Invasive non-native species pose a very serious threat to native plants and animals throughout Great Britain, and have been identified as the second biggest threat to biodiversity worldwide.

Many species have been introduced into England from other parts of the world. They include some of our most widespread and familiar plants and animals, such as sycamore, rhododendron, grey squirrel, pheasant and Canada goose. In urban situations, these may offer people valuable contact with nature, as with buddleia which is welcomed as a shrub with colourful flowers providing nectar attractive to native butterflies. These introduced species now form part of England’s biodiversity and have spread into our most isolated areas, so that there is unlikely to be a single SSSI without at least one.

Non-native species are introduced in many different ways, both accidentally and deliberately. Early examples include poppies brought in with arable plants when farming was introduced into England, and rabbits introduced for food. Currently, the deliberate release and accidental escape of aquatic garden plants and exotic animals are particularly significant.

Most introduced species either fail to survive in the wild or persist as rarities. Others live in native habitats without causing any threat to our native wildlife. A small fraction (perhaps 0.1%), however, can spread dramatically, increasing in range and abundance to such an extent that they cause marked effects. In some cases they can exist at low levels for some time before they expand to cause problems. Research has so far failed to explain how a species can become invasive.

All newly-introduced species have the potential to become problematic, especially where they can change their habitat preference and behaviour to suit their new home.

Freshwaters have seen dramatic transformations with the spread of Japanese knotweed, giant hogweed and Himalayan balsam on their banks, and with aquatic plants such as water fern, parrot’s-feather, floating pennywort and Australian swamp stonecrop in the water. These habitats are particularly vulnerable to invasion by non-native species which disperse by floating downstream and spreading in floods, including plants that would not normally produce seed in our climate but can grow from fragments. Such colonisation is often very rapid and difficult to control. Introduced animals can also transform habitats. Excessive grazing by deer (including several introduced species) is causing problems in woodland.

Predation Edit

Introduced species of animal can cause rapid crashes in the populations of some of our native animals through predation. Examples include the American mink, which is now widely established in England as a result of escapes and releases from fur farms. It is a specialist wetland predator, and has had catastrophic consequences for the water vole, which is now extinct in parts of its native range. In aquatic environments, introduced fish, like the zander, have caused significant declines in our native freshwater fish.

There are five main ways in which these non-native invasive species affect our native habitats and species:

Habitat transformation Edit

This is most obvious where introduced plants change the vegetation structure of our natural habitats, forcing out the native wildlife. One of the best known examples is rhododendron, which has spread into many western oakwoods and lowland heathlands. It forms dense stands that shade out native plants, and leaves toxic residues in the soil after clearance, which slows the recovery of native flora.


Water voles and mink Edit

Water voles favour watercourses with food plants such as reed canary-grass, sedges and bur-reed, in areas where water flow is relatively slow and constant, and where they can burrow into the banks. Water vole numbers have gone through two periods of heavy decline:

  • Decline in the 1950s onwards was due to land use changes. Intensive arable farming and heavy grazing encroached on and removed bankside vegetation, poaching of the banks damaged burrows, and hard engineering for flood defence prevented burrowing, and altered the flow and flooding regimes. Suitable habitat became linear and fragmented, and particularly vulnerable to damage that reduced water vole numbers to very low levels.
  • With the introduction in 1960s and escape of American mink in the 1980s, water voles were particularly vulnerable to predation. A breeding female mink and her kits can completely hunt out all water voles in their territory within two years, before moving on to alternative prey.

By 1990 sites occupied by water voles had reduced to only 32% of the number for the period 1900 to 1939. The English population has continued to decline, and is now only 20% of the 1989-90 figures.

Water voles now survive where heavy vegetation protects them from predation by mink, or in areas of high water quality where pest control by fisheries limits the numbers of mink.

Hybridisation Edit

Non-native species can threaten the genetic integrity of our native species, which are adapted to survive in their local environment. For example, our native bluebell is increasingly hybridising with the Spanish bluebell, commonly grown in gardens and discarded into the wild. The problem of the genetic impact of non-native species may be exacerbated by the release of genetically modified organisms into the environment.

Disease Edit

Many introduced species bring with them diseases to which they, but not our native wildlife, are immune. The best example of this is the spread of crayfish plague, introduced with the American signal crayfish in the 1970s and 1980s. This has decimated the native white-clawed crayfish population. In other cases, the foreign disease may be spread by native species, as with bark beetles spreading Dutch elm disease, which killed most mature elms in our landscape during the 1970s and 1980s.

Competition Edit

Many native species succumb through competition from more vigorous introduced species. For example, the Asiatic clam was first recorded in the River Chet in Norfolk in 1998, but in just five years has spread to at least four areas in the Norfolk Broads. It forms dense beds, crowding out native shellfish, and its filter-feeding can cause significant changes to the freshwater ecology by removing phytoplankton and other suspended particles. The clam could potentially out-compete our already threatened native mussel species, such as the depressed river mussel.

Conservation successes Edit

There have been many projects aimed at controlling non-native invasive species, and a number of specific examples are given to illustrate this.

Rhododendron control on heathlands Edit

Hardy’s ‘Egdon Heath’ is the largest of the initiatives under the Tomorrow’s Heathland Heritage Programme, supported by Natural England and the Heritage Lottery Fund. The project will restore 7,000 ha and re-create nearly 100 ha of heathland. Part of the work is to remove large areas of encroaching rhododendron, following which fencing and cattle grids will be introduced so that local farmers, supported by incentive schemes, can graze their livestock on the heath. This will provide the ideal management to restore a varied vegetation structure for species such as the smooth snake, Dartford warbler and sand wasp.

Coypu Edit

Imported for fur-farming in the 1930s, this large South American rodent soon escaped and established populations across East Anglia and beyond, with a peak population of about 200,000 in 1962. Significant agricultural damage then led to the start of control efforts in 1962. A full eradication campaign began in 1981, with the last animal trapped in 1989. This remains one of the few successful examples of the eradication of a non-native species from a large area.

North American bullfrog Edit

This large frog has been found at scattered locations in England, arising from releases or escapes of pets. Predation, competition and pathogen transfer by the frogs pose significant risks to our native amphibians. In the past, control measures at the sole confirmed breeding site, on the Kent-Sussex border, resulted in over 9,000 bullfrogs being removed from the wild, and further work will be required in coming years.

In 2000, a study of grey squirrel damage to beech, sycamore and oak estimated the total cost to the British timber industry as £10 million. Lessons learned from attempts to eradicate introduced species such as coypu and the North American bullfrog, support the clear message in the consultation document: that measures to prevent the introduction of non-native invasive species are far more cost-effective and environmentally desirable than measures taken after their introduction and establishment.

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Reference Edit

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