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What Actually are Invasive Species?

September 20th 2016

I was recently involved in an exchange on twitter where my understanding of the term “invasive species” was questioned, so I thought I would look into other points of view and share a little information.

In many places (particularly South Africa and Australia), “invasive species” is a term used exclusively to identify non-native species which cause damage to the environment, agriculture, infrastructure or health.

Recently in the UK, the code of practice for invasive species formerly known as “the Invasive Non-Native Species Code” (INNS Code) – has changed its name to the Invasives Code, showing that the terms “invasive species” and “invasive non-native species” are interchangeable to some degree in this country too.

Here’s a quick glossary that has served me well in the circles that I move in, (focussing on the treatment and eradication of both native and non-native plant species), which I find communicates most effectively.

A weed – is any plant growing where you don’t want it to.

An invasive species – is any species (plant, animal or fungus) which is invasive – that is: it spreads to a degree which causes damage to the environment, infrastructure or health.

A non-native species (or “introduced” or “alien” species) – is any species which has been introduced into an environment which is not its original range.

On this basis, the terms “invasive non-native species” and “invasive alien species” should be easily understood. The debate arises when we ask whether native species can also be invasive.

Wikipedia defines invasive species as exclusively non-native – although there is some debate among contributors about this.

Various UK sources including the GB Non-Native Species Secretariat, the Wildlife Trust and Plantlife UK, hold that native species can become invasive too. This is the terminology which I use but I am happy to accept the term “invasive” if someone uses it to refer exclusively to non-native species.

So We’ve Got That Sorted, Then?

The controversy doesn’t end there, though. Even if we agree on exactly what invasive behaviour looks like, and how to identify it, there are other questions which many people overlook.

Firstly: are non-native species only those spread by human activity?

It is possible that the ash dieback fungus migrated to the GB mainland on the wind or via other non-human activity like migratory birds. However, this is being treated by the UK as another invasive non-native species.

Another major question is “in what timescale” – because of course, many species are “non-native” in the sense that they have only been here for a limited time, having spread from another geographic location.

Some species of antelope, lemming and wolverine wouldn’t be widely considered as “native” to the British Isles, despite their presence here as recently as a few thousand years ago. Many people would say the same of the wolves, bears and beavers which were eradicated from the UK through human activity over the past thousand years or so.

And on the other hand, most people would consider rabbits to be native to the UK, although they were initially brought to these islands by the Romans only a few thousand years ago. In fact, rabbits may not have established wild populations until as recently as the 13th century. In 2010, rabbits were named by CABI as Britain’s most costly invasive species .

The UK government doesn’t always make the issue clearer, with often curious and sometimes contradictory positions about certain species made extinct in recorded history, such as wolves, lynx and Eurasian beavers – with some bodies overtly taking the position that these species are no longer native and should be eradicated, while other arms of government are explicitly consulting on the role of managed rewilding in improving the UK environment…

One definition for a non-native species is: anything that has been introduced ‘since the last ice age’ (or more specifically since the formation of the English Channel through rising sea levels (approximately 10-12,000 years ago).

Another definition, which applies only to plants which have become established in their non-native range, has the specific name ‘neophytes’, and covers any species introduced after 1492; non-native species introduced before 1492 would be known as ‘archeophytes’ – a definition which would include the cute little bunny wabbits, if it could be applied to animals too.

But Why Does It Matter?

While we can all quite easily move beyond differing interpretations of words like potato and tomato (or at least agree to disagree), the key point which shouldn’t be overlooked is that native species can cause damage by displaying the same sorts of behaviour as invasive non-native species.

Despite a bill estimated at £1.7bn or more, and with Japanese knotweed alone costing the UK economy an estimated £166m, the UK’s problems with invasive non-native species are less pronounced than countries like Australia and New Zealand. UK agriculture is arguably under greater pressure from native plant species than non-native ones.

Even long-established native species can become an environmental problem when man causes changes to the environment. This can be seen in particularly vivid examples across Vietnam, after the massive deforestation caused by Operation Ranchhand during the Vietnam War was followed by the colonisation of formerly dense broadleaf woodland by species of grass and bamboo which are very difficult to displace. These areas may never return to their former, more biodiverse state without human intervention.

Less visible but more economically-damaging problems can be seen in UK agriculture, where managed agricultural land is subject to a variety of native diseases and pests (those darn bunnies are back!) as well as invasive plants which can cause harm to cropland, pasture and grazing animals.

Admittedly, agricultural land could be called an “unnatural” environment, low in biodiversity and often tending towards a monoculture. It is also heavily reliant on human intervention of one form or another to keep it in its desired state. Nevertheless, there can be no disagreement that agriculture is essential for human civilisation, and that this relatively new but hugely valuable “environment” needs to be protected as well as carefully managed.

The Weeds Act 1959 (along with the Noxious Weeds (Northern Ireland) Order 1977 and the Ragwort Control Act 2003) lists a number of plants which can cause damage to agricultural environments in the UK: broad-leaved dock, curled dock, creeping thistle, spear thistle and ragwort. It gives various powers to official bodies to order the control of these species, and in doing so, UK law acknowledges the damage that these native plants can cause.

What’s more, UK farmers battle bracken , nettles and a variety of other native species in the course of managing their land – especially pasture and some of these species can become invasive due to factors like nutrient run-off or the removal of previously-existing ecosystems in areas like sheep pasture where broadleaf forest has been replaced by virgin grassland.

So even if you don’t agree that this behaviour of colonising areas at the expense of other species is “invasive”, or if you steadfastly insist that only non-native species can be considered “invasive”, my point is that there are native species in the British Isles and other places around the world which can displace other species and cause damage to a variety of valuable environments.

An INNSA member will be able to advise you on invasive species, whether native or non-native – and the INNSA Member Search allows you to search for specialists based on the species that you need help with.

Chris Oliver
Operations Manager