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Conservation adviser - tree and woodland health
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.
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.
Credit: GB Non Native Species Secretariat
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:
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.
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.
Credit: John Bridges / WTML
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:
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.
Credit: Trevor Renals
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.
Credit: Countryside Council for Wales
We can all help reduce the spread of invasive non-native species.