Problem species

From invasive plants to riverbank demolishers, we give the lowdown on the species wreaking havoc on the ecosystem.

We’re blessed with verdant hedgerows and riverbanks, but there are an increasing number of flora and fauna furrowing the brows of many conservationists. Whether they've arrived on our shores by accident, spread from botanical gardens or deliberately released, non-native species are proving to be a challenge. We give the lowdown on eight rather troublesome invaders.


Japanese knotweed, Fallopia japonica

Appearance: Roughly triangular leaves and an extensive system of rhizomes (horizontal stems)

How did it get here? Native to Japan, Taiwan and China, this weed was first brought to Britain as an ornamental plant in the 19th century.

What's the problem? Japanese Knotweed grows voraciously at a rate of 2cm a day, crowding out other vegetation and obliterating riverbanks and walls. It can grow from tiny fragments as small as 10mm and has no natural predators, making it a super-invincible weed.

How is it being controlled? Valiant volunteers across the country are digging out individual plants and injecting stumps with herbicides while sieving the soil to remove any traces of root, rhizome or stem it can sprout from. It's estimated it would cost £1.5bn to eradicate knotweed from the UK.


Killer shrimp, Dikerogammarus villosus

Appearance: Larger than our native shrimps; it grows up to 30cm in length and usually has a striped appearance.

How did it get here? This stripy critter is native to the Black Sea and spread across Western Europe over the past 20 years. It was first discovered in British waters in 2010.

What's the problem? A common problem among invasive species is they are fast breeders and Killer Shrimp are no exception. Females can produce up to three broods in their lifetime, each with an average of 150 eggs per brood. They prey on a range of our native species, from native shrimp to young fish, altering the habitat ecology. They're not even that great to eat.

How is it being controlled? Kayakers, anglers and others who come into frequent contact with water are advised to maintain good biosecurity - to check, clean and thoroughly dry their equipment, footwear and clothing. We don't want shrimpy hitchhikers getting about!


Zebra mussel, Dreissena polymorpha

Appearance: Grows up to 5cm long with brown and yellow stripes.

How did it get here? Another Black Sea native, Zebra Mussels were introduced to our waterways on the hulls of ships.

What's the problem? As well as the usual problem of competing for space with native species, Zebra Mussels form large colonies that attach to almost any hard surface, impeding the smooth running of canal gates and sluices. They also feed on plankton, which allows more sunlight to penetrate the water, stimulating the growth of weeds.

How is it being controlled? Divers remove the mussels by hand. Anglian Water alone spends £500,000 a year on this laborious process. A new toxin-filled pellet has been created that tricks the mussel into eating it, which should help keep numbers under control.


Chinese mitten crab, Eriocheis sinensis

Appearance: This greedy critter takes its name from its large furry white-tipped claws.

How did it get here? First recorded in British waters in 1935, brought in the ballasts of ships.

What's the problem? A voracious breeder with a voracious appetite, Chinese Mitten Crabs feast on a wide range of native invertebrates and eggs of fish. They're also rampant burrowers, leaving fragile canals and riverbanks vulnerable to collapse.


Himalayan Balsam, Impatiens glandulifera

Appearance: Stout red stems and deep purple to pink flowers.

How did it get here? Desired for its exotic beauty, the plant was brought to Kew Gardens from the Himalaya in 1839. Victorian botanists have a lot to answer for.

What's the problem? Despite its aesthetic qualities, Himalayan balsam is proving to be a bit of a nuisance, forming dense thickets that shade out other plants and create problematic monoculture habitats. Each plant is able to produce around 800 seeds, which they can shoot and spread up to seven metres away.

How is it being controlled? Removing the plant by hand on a regular basis and cutting it back before it seeds seems to be the best way of keeping it at bay.


New Zealand Pygmyweed, Crassula helmsii

Appearance: Yellow and green succulent leaves.

How did it get here? This weed from Australia and New Zealand was recommended as a good oxygenator in fish ponds back in the 1950s.

What's the problem? Much like Japanese Knotweed it grows fast and can germinate from tiny fragments in a variety of habitats. It forms a dense mat over the water's surface, blocking out light, preventing photosynthesis and killing life beneath it.

How is it being controlled? This weed is almost impossible to eradicate. Advice for those unfortunate enough to have it in their pond is to simply fill in the infected pond and dig a new one.


Water Fern, Azolla filliculoides

Appearance: Floating rosettes about 1cm in diameter.

How did it get here? Water Fern was first recorded in Middlesex in 1886. It spread into the wild after being cultivated in botanical gardens.

What's the problem? Don't be fooled by the delicate name. This waterway clogger is pure mischief. It can achieve up to 100% water cover and a thickness of 30cm, blocking out light and suffocating all beneath it.

How is it being controlled? It's difficult to control because spores can be carried in waterway workers' clothing and germinate elsewhere, however a frond-feeding weevil is helping keep levels under control.


Mink, Mustela vison

Appearance: Often mistaken for our native otter, minks have dark brown fur often with a white patch on the chin.

How did it get here? American mink first arrived in Britain in 1929 for commercial fur farms. They were first reported to be breeding wild in 1956 as a result of escapees and deliberate releases.

What's the problem? Mink numbers have risen rapidly over the last 30-40 years and it's virtually impossible to gauge how many there are in the UK today. The largest casualty has been the water vole, whose numbers have declined by 95% because of loss of habitat due to mink invasion. They also prey on domestic ducks, poultry and fish, and take eggs and chicks from gull and tern colonies.

How is it being controlled? Targeted trapping is proving effective in many areas. They're now almost eradicated in the Outer Hebrides.

Last date edited: 23 October 2014

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