The introduction of non-native species into ecosystems can pose a threat to biodiversity. The majority of introduced species are benign, but non-native species become invasive when they out compete rival indigenous species for the same resources, introduce new diseases, or directly prey upon indigenous species.
BRITISH SPECIES UNDER THREAT
In Britain biodiversity is threatened by a number of invasive species, including the American signal crayfish, which was introduced in the 1970s to be farm-bred for use in the restaurant trade. Its subsequent escape into British waterways has seen it decimate the native white-clawed crayfish, which it out competes through its larger size and ability to reproduce more quickly, as well as through the disease it carries known as crayfish plague. The IUCN estimate that the white-clawed crayfish population has been reduced by up to 80% in the past ten years, and it is now on their red list of species vulnerable to extinction.
It is not just the threat of losing our native crayfish that is of concern but also the negative impact of the signal crayfish upon wider ecosystems, through the damage it causes to river habitats by burrowing into riverbanks. Of equal concern to waterway ecosystems is an accidental addition to Britain's rivers and estuaries - the Chinese mitten crab, which first appeared in the River Thames in the 1930s via the ballast tanks of ships. It migrates into freshwater habitats for part of the year before returning to estuaries to breed, preying upon a wide range of native species along the way, posing a significant threat to biodiversity.
THREAT TO GRAZING LAND
Invasive plant species also threaten ecosystems, as well as negatively impacting on human health and livelihoods. An IUCN Invasive Species Group project from 2010 identifies a number of invasive plant species of great concern in parts of Africa, such as parthenium in Ethiopia, Kenya, Tanzania and Mozambique. Parthenium invades agricultural and pastoral land, where it substantially reduces crop yields and threatens the viability of grazing land. It is resistant to commonly available herbicides and causes skin and respiratory allergies to farmers trying to weed it out. Parthenium like other such invasive plants degrades ecosystems by displacing indigenous species and reducing biodiversity. Two invasive plant species of particular concern in Britain are Japanese knotweed and Rhodedendron ponticum, both of which were introduced as ornamental garden plants. They threaten biodiversity by out competing native plants due to their rapid spread and the difficulty in controlling and eradicating them.
One of the most high profile invasive species in Britain is the North American grey squirrel, which was imported into England in the late nineteenth century. Grey squirrels impact on biodiversity by causing damage to a wide range of tree species, and consuming unripe tree seeds, adversely affecting other seed-eating species such as dormice. Their most noticeable impact however has been on the native red squirrel population, which retreated with the rapid spread of the greys to the point where they have now disappeared from all but a few areas of Britain. Red squirrel numbers continue to decline, in part due to the parapoxvirus carried by the greys. Their disappearance would be a sad loss to biodiversity in Britain, and efforts to preserve the remaining populations are ongoing.