A guide to the UK's alien invaders

A guide to the UK's alien invaders

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Alien invaders are at large in the UK and have been for hundreds of years. However, we're not talking about little green men. The aliens in question are a diverse group of animals, some of which will be very familiar to you. 

The term alien is often applied to species that do not naturally occur in a habitat, but have established successful populations after being introduced as a result of human activity. A wide variety of these non-native creatures now call the UK home and even some animals that seem a quintessential part of the British environment are only here thanks to human intervention. 

Obvious invaders

Some alien species are more obvious than others. If you live in London and the south-east, there's a good chance you've seen noisy flocks of Ring-Tailed Parakeets flying across the skies. This exotic parrot species naturally occurs in Africa and south-east Asia, but escaped pets have flourished in the UK and the RSPB estimates there are now close to 9,000 breeding pairs in the country. 

Another obvious alien is the Canada Goose, which, as the name suggests, hails from across the Atlantic. First introduced several hundred years ago, most likely for hunting purposes, the birds are now a common sight on rivers, canals and waterways throughout the country. The RSPB puts breeding pairs at 62,000, with another 190,000 birds spending the winter months in the UK.

Canada Geese

Moving on to mammals, the Grey Squirrel is probably the most obvious invader. A native of the eastern USA, the species has spread rapidly since being introduced to the UK by 19th century landowners. Unfortunately, the rise of the Grey Squirrel led to a serious decline in Red Squirrel numbers, with conservationists still working hard to help the latter's population recover. Another well-known mammalian alien is the Mink, which has established a wild population after being released from fur farms around 50 years ago. 

Surprise aliens

There are quite a few species living in the UK that you'll surprised to learn are actually non-native. Take rabbits, for instance. A native of continental Europe, the species was actually introduced to Britain by the Romans who brought them across the channel as a food source. However, it wasn't until the Medieval period that firm wild populations became established. It is thought the Romans introduced Brown Hares to the UK too.

Indeed, the Romans had a surprisingly large impact on Britain's wildlife, as they were also responsible for the introduction of Fallow Deer. Of the UK's six deer species, only the Red and Roe deer are native - the Muntjac, Sika and Chinese Water Deer were all introduced over the past 200 years or so.

In the bird world, you may be surprised to know that Pheasants are also non-native. Exactly when the species found its way to the UK is a subject of debate, but it's thought the Romans may be responsible for this introduction too. Hunters have continued to release Pheasants into the British countryside, with the RSPB estimating their are well over two million females alone living in the UK.


Species such as the Rabbit, Fallow Deer and Pheasant have been living in the country for so long that they are not regarded as invasive species in the same sense as more recent introductions and are largely accepted as a natural part of the environment. Indeed, even animals as common and numerous as the House Mouse and Brown Rat are thought to have been brought, unintentionally, to the UK by people, with the former potentially dating as far back as 1,000 BC.

Exotic invaders

The UK is also home to some small populations of very exotic creatures. Red-Necked Wallabies can be found living wild in pockets of England, Scotland and the Isle of Man, having escaped from zoos and wildlife parks. It's generally accepted that a small number of Striped Skunks are now living wild in Gloucestershire's Forest of Dean, while numerous sightings of Coatis - a South American relative of the Racoon - have been reported in the Lake District.


Exotic creatures such as this are only present in very small numbers and whether their populations are sustainable remains to be seen. However, it is interesting to know just what a wide variety of alien species have been able to make the UK their home and the impact humans have had on the environment.

(Images from iStock)