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As an island nation, the United Kingdom’s geographical isolation makes it challenging for animals to naturally migrate here. Typically, non-native species are brought here via other means, and today there are a plethora of alien species living among us.
In this article, we delve into the diverse array of invasive non-native animal species that have made the UK their home. We will explore their origins, assess their ecological impacts, and examine the ongoing initiatives aimed at managing and mitigating their effects on the local ecosystem.
How Were Non-Native Animals Introduced to the UK?
When non-native animals come to the UK, it’s usually because of human activity. One such example of this are animals that are brought over for the pet trade. It only takes a few individuals from a species to escape their confines and, if the conditions are right, they can thrive in the wild.
In other cases, animals are brought over through shipping and global transportation by accident. It’s also not unheard of for animals to hitch a ride on commercial vessels taking humans around the world.
At times throughout history, humans have brought new species to the UK to help with agriculture and other reasons. For example, the gray squirrel was introduced to the United Kingdom to be released within private country estates. However, their populations grew dramatically and have now taken over the native red squirrel.
Are Invasive Non-Native Animals Bad?
There are lots of non-native species in the UK that cause little to no problems and have blended into our ecosystems seamlessly. But this is often not the case and non-native species have wreaked havoc in many places.
One of the biggest threats of these alien species is the damage they can do to biodiversity. Many species will take over an area because they’re a top predator or because they outcompete native species for resources. When links are removed from the food chain, this has a knock-on effect all the way down and can even affect the biodiversity of plant life as well.
What’s more, a lot of non-native species are considered to be pests as they attack agricultural crops and can even damage infrastructure, resulting in billions of pounds worth of damage every year.
There is also concern over the potential diseases that non-native species may bring to the United Kingdom. Not only can this have a negative impact on the wildlife but, in some cases, these diseases may also be passed to humans.
Invasive Non-Native Animals in the United Kingdom
It is estimated that there are around 3000 non-native species of plants and animals in the UK at the time of writing. Let’s take a look at some of the most notorious alien animal species here and the problems that they may (or may not) cause.
1. Gray Squirrel (Sciurus carolinensis)
The gray squirrel was introduced to the UK in the 1800s to be released into the gardens of private estates. While the red squirrel had been native to the UK for more than 10,000 years, its numbers have now dropped to just 247,000, while there are now an estimated 2 million gray squirrels that have taken over the habitat.
Gray squirrels are native to North America. They are stouter and have a less bushy tail than their red cousins. It took them just 25 years to spread more than 300 miles (483 km) in Scotland alone.
Not only did they outcompete the red squirrel because of how quickly they breed, but gray squirrels are also known to carry diseases that kill red squirrels. It’s proven very difficult to eradicate them because populations grow so quickly. While there are several scientific methods of removal being proposed, none are actually in effect as yet.
2. Chinese Mitten Crab (Eriocheir sinensis)
Chinese mitten crabs have grabbed a spot on the list of the 100 most invasive species in the world. Not only do they cause serious damage to fishing gear, which can have an economic impact, but they also burrow into sediment, causing flooding and have drastically impacted local wildlife, including white-clawed crayfish.
Originating from eastern parts of Asia, the Chinese mitten crab has long legs with bristles on the claws. They’re a problem in many places in Europe, and in the UK, they’re often found in the Thames River, where it was first recorded in 1935.
There have been studies into whether the crabs can be fished for meat to reduce numbers, but the results showed that the meat was not up to European standards. However, the public is encouraged to report sightings of the species so that authorities can monitor their spread.
3. Muntjac Deer (Muntiacus reevesi)
The muntjac deer is a small deer with a stocky build and brown fur which turns gray in winter. This species is one of the newest invasive species in the UK, having only been deliberately introduced in the 20th century. While initially introduced in Berkshire, they are now widespread in the southeast and continue to flourish.
The problem with these deer is that they are known to overgraze in forests and strip tree bark, which has a detrimental impact on local species and their habitat, including soil erosion. What’s more, they can carry diseases that affect cattle.
Hailing from China and parts of Taiwan, the muntjac deer has no natural predators, so it’s able to thrive in the UK. However, since the species is known to be problematic, several culls have taken place.
4. Chinese Water Deer (Hydropotes inermis)
The Chinese water deer has something of a teddy bear-like appearance, and the males boast large tusks which makes them easily identifiable. As their name suggests, they originally come from China and other parts of East Asia, including the Koreas.
This species was first introduced to the UK in the late 1800s and was initially kept in zoos. However, several individuals escaped from Woburn Abbey and spread in the wild. What’s more, in 1901, Chinese water deer were intentionally released into the wild and are mainly found in East Anglia and the surrounding counties.
While the muntjac deer has caused serious ecological impacts, the same cannot be said for the Chinese water deer. In fact, as things stand, they feed very selectively and are not outcompeting native species.
5. Ring-Necked Parakeet (Psittacula krameri)
The ring-necked parakeet has caused quite a stir in the UK as residents were not accustomed to seeing brightly colored tropical birds in our skies. However, this species was introduced to the UK from India in the 1970s when pet birds either escaped or were released. Although there were sightings as early as 1855.
These birds are unmistakable with their bright green plumage and dark rings around the neck. While they might be beautiful, they’re actually on the list of the top 100 invasive species and are problematic around Europe.
In the UK, the ring-necked parakeet is known to be aggressive towards native birds and easily outcompetes them for food. Since they’re cavity nesters, they also compete with native cavity nesters for spots to raise their young. However, the government has created frameworks to look at how the species is affecting local wildlife, and it has been suggested that culls should be made.
6. Monk Parakeet (Myiopsitta monachus)
The monk parakeet is another tropical bird species, this time from South America. They have green coloration with white faces and are typically smaller than the ring-necked parakeet. The first wild sightings of these birds came in the mid-1990s, but populations remain limited to London, Greater London, and Hertfordshire. Luckily, it’s thought that there may only be up to 60 individuals in the wild at this time.
Unlike the ring-necked parakeet, the monk parakeet is not known to compete for nesting spots or materials with native species. However, there is still much to be discovered, and assessments on their impact are still ongoing.
7. American Mink (Neovison vison)
Native to North America, the mink is a species of mustelid (weasel) that found its way to the United Kingdom in 1929 where they were farmed for their fur. However, after intentional releases into the wild as well as individuals escaping, reports started to be made in 1956.
As things stand at the moment, there are no official programs for managing the growing mink populations in the UK. What’s more, we currently have no idea of how many mink inhabit UK waterways.
Mink are voracious carnivores, and one of the main problems is that they outcompete native species. What’s more, since they feed on seabirds and water voles, they are having a direct impact on these species’ populations. That’s sad news for the water vole that is already listed as being endangered.
8. Egyptian Goose (Alopochen aegyptiaca)
At the moment, colonies of Egyptian geese are established in the southern and eastern parts of England. While their numbers are thriving, they don’t currently pose a threat. Although it did take some time for them to acclimatize to the cold weather. However, in their native range, they are known as crop pests, and this could become a problem in the UK if their distribution spreads.
The Egyptian goose is markedly different to other species, with an apricot breast and distinct brown markings on the face. It is thought that there are around 1850 breeding pairs in the country at the time of writing. Monitoring their numbers will help us to understand if they may become a threat.
But this species was never intended to be in the wild. It was brought to the UK in the 17th century as an ornamental species in private estates. Of course, several individuals got out into the wild and that’s where the spread began.
9. Mandarin Duck (Aix galericulata)
The Mandarin duck is a very distinct aquatic bird, with the males having orange plumage around the face and sail-like feathers along the back. Although the females don’t boast such an ornate design. This species was introduced to the UK from China and initially kept in captivity. However, as is the case with many invasive species, all it took was for a number of birds to escape.
Interestingly, this species, unlike other water birds, builds its nests in trees, and it can be spotted across the south of England. Although its distribution has spread further north and even into parts of Wales.
The good news is that this species has caused no ecological problems in the UK. This is because it does not compete for nesting sites with native species so they can live happily alongside one another.
10. Raccoon (Procyon lotor)
The raccoon is a species of mammal native to North America. It’s often depicted as being a ‘robber’ because of the mask on its face and is seen as a pest in its native region. They will raid trash cans and leave a mess everywhere, which is a pain for homeowners. However, there’s no denying that these intelligent creatures are pretty cute.
Raccoons first came to the UK in the middle of the 20th century to be used in the fur trade and in zoos. While a small number of racoons have escaped, numbers are not yet high enough for them to be considered a risk. However, if they were to distribute across the country, there would be many problems as a result.
The issues with raccoons are that they could carry diseases and being omnivores, may prey on the eggs and young of native species. What’s more, their curious nature means that there is potential for them to cause damage to both human and natural structures.
11. American Signal Crayfish (Procambarus clarkii)
While there are six crayfish species that have been introduced to the UK, the American signal crayfish is the most abundant. The species was introduced to Scandinavia in the 60s as a food source, but it soon spread to the UK.
The American signal crayfish has posed a threat to our native white-clawed crayfish by outcompeting it in the wild and spreading a disease called crayfish plague that kills white claws. However, it was long advised for the public to catch these crayfish and eat them but unfortunately, this control method seems not to be working. Even worse, reports suggest that it could only be making things worse. What’s more, they breed like rabbits!
Not only are they a threat to local species, but the signal crayfish has also caused massive problems with erosion due to its burrowing habits.
12. Asian Hornet (Vespa velutina)
When most people hear the term ‘Asian hornet’, they panic because the species is notorious for its painful sting. While the species was only first recorded a few years back in 2016, Asian hornets are already causing problems for native bees because they eat honey bees. It’s thought that the species turned up in France through shipping and eventually made its way to the UK.
The public is encouraged to report sightings of Asian hornets (they grow to around 1.8 inches (4.6 cm) and have a black abdomen with a yellow band on the fourth section.) However, so far there have only been 28 confirmed sightings, so numbers aren’t too much of a threat at the current time.
Originally from parts of East Asia and India, the Asian hornet may not be successfully established in the UK due to the cooler climate. However, there are concerns that it may thrive due to climate change, as has been the case in parts of Europe.
13. Ruddy Duck (Oxyura jamaicensis)
The ruddy duck is native to both North and South America and wasn’t introduced to the UK until the 1960s. It was initially introduced to Gloucestershire as a collectors bird but after some individuals escaped, it only took them until 2001 to establish their numbers all over the UK and even into Europe. However, at this point, there were still only around 6000 ruddy ducks in the UK.
With a black cap on a white face and a blue bill, this species is easy to identify, and the UK government spent time researching how best to eradicate them. While various trials were had, it appeared that shooting the ducks would be the best method. Culls have been made and the public is encouraged to report sightings so further action can be taken.
So, what are the risks of having ruddy ducks in our ecosystem? Well, one of the main concerns is that the species will breed with white headed ducks causing hybridizations. What’s more, the cost of killing a single bird can be as much as £3000, so their presence has a significant economical impact.
14. Midwife Toad (Alytes obstetricans)
The midwife toad is native to northern Europe, and individuals made their way to the UK in shipments from France. Although interestingly, there is fossil evidence that this species may have once been native to the UK. Currently, there are only wild populations in Bedford where the original shipment of plants from France was sent.
That said, it is thought that further colonies may have been established in Yorkshire, Devon, and Worksop as a result of the species being kept as a pet and either being released or escaping.
Midwife toads take their name from their breeding behavior in which the males tend to the eggs. They’re smaller than native toads and have a call that sounds like a smoke alarm! The good news is that the midwife toad doesn’t appear to pose any significant threat to the UK wildlife.
15. Siberian Chipmunk (Tamias sibiricus)
The Siberian chipmunk is a colorful species of small squirrel that measures up to 9.8 inches (25 cm). It’s native to parts of Russia, Korea, Japan, and China but has been introduced to several areas in Europe, most notably, Paris.
In the UK, the first wild sighting was in 1999, and it’s thought that this was an escaped pet. While several other sightings have occurred, individuals have either been killed or recaptured, so there isn’t currently a widespread wild population.
That said, problems in France are significant, with around 100,000 Siberian chipmunks thought to be in the wild. Since they carry diseases such as Lyme disease, having them in the UK is a big no-no. But the fear is that they’ll soon start hitching a ride on shipments from Calais, so the French wildlife experts are encouraging the country to ban the sale of this species as pets to prevent further accidental releases into the wild.
16. Carpet Sea Squirt (Didemnum vexillum)
The carpet sea squirt might not seem like much of a threat when you consider that this tube-like invertebrate can’t even move. But numbers of the species were first found along the Kent coast in 2011, thought to have come from ship ballast water.
However, when you also consider that the species can grow rapidly and cover vast areas, it’s easy to understand how this can pose a threat to local species and impact biodiversity. What’s more, since the carpet sea squirt likes to attach to solid surfaces, it can be problematic to the marine industry.
The species is native to Japan, but since first sighted in the UK, numbers have now spread around coastlines on the south of England and even as far north as Scotland. While exact numbers are unknown, the public is encouraged to report sightings and locations.
17. American Bullfrog (Lithobates catesbeianus)
The American bullfrog is something of an iconic species in its native North America. However, after being introduced to the UK in the mid-90s, it has become a problem. The first wild sighting was in 1996 and it’s believed that individuals are being released by pet owners.
The issue with the American bullfrog is that it preys on native species which has a direct impact on biodiversity. It is also known to carry diseases such as chytrid fungus, which is lethal to our native amphibian species and also breeds very rapidly.
Because of the potential threat, the import of American bullfrogs to the UK has now been banned and the species has been added to the Union List Of Invasive Alien Species, which will ensure that the right action is taken to remove it.
18. Topmouth Gudgeon (Pseudorasbora parva)
The topmouth gudgeon is a species of Asian fish with silvery scales, an upturned lower jaw and rounded fins. Originally hailing from South East Asia, the topmouth gudgeon is thought to have established itself all over England and Wales as a result of being released by pet owners.
As a result of this, people are being encouraged not to release unwanted pets into the wild, and the EU has now banned the trade of the topmouth gudgeon, although this isn’t in effect in the UK.
This species poses several risks to our native aquatic life, including competing for resources (food and habitat), preying on native species, the spread of disease, and may even become a nuisance to anglers, meaning it has a human impact as well.
19. Harlequin Ladybird (Harmonia axyridis)
The first sighting of the harlequin ladybird in the UK was in Essex in 2004. Since then, the species has been dubbed the fastest spreading non-native creature, as it managed to colonize the entire UK within just ten short years!
It can be difficult to identify the harlequin as it is very similar in appearance to our native ladybirds but poses many more risks. In fact, they’re known to enter UK homes in search of shelter, but they produce a yellow liquid which can stain clothes and furniture. That might seem like a minor inconvenience, but there is concern that the harlequin will threaten native species, although at this time, no real issues have been noted.
Additionally, since the harlequin ladybird eats aphids, many people welcome it to their garden as a pest controller. Just make sure it doesn’t get inside the house!
20. Sacred Ibis (Threskiornis aethiopicus)
The first record of the sacred ibis in the UK was in 1995, although the species is native to parts of Africa and the Sahara. While some vagrants were noted in France, it is thought that the sacred ibis got into the wild here after being intentionally released.
The sacred ibis is a species of wading bird with a rotund white body, black head and tail, and a long slender bill. In the UK, the species currently hasn’t caused many problems, but there is a serious potential of egg predation of native species, which has become apparent in France.
If you spot a sacred ibis, you’re encouraged to report this with photographic evidence. What’s more, the UK’s Non-Native Secretariat does have plans to prevent the sacred ibis from properly establishing in this country.
21. Killer Shrimp (Dikerogammarus villosus)
Sometimes called the demon shrimp, the killer shrimp sounds like the stuff of nightmares, and that’s right since the UK Environment Agency has dubbed this as the worst invasive species in the country!
Native to the Caspian and Black Seas, the killer shrimp was first spotted in Cambridge in 2010. It’s thought that it got here through the transportation of ballast water, but it’s also possible that it got mixed in with fish shipments.
The main threat caused by the killer shrimp is unsurprising; it preys on many native species, which can drastically reduce their numbers. Although for some species, this is beneficial as the killer shrimp caused their predator populations to die off. Not only this, but it’s thought that their impact on biodiversity could affect water quality and even have a knock-on effects for fisheries.
22. Quagga Mussel (Dreissena rostriformis bugensis)
The quagga mussel is found in the Caspian Sea and has already caused major problems in North America after being introduced there. These pesky mussels are known to clog water pipes and interfere with local biodiversity.
The species has been found in Rutland and the River Trent, and anglers in the area are being enlisted to help prevent them from spreading further by ensuring that they properly clean and check their equipment after fishing. This seems to be the best method of controlling the spread and is therefore being encouraged by all water users.
Quagga mussels are prolific breeders, and once they have established, there’s no way to get rid of them, we can only try to stop them spreading further. What’s worrying is that they’ll often sit on native mussels effectively smothering them.
How Does the UK Government Manage Non-Native Animals?
The UK Government appears to be making significant efforts to control or even cull populations of non-native invasive species that are entering the country. But before I talk about what they’re doing, it’s important to point out that the public can also play a role.
We are being encouraged to report sightings of invasive species and provide photos and locations where possible. If everyone does this then we stand a greater chance of controlling their numbers and preventing them from becoming a threat.
In any case, the UK government has developed a couple of policies that are designed to manage invasive species.
There is the Invasive Alien Species (Enforcement and Permitting) order 2019 which is designed to prevent and manage the introduction of non-native species. This includes licensing for the possession of certain species and making it a criminal offense to release or permit the escape of a non-native species into the wild.
Furthermore, we have the Non-Native Species Secretariat (NNSS), which is a body that coordinates the approach to managing non-native species in the United Kingdom.