What is an invasive species? And why are they a problem?

An invasive species is a plant or animal that has been introduced to a habitat from somewhere else. This by itself isn’t a problem. It happens all the time! Usually, if a species is introduced, it can’t survive in a new habitat that it isn’t adapted to. However, if a species does manage to survive and even reproduce, then things get trickier. In its natural habitat, a species will have predators, parasites, and diseases that keep them in check. However, when a species is introduced to a new environment – it doesn’t have these natural population controls.

Muntjac Deer are small, fanged, deer native to Southeast Asia. In their native range, Muntjacs are predated by leopards, tigers and pythons. These predators are notably absent from the UK. Without their native predators, Muntjac populations have increased since their introduction in 1894. In fact, since 1995, Muntjac populations have increased by 360%! But why is this a problem? Habitats are in a delicate balance. If you add something to an ecosystem where it doesn’t belong, there are often knock-on effects. Muntjac significantly change woodland ecosystems. By feeding on tree saplings they prevent woodland re-generation and change species compositions.

How do invasive species get to new places? 

Sometimes it’s accidental. The New Zealand Flatworm was accidentally introduced from imported plants. This flat worm is a predator of earthworms munching on many of our native earthworm species. The New Zealand Flatworm is also a problem for our native flat worm Microplana scharffi, competing with them for the same prey species. Over time, people have also intentionally brought in invasive species. During the Victorian era, people were mad for parks, gardens, and interesting plants. It was fashionable at this time to have ‘exotic’ plants from all over the British empire in your garden. Having a Balsam from the Himalayas or Knotweed from Japan was the perfect way to show off to your neighbours. Himalayan Balsam has delicate pink/white flowers and a strong perfume. Personally, I don’t think Japanese Knotweed looks that great but I’m not an 18th century landscaper, so what do I know. Himalayan Balsam and Japanese Knotweed are now some of our most pernicious invaders in the UK.

The harm caused by invasive species depends on the species in question. American Mink were introduced to the UK in the 1960s and since then have been causing all kinds of trouble. Mink feed on a variety of different prey including ground-nesting seabirds and water voles. Our native seabird and water vole populations are both in decline. Invasive non-native predators are one of the greatest threats to our seabirds, water voles and to many other species. Japanese knotweed causes an estimated £1.7 billion in property damage each year! By growing up through walls, floors, and pavement, Japanese Knotweed causes serious damage. Japanese knotweed is also extremely difficult to remove. Entire plants can regrow from the tiniest fragment left in the soil. This means that if Japanese knotweed is present, you need an expert to help you remove it. Signal crayfish are now common in our rivers and streams after being introduced from North America in the 1970s. Invasive crayfish destabilise with their burrows and feed on native species. Our native crayfish species, the white clawed crayfish is also in trouble. Signal crayfish are bigger and more aggressive and carry a disease which is deadly to native crayfish.

Introductions of invasive species are continuing to happen. As temperatures increase due to climate change, many species will increase the areas they are able to colonise. One species predicted to become more prevalent in as an invasive in the UK is the North American Racoon. There are currently only 27 records across the UK of raccoons (Including 1 in Wales!) and most of these are thought to have escaped from private collections. Raccoons are now locally abundant and reproducing in 8 European countries! In their native ranges racoons have a very varied diet. Racoons eat fish, shellfish, fruit, seeds, frogs, songbirds, small mammals, snakes, and frogs! In the UK, where our wildlife has not adapted to face these new predators. Many species will be more vulnerable, many of which, are already threatened. Native predators such as badgers, foxes, and even birds of prey, are also at risk, losing precious resources to racoons.