Deadly Invaders: the Non-Native Species Threatening British Wildlife

Britain's island status may have once been an asset against invasion – but these unwelcome guests made it to our shores and are quietly wreaking ecological havoc.

By Jonathan Manning
Published 29 Jul 2019, 08:00 BST
The Signal Crayfish is native to the Americas and Canada – but is now found is ...
The Signal Crayfish is native to the Americas and Canada – but is now found is British waterways, too.
Photograph by Adrian Weston, Alamy

A perfect storm of international trade, human carelessness and climate change are bringing new species to the UK that can out-compete native flora and fauna. These factors endanger biodiversity and risk changing forever Britain’s countryside, forests, skies and waterways. 

When non-native species invade, they frequently arrive without the controls that kept them in check in their original habitats. Free from predators and disease they can grow and spread rapidly, overcoming native wildlife and plants. Throw into the equation the effects of climate change, which is allowing creatures as exotic as terrapins to survive Scottish winters in the wild, and it is little wonder that the IPBES Global Assessment Report identified invasive alien species as one of the top five global culprits for damaging biodiversity. 

The Scale of the Invasion

According to the Joint Nature Conservation Committee (JNCC), there are 3,163 non-native species in Great Britain, of which 1,980 are established (i.e. they reproduce in the wild). The JNCC has identified 190 non-native species that it considers to be exerting a negative impact on native biodiversity. These include 46 freshwater species, 36 marine species and 108 land species.

Clearly, with a number of species able to ‘invade’ under their own steam, whether carried on the breeze, ocean or their own energy, the question of what is and isn’t native is a moot point.

“Free from predators and disease, non-native invaders can grow and spread rapidly, overcoming native wildlife and plants.”

The JNCC identifies non-native species as those that have reached this country by “accidental human transport, deliberate human introduction, or which arrived by natural dispersal from a non-native population in Europe.” And it backdates this arrival to the year 1500.

For the UK’s largest woodland conservation charity, the Woodland Trust, year zero is the end of the Ice Age, 12,000 years ago. Plants that bloomed as the ice receded are considered native, says Matt ElliottWoodland Trust Conservation Advisor, tree and woodland health. 

The RSPB draws a distinction between birds that arrive naturally on British shores, such as the great white egret, which should not be considered as non-native, and escapees from zoos and collections that would never have arrived without human assistance, whether deliberate or accidental.

“The essential physical roughness of the planet – oceans, mountain ranges, deserts, currents in the sea – means that evolution proceeds independently in different regions of the world, so we end up with antelope as plains grazers in Africa and kangaroos as plains grazers in Australia; an the tiger as the lowland jungle cat in south-east Asia and the jaguar as a lowland jungle cat in South America,” says Dr Paul Walton, Head of Habitats and Species, RSPB Scotland.

“What is happening increasingly and at an accelerating rate is that human beings, through moving around and through international trade, are increasingly moving animals and plants in ways that they would never naturally have moved and at rates that they would never naturally have moved.” 

The result is the introduction of new predators and diseases to which native species have no immunity, and there’s only one consequence – a depletion of biodiversity. The GB Non-native Species Secretariat calculates that invasive non-native species not only damage the environment, but also cost the domestic economy £2 billion per year in terms of their impact and the action taken against them. The solution, says Dr Walton, lies in prevention rather than cure, in better bio-security rather than eradication programmes.

Six Non-Native Invaders

Hardy, omnivorous Ponto-Caspian species such as the killer shrimp can cause 'invasion meltdown' of an ecosystem when enough varieties appear in the same place.
Photograph by Arco Images GmbH, Alamy

Invader: Ponto-Caspian species

With a name like a Bond villain, this group of invaders launch hostile takeovers of every fresh watercourse they reach. It’s not a single species but a collection of 24 different crustaceans, molluscs and fish that together introduce their own all-conquering eco-system.

Originating in the Ponto-Caspian region of south-east Europe, between the Black, Azov and Caspian Seas, these species have swum and ‘hitchhiked’ westwards. They now represent a formidable threat to British freshwaters.

An assessment of Lake Zurich found that 90% of the living biomass is now Ponto-Caspian, says Dr Paul Walton, Head of Habitats and Species, RSPB Scotland.

“These species are invasive on their own but when they occur with the other 23, you get a situation called ‘invasion meltdown’,” he says.

The Ponto-Caspians display a great tolerance to both wide temperature and salinity ranges, reproduce rapidly, and as omnivores will opportunistically feast on whatever food is available. They drive out native species through predation and competition, and can even change the energy flow of ecosystems. 

While diminutive, the killer shrimp (Dikerogammarus villosus) lives up to its name.
Photograph by The Environment Agency

“This is a really big threat. Once they get a toe hold it’s extremely difficult to combat them.”

Dr Paul Walton, RSPB

Four of the Ponto-Caspian species have arrived in the UK, including the killer shrimp (Dikerogammarus villosus),the bloody-red shrimp (Hemimysis anomala), the Demon shrimp (Dikerogammarus haemobaphes), and the Caspian mud shrimp (Chelicorophium curvispinum).  

“But there are 20 more of them in the Dutch ports and there is a real risk that more of these species will arrive in the UK, where we have some of the most valuable fresh water systems in Europe. This is a really big threat. Once they get a toehold it’s extremely difficult to combat them,” says Walton.

According to one study by Cambridge Environmental Consulting, the time lag between the first recorded sighting of a Ponto-Caspian species in The Netherlands and its arrival in Great Britain has shortened to five years, compared to 30 years in the 20thcentury. 

The same report said, “Ponto-Caspian species can disperse at very high velocities (on average 87 km/year upstream and 80 km/year downstream) aided by human activities such as boating or fishing, through hull fouling, attachment to boat material, trailers or fishing gear.”

As a result, the GB Non Native Species Secretariat has launched a campaign called ‘Check, Clean, Dry,’ to halt the spread of Ponto-Caspian species.

Rhododendron, like purple stars amongst the greenery on the shore of Lochan Urr in Glen Etive, Scotland. Introduced as ornamentals by the Victorians, this shrub has run wild into native woodland.
Photograph by Derek Croucher, Alamy

Invader: Rhododendron

The kaleidoscopic colours of the rhododendron (Rhododendron ponticum), which range from light pink to dark purple, belie the more sinister impact of this invasive shrub. Introduced as a garden plant by the Victorians, the rhododendron has escaped and flourished, colonising alarming acreage of precious native woodland.

In Scotland’s oceanic rainforest on the west coast, for example, invasive rhododendron is present in 40% of sites, threatening trees, and rare lichens, moss and liverworts. Almost 30,000 hectares of rare Atlantic woodland now has rhododenron present, a major concern for conservationists given the plant’s prolific seed production and domineering growth patterns.

Matt ElliottWoodland Trust Conservation Advisor, tree and woodland health, says, “One of the problems that we only realised 20 years ago is that rhododendron is a host for a particular tree disease, Phytophthora ramorumThe diseaseinfects the rhododenron, but does not kill it, and the spores then spread to the trees around it.”

Its impact has been severe on larch, causing bark and foliage infections; symptoms include dark, oozing lesions and blackened needles. As a consequence, hundreds of thousands of trees have had to be felled, says Elliot. “On top of that, rhododendron shades everything else around it, so no ground flora can grow,” he adds. 

Clearance involves a labour intensive process of cutting down, drilling holes into the trunks, and injecting herbicide. And even when the first wave has been cleared there’s a rhododendron seedbank in the soil ready to germinate. “Essentially it causes havoc and it’s very expensive to clear. It takes a lot of effort over a long period of time,” says Elliot.

Over the past five years the Woodland Trust has spent £1.25 million clearing up rhododendron and other invasive species.

The brown rat is likely an invader to UK shores from Europe – but it's rat presence on isolated islands that have caused the decimation of native wildlife.
Photograph by David Chapman, Alamy

Invader: Rat

It’s rare that rats ever get good press: since the days of the Black Death plague these long-tailed rodents have been blamed for spreading disease and destruction.

The black rat may be a native species (and one that’s actually under threat from its own invader, the larger European brown rat), but both become highly problematic in the UK when they invade off-shore islands where they are not native.

This country is home to internationally important seabird colonies, many of which nest on islands.

“Seabirds tend to be ground or cliff nesting birds and are highly susceptible to ground-based mammal predators,” says Dr Paul Walton, Head of Habitats and Species, RSPB Scotland.

On the Shiant Islands, a rat eradication initiative was introduced to control the effect of the invasive rodent on seabird populations.
Photograph by Jim Richardson

The birds have established their nest sites on islands for centuries precisely because of the absence of ground-based predators, but shipwrecks and imports can lead to the accidental introduction of rats, which subsequently prey on seabird eggs and chicks.

“At least 13 colonies of the Manx shearwater, of which 80% of the world’s population is in the UK, have gone extinct over the last century largely because of the human-assisted introduction of rats,” says Walton.

rat eradication programme on the Shiant Islands in the Hebrides, during the winter of 2015-16, is already starting to bear fruit, with storm petrels returning to breed on the islands for the first time in living memory.

“We are trying to restore these islands to be suitable for breeding seabirds by removing non-native predators and maximising the birds’ breeding opportunities in a bid to build their resilience to climate change challenges,” says Walton.

Aggressive in proliferation of seeds and competitive in its determination to shade out other plants, Himalayan balsam is choking the life out of plants in the habitats it invades.
Photograph by Michael Grant Plants, Alamy

Invader: Himalayan balsam

With the ability to reach a height of 2.5m from seed in a single season, it’s easy to see how Himalayan balsam (Impatiens glandulifera) is running rampant through the British countryside.

This non-native invasive plant not only forms dense thickets, shading out and smothering other plants, but can also cast its seeds as far as seven metres as it colonises new territory.

“It out-competes native plants,” says Matt Elliott,Woodland Trust Conservation Advisor, tree and woodland health. “It’s a prolific seeder with explosive seed heads that spit their seeds a long way.”

The pink-purple flowered plant (colloquially known as Policeman’s Helmet due to the shape of its flowers) favours the banks of rivers, streams, ponds and lakes, effectively choking the life out of other plants along these waterways and not allowing other species to grow.

Clearance and spraying takes a concerted effort over a long period of time, with return efforts required to ensure seeds do not take root.

The Signal Crayfish is native to the Americas and Canada – but is now found is British waterways, too.
Photograph by Adrian Weston, Alamy

Invader: Signal crayfish

Introduced to British waters by commercial fisheries, the lobster-like American signal crayfish (Pacifastacus leniusculus) soon escaped its nets and cages and began a rampant march through Britain’s rivers and stillwaters, outcompeting the native white-clawed crayfish (Austropotimobius pallipes) for both habitat and food.

The invader is also a carrier of crayfish plague, which is lethal to white-clawed crayfish, says Peter Haynes, Biodiversity Officer at the Environment Agency in the East Midlands.

“Where mixed populations occur the white-clawed crayfish is very quickly eliminated,” he says. “It is estimated that in a mixed crayfish population where crayfish plague is not present in the signal population, it takes three to five years for the signal crayfish to eliminate the white clawed population through direct competition.”

One research paper, published by English Nature, estimated that signal crayfish populations can advance by 1km per year. The species is currently progressing northwards from the south of England, although some populations of white-clawed crayfish are protected in isolated refuges, known as ‘ark sites’ in reference to Noah, says Haynes.

A poster designed to prompt biosecurity awareness amongst anglers abroad of both contamination and invasives.
Photograph by Angling Trust

“The lobster-like American signal crayfish soon escaped its nets and cages and began a rampant march through Britain’s rivers and stillwaters”

He is leading a project to control signal crayfish from Nottinghamshire waters.“Traps are put out and the crayfish processed with the larger males being castrated and returned to the water body,” he says.

The theory is that the larger males will suppress juveniles either by cannibalising or out competing them, preventing a population explosion that occurs if all the larger individuals are removed from a population.Any other signal crayfish caught in traps are humanely dispatched. The voracious appetite and vigorous foraging behaviour of signal crayfish make them a threat to wider ecosystems, too.

“Once signals take over a river they can have a big impact on water quality though burrowing into banks and causing siltation, so if a once pebbly river has turned silty this may also be a sign that signals are there in large numbers. Signals can also impact the ecology of rivers through increased predation on invertebrates, this will therefore impact the ecosystem of the river reducing the variety and density of species fish and macrovinvertebrates within it,” says Haynes. He is pessimistic about the prospect of interrupting the relentless march of the signal crayfish, despite concerted efforts by a number of conservation organisations to protect native crayfish through secure ark sites.

“It is extremely important that as individuals we do not add the spread of the signal crayfish through moving them out of rivers and into other river catchments,” says Haynes. “It is also important to stress that when people are active in or around rivers and stillwaters that they apply biosecurity measures (checking, cleaning and drying) to their equipment. This is essential to stop the spread of crayfish, crayfish plague and other invasive species.”

Thought to have established in the UK after being released by collectors, the ruddy duck migrated and bred – and has since caused a hybridisation problem with the white-headed duck.
Photograph by Papilio, Alamy

Invader: Ruddy duck

Non-native invaders don’t always out-compete and kill native species. Sometimes it’s their over-friendly fraternisation that’s the problem. 

This was the case with the ruddy duck (Oxyura jamaicensis) in its relationship with the globally endangered white-headed duck (Oxyura leucocephala). 

Imported from North America as an ornamental wildfowl prized for its striking plumage, the ruddy duck escaped from collectors in the UK and started replicating its migratory behaviours by heading to Spain and North Africa. There it encountered the white-headed duck, a species that had recovered from the very brink of extinction in Spain, and that’s when hybridisation problems began. 

The two ducks have, “a natural tendency to hybridise and produce sexually viable individuals that can breed,” says Dr Paul Walton, Head of Habitats and Species, RSPB Scotland. “The white-headed duck is largely a monogamous bird and once mated tend not to mate again, whereas the ruddy duck is far more promiscuous.”

This means the introgression of genes from one species into the other is virtually a one-way street, because the mating success of the ruddy duck is much greater.

“So you end up with the population just becoming ruddy ducks,” says Walton.

This prompted an international conservation effort, which involved a control programme to eradicate the ruddy duck from the UK, reducing its population from 6,000 at the turn of this millennium, to barely 40 birds by 2014.

Invasive Species 101
Invasive species cost the global economy over a trillion dollars each year. Find out how these non-native organisms are introduced into an ecosystem, how they impact local communities, and which measures can be taken to help prevent the introduction of invasive species.

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