Around 2000 non-native plant and animal species have been introduced into the UK. They have caused irreparable damage and changed landscapes forever. In recognition of this, Invasive Species Week brings organisations together to raise awareness of invasive non-native species. Led by the GB Non Native Species Secretariat, the annual event began in 2015.

What is an invasive non-native species?

Put simply, a non-native species is one that has been transported from its native range to a new region with help from humans.

Most non-native species are harmless. An invasive non-native species (INNS) is one that has a negative impact on the environment, economy or our health and way of life.  Around 10-15% have become invasive and have a negative impact in some way.

Why should we care?

The overall impacts of INNS are startling. They cost the UK economy at least £1.8 billion a year. They have also led to dramatic declines of some native species, including:

  • water vole
  • white-clawed crayfish
  • red squirrel.

30% of the UK’s Important Plant Areas have been found to contain invasive non-native species. Globally, invasive non-native species are one of the main drivers of biodiversity loss. They have contributed to 40% of animal extinctions that have occurred in the last 400 years. On islands in particular they are the primary driver of biodiversity loss.

The impact on our woods

Our woods have been affected by invasive species in a number of ways. For example, the introduction of non-native deer species for hunting has become a serious issue. Lack of natural predators means deer numbers can grow unchecked. The increased population feeds on tree saplings and hampers the process of natural regeneration. These deer are preventing the next generation of trees developing.

Grey squirrels, introduced from America in the 1870s, strip the bark of trees, especially fresh growth. They have outcompeted native red squirrels. They also carry the squirrel pox disease which our red squirrels are particularly susceptible to. As a result, red squirrel populations have plummeted to the point where they can now only be seen in small areas of Great Britain.

The threat from Japanese knotweed

It is not always apparent from the outset that a species will go on to become invasive. Many of the more harmful INNS plants started out as ornamental garden plants. But without natural predators from their home range, they grow uncontrollably.

Japanese knotweed (Fallopia japonica) was introduced as an ornamental garden plant by the Victorians. It’s now common in urban areas, particularly on wasteland, railways, roadsides and riverbanks.  But it affects our woods too. For example, it:

  • outcompetes native species
  • contributes to river bank erosion and increases the likelihood of flooding
  • can cause significant delays in development, as it grows through asphalt and other surfaces, leading to structural damage and increased costs.

Even the smallest fragment of root can grow a new plant, so it also spreads easily, making it challenging and expensive to manage. In fact, Japanese knotweed is so problematic that it’s a criminal offence to plant it or cause it to grow in the wild.

Devil in disguise: rhododendron

Invasive plant species can also harbour or spread diseases onto native plants. For example, rhododendron (Rhododendron ponticum) is an ornamental shrub introduced by the Victorians for its spring flowers.

It may look pretty, but its effect on the landscape is significant, particularly in the wetter western parts of the UK where it thrives. It is poisonous to livestock and casts such a dense shade that nothing will grow near it.

It can also carry the tree disease Phytophthora ramorum, which has killed thousands of larch, beech and oak trees in the UK. The rhododendron doesn’t die immediately - instead it acts like a reservoir for the disease, allowing the Phytophthora to release spores onto nearby trees.

What can we do?

We can all help reduce the spread of invasive non-native species.

  • Like to paddle, fish or sail? Follow the Check, Clean, Dry process for equipment and clothing after leaving the water. This will help avoid spreading invasive aquatic species.
  • Dispose of plants responsibly, including any from your pond or aquarium. Don’t dump them in the wild – remember to Be Plant Wise
  • Take care of your pets. Never release them - or allow them to escape - into the wild.
  • Check the latest species alerts from the Non-Native Species Secretariat (NNSS) website and be aware of what to look out for. For example, this spring and summer, keep an eye out for Asian hornets. Report sightings via an online form or by emailing a picture to
  • Enjoy being outside and want to try something new? You could volunteer with a Local Action Group working on the management of invasive non-native species.
  • Or volunteer with us – some of our roles involve monitoring and managing species in our woods. Check our latest volunteering opportunities.

Learn more about invasive species