What is an invasive species?

by guest blogger Owen Leigh

If you live in the UK, you’ve probably heard someone call grey squirrels an invasive species. Who hasn’t heard the story of how grey squirrels hopped over from North America and sent Britain’s native red squirrel scampering into rarity? But not all non-native species are invasive, in fact, some are very important for British farming. So, what exactly is an invasive species?

Grey Squirrels came to Britain in the 1870s but are also found as an invasive species across Europe © Owen Leigh.

Now limited to just a few areas around the British Isles, the red squirrel is a reclusive critter that many people have never seen. The grey squirrel, on the other hand, seems to be everywhere. Though they may look cute and fluffy, grey squirrels are a bit of a problem. They were first brought to Britain in the 1870s, during the Victorian fascination with keeping exotic animals on estates, but it wasn’t long before they escaped over and invaded the British countryside.

An invasive species is one which is not native to the habitat it has been introduced to, and which causes ecological or environmental harm to that new habitat. That habitat doesn’t have to be a whole new country, it can also be a new area of the same place but with different animals and plants. When the species enters the new habitat they can bring new diseases that those already living there, the native species, aren’t used to. They might also become a new predator for the native species or suddenly find themselves without their own predator, making it easier for them to live in the new habitat. The new species’ also cause competition for food and space that didn’t exist before. As a result, some native species find themselves unable to adapt quickly enough and are overtaken by the new species.

Pine martens were once common in Britain, but the fur industry and loss of habitat caused them to decline in the 1800s. (Image source: Pixabay)

This is what happened with squirrels in the UK. When they escaped from the Victorian estates they found that their usual predators, pine martens, were rare in Britain. Without a predator to slow them down, grey squirrels quickly spread through British woodlands. Where they went, populations of native red squirrels dropped as they became sick with new diseases they weren’t used to and the trees they lived in got damaged by grey squirrels who like to strip bark of tree trunks. With fewer places to live, less food and a reduced population, red squirrels retreated to small areas where greys hadn’t yet arrived. They are now only found in a few woodlands, mainly in Scotland and Ireland. Grey squirrels have caused so much damage to woodlands and to red squirrel populations that they are definitely an invasive species.

But it isn’t just animals that can be an invasive species. Some plants, like Himalayan balsam, can cause just as much damage. Originally brought to Britain to decorate the gardens of the rich, some seeds found their way into the countryside and started to grow along rivers and lake shores where it causes the soil to deteriorate.  It also grows very tall and thick, blocking light from reaching smaller or slower growing species. Without light, those species die, leaving even more space for Himalayan balsam to take over.

Bees and other pollinators are attracted to the bright pink flowers of Himalayan balsam, helping it to spread. (Image source: Pixabay)

Himalayan balsam also has another trick up its sleeve – it can ‘shoot’ its seeds from exploding pods. The seeds are flung far away from the mother plant and stick to the fur of animals and insects, people’s clothes, or even car tyres. This allows it to spread over large distances very quickly. It has become so difficult to stop it spreading in the UK, that those who come across it while walking in the countryside are encouraged to pull it up!

Not every non-native species is considered invasive though. Some non-native species arrive in a new habitat, only to find that while they have left their old predators behind, there are new predators to deal with and enough food or space for them to co-exist with native species. These are sometimes called exotic species, which just means they did not evolve in the habitat they are now found in but came to be there through migration or introduction by humans at some point in history. Many of these do no real harm to their new home at all. Some can even be beneficial.

You might recognise some exotic species like wheat and sheep. Neither of these is originally from Britain but they make really good species for farming. They help to feed and clothe us, and they don’t upset our local species because their populations are controlled. In fact, sheep that are allowed to wander and graze on grasslands can help encourage native plant growth. They help to keep some species of plant in check so that others have the space to grow and their poo acts like a fertilizer.

Even though not all non-native species turn out to be invasive, it is still risky to introduce any species to a new area. It isn’t always possible to guess what the effects might be on the environment or the species that already live there and it isn’t easy to solve the problem once it starts. Sometimes, in order to try and deal with an invasive animal, like grey squirrels, another species that would be a natural predator is brought in to try and control the population.

While it might sound like this should work, sometimes other native species are easier to catch, and the predator becomes an invasive species too. So, you might think that because pine martens, the natural predator of grey squirrels, were once common in Britain, it would be a good idea to bring them back to help control the squirrels. But pine martens have been rare for a long time and increasing their populations is a difficult task that might upset ecosystems that have become used to their disappearance if not done carefully. Both pine martens and red squirrel populations are starting to recover though, thanks to conservation work across the UK. Little by little, they are making a come-back and one day might reclaim their home from the invasive grey squirrel.


 About the author

Owen Leigh is a multimedia science communicator. You can find more about him and his work here.

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Posted in Biology, Exploring Science