Alien invasion: Britain’s most wanted


Invasive species are becoming an increasing problem in and around the UK.

Whilst publicised ecological research continues to provide us with valuable insights into some of the environmental issues connected with invasive species, for many invaders we are still unaware of the full ecological implications of their establishment here in Britain. In addition, the identities of many of our alien species are not widely known, and as a result, counteractive measures are often ignored.

What has become abundantly clear over the last few centuries however is that non-native species significantly alter the composition and functioning of native ecosystems and the status of species residing in them. In some cases, the presence of invasive species has directly contributed to the extinction of native ones.

So what makes a species invasive?

  • Invasive species can be described as those which have become established in a region that they did not evolve in, either through natural or human induced introduction
  • Invasive species are often hard to control because they lack the natural predators they co-evolved with in their native environment

How have invasive species got into Britain?

  • Species have been deliberately introduced into the UK– In the late 19th century the grey squirrel was deliberately released into Britain
  • Species have been accidentally introduced into the UK– the Chinese mitten crab for example, was accidentally introduced into the UK through transportation of larval crabs in ballast water

Entry by escape– some UK invaders are thought to have become established in wild following escape from contained facilities; one such example is the American mink (Mustela vison), which has colonised Britain following escape from fur farms



Mink contained at a fur farm

Why are invasive species problematic?

Different invaders present different threats to native wildlife and ecosystems. Some of the common problems associated with invasive species include:

  • Population reduction or total extinction of native species due to predation, competition, disease transmission and habitat destruction
  • Loss of genetic identity (and subsequently of biodiversity) through hybridisation of native and non-native species
  • Alteration of ecosystem functioning
  • Difficulty of control; invasive species are generally hard to control/eradicate and tend to possess traits that favour expansion such as high growth, dispersal and reproductive rates. Invasive species are also typically very adaptable in terms of diet and tolerance of varying climatic conditions; this contributes to their success at outcompeting native species
  • Economic costs; invasive species can cause significant amounts of damage to farmed species and equipment

Species focus: The American Signal Crayfish



The signal crayfish (Pacifastacus leniusculus)

The signal crayfish (Pacifastacus leniusculus) is a species of crayfish native to North America. The species was deliberately introduced into Britain in the 1970s to enhance a rapidly depleting population of European crayfish for the food industry. Decline of European crayfish was attributed to a fatal

disease caused by the fungal pathogen Aphanomyces astaci, known as the crayfish plague.  This too is an invasive organism, introduced into Europe in the 1800s and spreading into Britain in the late 20th century. Originally, the signal crayfish was confined to secure (or not so secure) farming facilities but became established in the wild following escape.  It was only after the signal crayfish was introduced into Britain that scientist’s discovered this species is actually a vector of Aphanomyces astaci. This has resulted in decline of Britain’s only native species of crayfish, the white clawed crayfish (Austropotamobius pallipes). To make matters worse for the white clawed crayfish and other European species, signal crayfish are not susceptible to the disease themselves, and are thus able to maintain large populations.




The white clawed crayfish, Austropotamobius pallipes

This is further problematic because signal crayfish are larger and more aggressive than European species, and are thus able to successfully compete against them for food and habitats.

Alongside crayfish, many other native animals are adversely affected by the presence of the invasive signal crayfish.  Signal crayfish are not fussy when it comes to food, possessing a wide diet that includes plants, fish, fish eggs and even their own young. With large volumes of vegetation being consumed by these animals there are fewer egg laying sites for aquatic insects and fish, contributing to declines in those affected.  Due to their impressive egg laying capabilities and burrowing habitats, signal crayfish are also thought to be responsible for the collapse of river banks in some areas.

What control is there?

Limited control measures exist for the signal crayfish. Whilst trapping may seem like a feasible idea and has been adopted by some fishermen, this has been found to enhance the dispersal of invasive crayfish and further spread disease. In addition, young crayfish are likely to avoid traps due to their small size. The idea of commercially hunting the signal crayfish is of major concern to conservationists who fear that the few remaining white claw crayfish could be mistaken for signals and wiped out in the process. Further suggestions for control include electrocution, the use of chemical pesticides and implementation of male sterilization techniques.

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