The second-largest contributor to the extinction of species after habitat destruction are invasive species. Invasive species are any organisms which have been artificially introduced to an area by humans which thrive and out-compete native species. Invasive species disrupt established ecosystems and decrease biodiversity.
A prime example of an invasive alien specie is the Sacred Ibis, a waterbird native to sub-Saharan Africa and Madagascar. In the 1970s, European countries such as Spain, Italy, and France began to raise ibises in free-flying groups in zoological gardens. As a result, many ibises escaped and established themselves in wetland environments throughout Europe. They feed upon invertebrates and fish and prey upon the eggs and young of other bird species. In France, this predation has wiped out entire colonies of Terns and Mallards and has devastated populations of newts.
One of the impacts of low biodiversity is that environments become more susceptible to the spread of dangerous organism. Perhaps the most notable example of this is the Irish Potato Famine. The potato was non-native to Ireland, but when it was introduced there it was so calorie dense per square foot of land that it quickly became the only crop famers planted. Then in 1845 an airborne fungus was carried in ships from North America to England. The resulting blight infected and rotted potato plants and because of the low diversity of crops, this caused a mass starvation within Ireland. Millions died and millions more emigrated out of Ireland as a result.
Once firmly established, invasive species become very difficult to eradicate. Instead, efforts must be focused on decreasing the spread of these species. Organizations such as the National Wildlife Foundation work to stop the spread of invasive species by monitoring for new infestations, working quickly to eliminate them, and enacting legislation to restrict the importation of species where they could be potentially invasive.
IUCN - Invasive Species
Birding World - Sacred Ibis: A New Invasive Species in Europe
The Guardian - Invasive Species: Can they be Stopped?