the survival of hundreds of different bird species worldwide, but other species can also have devastating impacts (Native birds on Gough Island are being devastated by house mice, Endemic birds on Christmas Island are being devastated by introduced ants, Bird populations in the Northern Mariana Islands are being decimated by brown tree snakes).

Invasive diseases are a growing problem
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Diseases carried by invasive pathogens and parasites are already implicated in the decline and extinction of many bird species (Native forest birds in the Hawaiian Islands are limited by introduced avian diseases ). Some diseases, such as West Nile Virus, appear to be spreading to populations previously unaffected, including to species already threatened by other factors (Avian diseases are spreading to impact hitherto unaffected populations , West Nile Virus is spreading throughout the Western Hemisphere). ‘Bird flu’ is of particular concern, not because of direct impacts on wild birds, rather because it is leading to negative public perceptions towards birds (The H5N1 avian influenza virus: a threat to bird conservation - but indirectly).

Global travel, trade and a changing climate encourage the spread of invasive species
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Increases in human mobility and expansion of global trade encourage the spread of invasive species. Global climate change creates conditions suitable for new invasives. For example, increased temperatures potentially enable disease-carrying mosquitoes to expand their ranges. These factors, together with the degradation and fragmentation of natural habitats which make it easier for invasives to establish themselves, mean that invasive species are likely to become an increasing threat.