Alien invasive species, including diseases, are spreading

Martin Hale A recent outbreak of avian botulism killed more than 7% of the world population of Black-faced Spoonbill Zoom In

Invasive species of animals, plants and disease-causing micro-organisms are a particular threat to birds on islands, and have already caused numerous extinctions. Global travel, trade and a changing climate are encouraging the spread of invasives world-wide, including damaging diseases.

Invasive species are the main cause of recent bird extinctions

Humans have been transporting animals from one part of the world to another for thousands of years, sometimes deliberately (e.g. livestock released by sailors onto islands as a source of food) and sometimes accidentally (e.g. rats escaping from boats). In most cases, such introductions are unsuccessful, but when they do become established as 'alien invasive species', the consequences can be catastrophic. Invasives can affect native species by eating them, competing with them, hybridising with them, disrupting or destroying their habitat, or introducing pathogens or parasites that sicken or kill them. Over the last five hundred years, alien invasives 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 (see box 1).

Invasive species are a particular threat on islands

Currently, nearly 30% of Globally Threatened Birds (GTBs) are affected by alien invasive species, making this the third most important threat after habitat destruction and over-exploitation (see pp. 30–31). Island species are particularly susceptible because of their isolated evolutionary history, with 67% of oceanic-island GTBs affected by invasive species (box 2). The arrival of new invasives in the near future is a very real threat for almost 50 island species. There is particular concern for bird species in Micronesia. The brown tree snake has already caused severe ecological, economic and health problems on Guam since it was accidentally introduced shortly after World War II (including the local and global extinction of several native bird, bat, and lizard species).

Globalisation and climate change encourage the spread of invasives

Increases in human mobility and expansion of global trade encourage the spread of alien invasive species. Global climate change creates conditions suitable for new invasives. For example, increased temperatures potentially enable disease-carrying mosquitoes to expand their ranges (see p. 47, box 4). 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.

Invasive diseases are a growing problem

Diseases carried by invasive pathogens and parasites are already implicated in the decline and extinction of many bird species (box 3). Some diseases appear to be spreading to populations previously unaffected, including to species already threatened by other factors (box 4).

Boxes: case studies and scientific analyses

Download SOWB pp.44–45 (PDF, 308 KB) containing the following:

1. Invasive species have been implicated ion nearly half of recent bird extinctions
The major threats contributing to bird extinctions since 1500

2. Small island species are most at risk from invasives
a) 67% of GTBs on oceanic islands are affected by invasive species, a much higher figure than on continental islands or continents
b) 95% of GTBs threatened by invasives are affected by invasive predators

3. The distribution and abundance of native forest birds in the Hawaiian Islands are limited by introduced avian diseases
On Mauna Loa Volcano, Hawaii, avian pox is most common in mid-elevation forests where there are relatively high densities of both native birds and mosquitoes

4. Avian diseases are spreading to impact hitherto unaffected populations
Examples include Avian botulism, West Nile Virus and Avian cholera