Invasive species

It’s the end of the 1800s and red squirrels in the United Kingdom are thriving. They have plenty of food and forests to roam around in. But things are about to change. An army of grey ‘super’ squirrels is invading, heading to the UK all the way from North America! These squirrels are bigger and can adapt fast to new environments.

And they carry a secret weapon — a disease usually fatal to red squirrels. Fast forward to the present day. The result? Nearly ten times as many grey squirrels as red ones in the UK. Because grey squirrels haven’t always lived in the UK, but were brought from a different country, they are an invasive species.

Invasive species cause harm to other animals or plants, and the new environment they find themselves in. But how do animals or plants travel long distances to invade new places? Most are transported by humans. Humans have been dispersing plants and animals for millenia. But the real problems started with Europeans establishing colonies around the world.

So plants and animals often ended up in new areas along with Europeans, either deliberately or by accident. This is how cats, rabbits and rats invaded New Zealand, for example. While some of these new plants and animals died out quickly, leaving no impact, others persisted and became destructive to ecosystems and species already there. Spreading species around the world is not a thing of the past though. It still happens today!

This ship just unloaded some of its cargo. It is much lighter now, so it needs to take on water from its surroundings, before it moves to another port. At the new port it picks up new cargo, so now it needs to pump out the ballast water. But this dumped ballast usually contains more than just water — it can contain algae, mussels or even fish. If these living things are new to the area and manage to survive in their new environment, they become invasive species.

One of the biggest problems with invasive species is that they are hard to control and get rid of. This is especially true for invasive plants. For example, Japanese knotweed — a plant originally brought from East Asia to Europe and North America. It forms thick, dense bushes which prevent any other plant growing nearby. It is resistant to nearly everything — it can survive different soil types, low nutrient levels, even extreme cold.

Its roots can spread almost ten metres deep, so it can’t be easily uprooted. Cutting it above ground won’t help much. It grows back very fast. Getting rid of it is expensive and practically impossible, which causes many problems. Despite their bad reputation, some invasive species can, however, benefit some areas they invade.

This is a European green crab, which has invaded coastal areas all over the world. In most places the green crab causes destruction because it eats anything it comes across! But when this crab invaded New England, the environment there was already in trouble because native marsh crabs were causing soil erosion. When the European green crab came and started eating the marsh crabs, the soil and the ecosystem could recover! But this is an exception.

The vast majority of invasive species harm their new environment and other living things there. To limit destruction, we need to find better ways to stop the spread of species, and to manage their impact when they do invade.