A-Z Index











May 2006 - Non-native species


SNH is working with others to combat the threat of invasive non-native species.

The introduction of non-native species, either deliberately or accidentally, has been going on for a long time. Some introductions remain as discrete, isolated populations and do little harm, but others are highly invasive and detrimental to native wildlife:

• American grey squirrels displace the smaller native red squirrel
• American mink take native water voles and ground nesting birds
• Rhododendron ponticum smothers the native underflora of west coast oakwoods.

Once established, non-native species can be extremely difficult, if not impossible, to eradicate and very expensive to keep under control.

Non-Native species

Rhodododendron, the delightful pink-flowering bush in the May photograph, was introduced, probably from Iberia, as an ornamental plant for parks and gardens about 250 years ago. It is attractive to us when flowering, and useful to bumble bees, but has spread into the wild where in woodland, especially in the west of Scotland, it replaces the flowers of the woodland floor, the internationally-important assemblages of lower plants growing on the tree trunks and without management, ultimately would destroy the woods themselves by preventing tree regeneration.

Scotland’s native biodiversity is made up of plants and animals which populate the habitats and landscapes we enjoy all around us. But there are many non-native species, like Rhododendron, which have arrived from other parts of the world as a result of human activities. Many were introduced to Scotland intentionally, for instance as crop species for agriculture or forestry, or as ornamental specimens by Victorian plant hunters. Other species have appeared unintentionally; many are introduced accidentally with commercial imports, or, in the marine environment, attached to ships hulls or discharged with ballast water. As a result nearly half of Scotland’s wild vascular plant species, for instance, are now non-native species, together with more than half of Scotland’s freshwater fish species and almost a third of our mammals (SNH Trends Series).

The introduction of non-native species, deliberately or accidentally, has been going on for a long time and many have a range of social, economic and cultural values; they add to the brightness of our gardens, the design of our built environments, and the diversity of our landscapes. Once in the wild some remain as discrete and isolated populations doing no obvious harm to our native biodiversity. However there are others that, without the checks provided by their natural predators, are highly invasive here and can be detrimental to natural habitats or native species. In severe cases they can lead to the extinction of populations especially on islands. Some have very significant economic impacts as weeds and animal pests in agriculture, aquaculture and forestry. Introduced fungi and micro-organisms may cause human and livestock health problems or crop diseases.

Once established, non-native species are costly to control. Species, like the giant hogweed, are costing Local Authorities large sums to control and this will continue in perpetuity. Only two widely-established non-native species have been eliminated from the wild in Britain – the muskrat and the coypu. Successful eradication is most likely in the early stages of invasion but at that stage they often go completely undetected in the countryside.

So the best means to limit the number of new species appearing in Scotland, with unknown consequences, is to prevent introduction. The answer lies in the hands of ordinary people like you and I. Increasing public awareness of this and providing guidance on how to behave is therefore an important plank in the management of invasive non-native species. A first step was taken when the Scottish Executive, Defra and Welsh Assembly Government launched a Horticultural Code in 2005 to provide advice and guidance on the safe use, control and disposal of invasive non-native plants for everyone involved in horticulture and the use of plants.

Some Examples:
Ruddy Duck – introduced to Britain as an ornamental waterfowl in the 1940s from North America, birds from Britain is now threatening the European white tailed duck with extinction in its home in Spain. The programme to eradicate the species is costing £3.3million (Defra News).

American mink - a member of the weasel family introduced to Britain from North America for the fur trade. Mink affect ground-nesting birds by taking eggs and young chicks. Water vole populations can be exterminated by mink and this has caused a UK-wide decline in this species. The mink can also has a significant impact on fish farming, river fisheries and poultry farming. The programme to eliminate this species from the Western Isles only has so far cost £1.6m (Mink Project). A Species Framework target in Scotland.

Grey Squirrel – introduced into Britain from North America in the late 19th century. It has become the most common species of squirrel in the UK outside Scotland replacing the native red squirrel by dominating the food sources which they share and spreading a virus disease which is lethal to the red squirrels. Without targeted action against this species now there is no doubt that the red squirrel will become extinct in Scotland as it has in most of England (Red Squirrel News).

North American Signal Crayfish – a lobster like creature introduced to Britain about 20 years ago for fish farming. They cause damage to river banks and fish spawning beds and are voracious predators feeding mainly on fish eggs, small fish, other crustaceans and river vegetation posing a threat to many species of our fish and other freshwater life (Scottish Marine Labs). A Species Framework target in Scotland.

New Zealand Flatworm – an invertebrate like a flat pink and black worm which lives on damp soils. It was introduced to Scotland in about 1960 in the pots of garden plants and because of that mode of transport is now widespread in gardens. The flatworm preys on earthworms and has devastating impacts on garden populations. Earthworms are vital to the health of the soils and the effect of this species in the wild may be very significant (Defra Pest Note).

Rhododendron – a native evergreen of Turkey and Spain introduced to Britain in the 18th Century as a garden ornamental and used for game cover on sporting estates. The bushes spread very quickly in woodland and lightly grazed heathland smothering the native flora and therefore eliminating most of the diversity of native invertebrate animals. Rhododendron does provide cover for nesting birds and the flowers provide food for bees in their season but the damage to other species’ populations is considerable. Oakwoods in the west of Scotland are internationally important for their diversity of mosses and lichens: the dense shade cast by rhododendron on our woodlands and hillsides eliminates these. (Scottish Lower Plant Strategy). Because tree regeneration is prevented, if left alone the woods themselves would die out. A Species Framework target in Scotland.

Japanese knotweed – a native plant of Asia this was introduced to Europe as an ornamental plant in the nineteenth century. It is now widely established in British Isles, is extremely invasive and colonises most habitats – woodlands, river banks, grassland, coasts and can even grow through walls, concrete and tarmac! It grows very densely, out-competing, out-shading and suppressing growth of native plants although in time it may support an understory of woodland plants (Knotweed Alliance).

Dutch Elm Disease – this fungus was introduced to Britain in 1927 and caused little damage until a North American strain was introduced sometime before 1960. Since then it has eliminated many of Britain’s elms with a very significant impact on wildlife and landscapes. It is still spreading in Scotland (Forestry Commission).

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