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Invasive Species

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Many non-native species are already established in Ireland. Others are likely to arrive in the near future. While not all non-native species are damaging to our native wildlife, invasive non-native or ‘alien’ species are one of the biggest threats to Ireland’s native biodiversity.

Invasive, non-native or alien species can cause problems for our native wildlife in a number of ways, such as competition for food or space, predation (being killed and eaten), disease and loss of genetic purity through interbreeding. They can also damage the environment and the economy.

The best way to avoid problems with invasive alien species is to prevent their introduction in the first place. Once established, it may prove impossible to get rid of them.

Brown rat

Brown rat

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Harlequin ladybird

Harlequin ladybird

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Aquatic plants

Aquatic plants

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Muntjac deer

Muntjac deer

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New Zealand flatworm

New Zealand flatworm

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Grey squirrel

Grey squirrel

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Himalayan balsam

Himalayan balsam

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Sea squirt

Carpet sea squirt

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American mink

American mink

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American skunk cabbage

American skunk cabbage

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Ruddy duck

Ruddy duck

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Sika deer

Sika deer

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Brown rat

Image: Brown rat - Rattus norvegicus - Native to China. BELUM.Lh2010.11 © National Museums Northern Ireland
Brown rat - Rattus norvegicus - Native to China. BELUM.Lh2010.11 © National Museums Northern Ireland

The brown rat is probably the most successful ‘invader’ on earth. Arriving in Eastern Europe during the early eighteenth century, within a hundred years it had colonised every European country. It is now found on every continent except Antarctica.

There is estimated to be over sixty million rats in the United Kingdom.

A female rat can give birth to a litter of eight pups up to seven times per year – so, in one year she can produce over fifty young. Rats can breed when they are just four months old and live for about two years.

Brown rats carry many diseases that can be transmitted to people, including salmonella, Weil’s disease and tuberculosis. They also cause considerable damage to stored food, crops and property.

Rats are a huge threat to many species of wildlife, especially ground nesting birds whose eggs and chicks are particularly vulnerable.

Harlequin Ladybird

Image: Harlequin ladybird - Harmonia axyridis - Native to eastern China and Japan © Roy Anderson
Harlequin ladybird - Harmonia axyridis - Native to eastern China and Japan © Roy Anderson

The harlequin ladybird has been deliberately introduced in various countries around the world as a very efficient biological control for greenfly and other aphids. However, it is also a pest species - it can eat all the aphids in an area leaving our native ladybirds to starve. When the harlequin runs out of aphids it will eat the eggs and larvae of other insects including other ladybirds, butterflies, moths and hoverflies.

In the cold winter months large numbers of harlequin ladybirds may shelter together in houses and garages - in North America up to 20,000 hibernating adults have been found in one house. When disturbed they leak smelly yellow fluid from their leg joints that will stain walls and fabrics.

Up to 2011 the only confirmed sightings in Ireland were linked to batches of supermarket vegetables (Lisburn in 2007, and Cloghy, Ards in 2009). Since then there have been over 50 harlequin records mainly in counties Antrim and Down.

The harlequin ladybird is a very varible species making it hard to identify. Click here to learn more about ladybirds in Ireland.

Aquatic plants

Floating pennywort - Hydrocotyle ranunculoides - Native to North America © John Early
Floating pennywort - Hydrocotyle ranunculoides - Native to North America © John Early
Fringed waterlily - Nymphoides peltata - Native to Eurasia and the Mediterranean © John Early
Fringed waterlily - Nymphoides peltata - Native to Eurasia and the Mediterranean © John Early

Many of our invasive aquatic plants have escaped from garden ponds. Some are all too easily capable of spreading into local lakes and rivers, via plant fragments or by seed.

Once they become established they can take over a habitat and cause a wide range of problems such as:

  • overwhelming the native water plants
  • risk of flooding
  • disruption to leisure activities, such as fishing and boating

Some aquatic plants such as floating pennywort are now banned from sale in Northern Ireland.

Muntjac deer

Muntjac deer - Muntiacus reevesi - Native to China and Taiwan. BELUM.Lh2010.08 © National Museums Northern Ireland
Muntjac deer - Muntiacus reevesi - Native to China and Taiwan. BELUM.Lh2010.08 © National Museums Northern Ireland
Male muntjac deer showing long sharp canine teeth. BELUM.Lh2010.08 © National Museums Northern Ireland
Male muntjac deer showing long sharp canine teeth. BELUM.Lh2010.08 © National Museums Northern Ireland

Muntjac deer were introduced into parks in Britain in the early twentieth century. Many escaped over the years and became established in the wild – the estimated population is now thought to be over 50,000.

Unlike most deer species, muntjac do not have a specific breeding season and can produce up to three young every two years.

In 2009, a young male muntjac was hit by a car in County Down – the first confirmed sighting in Northern Ireland. Since then, there have been over 200 sightings mainly in counties Down and Armagh

Muntjac can cause severe damage to woodland. They can also cause road traffic accidents.

New Zealand flatworm

Image: New Zealand flatworm - Arthurdendyus triangulatus - Native to New Zealand © Roy Anderson
New Zealand flatworm - Arthurdendyus triangulatus - Native to New Zealand © Roy Anderson

The New Zealand flatworm was first recorded in Northern Ireland in 1963, where it may have arrived with a shipment of roses or bulbs from New Zealand.

It has a strap-like body, up to 15cm long, pointed at both ends and covered in sticky mucus. Adults are often found resting coiled up under stones or black plastic sheeting. The shiny, black oval eggs are laid in soil and are around 4 - 11mm long. Each egg contains 5- 8 tiny flatworms.

It preys on our native earthworm and has devastated local populations. The species has now reached all parts of Northern Ireland and is spreading into the Republic of Ireland, Scotland and England.

Grey squirrel

Image: Grey squirrel - Sciurus carolinensis - Native to North America. BELUM.Lh813 © National Museums Northern Ireland
Grey squirrel - Sciurus carolinensis - Native to North America. BELUM.Lh813 © National Museums Northern Ireland

Grey squirrels were intentionally introduced into Ireland in 1911. People at that time considered the grey squirrel to be an attractive addition to the native wildlife.

In less than 100 years, they have spread throughout most of Ireland.

Grey squirrels compete with our native red squirrels for food. They can also carry and spread a viral disease – ‘squirrel pox’ – that is fatal to red squirrels.

Grey squirrels strip bark from trees so they can feed on the soft inner layers. This can cause considerable damage and, in severe cases, the tree may die. Bark stripping can cause extensive damage to commercial timber plantations.

Himalayan balsam

Image: Himalayan balsam - Impatiens glandulifera - Native to the Himalayas © John Early
Himalayan balsam - Impatiens glandulifera - Native to the Himalayas © John Early

This attractive plant is closely related to the houseplant ‘Busy Lizzie’ and was introduced to Europe as a garden plant in the 1800s.

It is the largest annual plant in Northern Ireland and spreads easily by seed. When the seed is ripe it can be catapulted up to four meters away allowing the plant to spread and colonise new areas quickly.

Like many garden plants, it escaped into the wild and can be found growing in dense stands on riversides and in damp woods.

Himalayan balsam produces a lot of pollen over the growing season and competes with native riverbank species for pollinators such as bumblebees, and so reduces seed set in these other plants. It also forms dense thickets that shade and crowd out our native plants.

Himalayan balsam plants are easily pulled out but this must be done before the seeds mature in late summer. The seed can only live in the soil for around 18 months - so removing every balsam plant in an area for 2-3 years can get rid of it completely.

Carpet sea squirt

Image: Carpet sea squirt - Didemnum vexillum  © Julia Nunn
Carpet sea squirt - Didemnum vexillum © Julia Nunn

Non-native invertebrate species can find their way into coastal waters by a variety of means: attached to the hulls of boats, on floating debris, or as imported seafood, for example. Another significant route involves the deliberate loading and discharge of ballast water by large ships, to maintain their stability as they travel the open seas.

A species of sea squirt from the Didemnum group has recently been found in Ireland fouling boats and marina structures. Already known from many parts of the world, it grows either in ‘dripping’, candle-wax-like colonies that hang from hard surfaces, or as mats that encrust rocky seabeds. This fast-growing species has the potential to outcompete and smother many native marine species, such as mussels, scallops, oysters, seaweeds and sponges.

American mink

Image: American mink - Neovison vison - Native to North America and Canada. BELUM.Lh826 © National Museums Northern Ireland
American mink - Neovison vison - Native to North America and Canada. BELUM.Lh826 © National Museums Northern Ireland

Mink were intentionally introduced into Ireland for fur farming in the early 1950s. Within ten years, there were around forty mink farms or ranches across Ireland, housing thousands of animals.

Many mink escaped from these farms and, in some cases, they were intentionally (and illegally) released. As a result, mink have become firmly established in the wild throughout Ireland.

Mink farming was banned in Northern Ireland in 2003. There are, however, several mink farms still operating in the Republic of Ireland. In September 2010, up to five thousand mink were intentionally released from a fur farm in County Donegal by animal rights activists – some of these animals will starve to death but many will survive and are likely to have a severe impact on wildlife in the local area.

Mink pose a serious threat to ground–nesting birds, particularly colonies of seabirds nesting on islands.

American skunk cabbage

Image: American skunk cabbage - Lysichiton americanus - Native to North America © National Museums Northern Ireland
American skunk cabbage - Lysichiton americanus - Native to North America © National Museums Northern Ireland

The skunk cabbage was introduced into Europe in the early 1900s as an ornamental garden plant. The bright yellow flowers appear in spring and produce a horrible smell from which the skunk cabbage gets its name. The plant grows best in wet boggy ground.

The leaves are huge - mature plants can grow up to 1.5m tall.  The plant spreads via a thick fleshy underground rhizome - it also produces seeds.

Within a few years plants can quickly spread and build up dense layers of vegetation blocking out the light and killing everything beneath them.

Ruddy duck

Image: Ruddy duck - Oxyura jamaicensis - Native to North America. BELUM.Lg2009.09
© National Museums Northern Ireland
Ruddy duck - Oxyura jamaicensis - Native to North America. BELUM.Lg2009.09 © National Museums Northern Ireland

The ruddy duck was introduced into wildfowl collections in England in the 1940s and soon spread across Britain, reaching Ireland in 1973 when it was first seen at Oxford Island Nature Reserve, County Armagh. It has now colonised over 40 countries in Europe and North Africa.

The ruddy duck poses a huge threat to its very rare, close relative, the white-headed duck (Oxyura leucocephala), with which it can interbreed and produce fertile offspring. This is likely to result in the eventual extinction of the white-headed duck

Sika deer

Image: Sika deer - Cervus nippon - Native to Japan. BELUM.Lh2010.06 © National Museums Northern Ireland
Sika deer - Cervus nippon - Native to Japan. BELUM.Lh2010.06 © National Museums Northern Ireland

Sika deer were intentionally introduced into Ireland in 1860 by Lord Powerscourt, who released a single male (stag) and three females (hinds) into his estate in County Wicklow. The sika bred with native red deer and, within a few years, the herd of sika and sika hybrids had increased so much that Lord Powerscourt was able to supply deer to other estates in Ireland.

In the 1920s, all the sika deer escaped from the deer park at Baronscourt, County Tyrone and became established in the local woodland.

Sika deer are related to red deer and will interbreed with them. The hybrid offspring are fertile so interbreeding could destroy the genetic purity of the native red deer as a species.

Sika deer can damage trees by gouging the trunks with their antlers and by bark stripping. They may prevent the regeneration of native woodland by browsing on young shoots and seedlings.