Invasive species can cause a huge range of problems for humans and wildlife. The reasons for the success of the invader can vary, but the consequence is that it becomes a pest.
Competing for space and food
Invaders threaten native species if they are able to compete more effectively for resources, such as light, space, water or shelter. The invader’s ability to compete may be helped by being free from the predators, competitors, parasites and diseases that control them in their native habitats. Invaders can be so good at outcompeting native species that they overwhelm habitats. The growth of dense clumps of invasive vegetation, such as Japanese knotweed (Fallopia japonica, Houtt, 1777), prevents other species growing and by doing so changes the structure of the whole habitat.
Unnatural born enemies
Species that evolve in the same place can evolve defences against one another. Invaders can prove particularly deadly enemies because they have an array of attack strategies that are new to the invaded community. This is especially true of remote islands, where the wildlife may have evolved without pressure from certain types of animals, such as predatory mammals. Rats (Rattus spp. - three species; the ship or black rat Rattus rattus, the norway or brown rat, Rattus Norvegicus and the polynesian rat or kiore Rattus exulans are recognised as invasive), and cats, for example, have devastated the wildlife of many islands. Rats, introduced to 82% of the world’s island groups, are considered an important factor in 54% of island bird extinctions. Offshore islands of the British Isles can be just as vulnerable as tropical islands. Hedgehogs (Erinaceus europaeus, L.) were introduced to North Uist, South Uist and Benbecula, off the coast of Scotland, in the 1970s. Since then numbers have rapidly increased with the hedgehog population now estimated at between 5,000 and 10,000. Although hedgehogs mainly eat invertebrates such as slugs, they will also eat bird eggs and chicks. The high number of hedgehogs on these islands now threatens the breeding success of wading birds such as dunlin (Calidris alpina, L.) and redshank (Tringa totanus, L.).
Invaders do not have to be predators to be an effective enemy. Grazers such as goats and sheep have proved devastating enemies to the wildlife of remote islands such as Hawaii and New Zealand, where plants have never been grazed or trampled by mammals before.
Some invaders breed with closely related relatives causing problems at a genetic level. The ruddy duck (Oxyura jamaicensis) was first recorded as an escapee from Slimbridge Wildfowl park in 1954, 6 years after it was imported from the United States of America. The population of ruddy ducks in the British Isles increased rapidly and then began appearing in mainland Europe. In Spain, the ruddy duck competes and interbreeds with the endangered white-headed duck (Oxyura leucocephala, Scopoli, 1769). Isolated from one another by the Atlantic Ocean for 3-5 million years, the two ducks are genetically distinct. The invasive ruddy duck population threatens not only to outcompete the white-tails, but also to absorb the white-tail population by hybridisation.