Invasive Species

What is an invasive non-natural species?

An invasive non-natural species (INNS) is a species that has been transported out of its natural range (we can thank Victorian gardeners for a lot of the INNS plants). INNS have been classed as “those that upset the balance of the ecosystem because they are bigger, faster growing or more aggressive than the native species and also have no natural predators to control numbers” (Environment Agency, 2003)

Not all species introduced into new areas are a problem, indeed some of Britain’s best loved species have been introduced, for example the hare or the horsechestnut. However many other species introduced to the UK have caused major ecological and even economic problems. Japanese knotweed (Fallopia japonica) has an estimated, although impracticable, cost of £1.56 billion to eradicate the plant in Britain.

What can I do about them?

If, like a lot of us, you own a smartphone then you can download the plant tracker app This is an easy way to report findings (and reduces the risk of forgetting about it by the time you get home!)

By familiarising themselves with INNS, visitors to the rivers in the Don Catchment can help in the fight against some invasive species. Intelligence on the distribution and status of invasive species is vital if we are to combat their spread or mitigate their worst impacts. It is particularly important to discover the presence of a new colony of an invasive species as soon as possible, as while the colony is small it is often cheaper and easier to eradicate it. Given time however, an invasive species may become so established that it may not be possible to remove it.

Many invasive species are spread unwittingly by ordinary people enjoying rivers and streams. Numerous eggs and seeds are too small to see even with the naked eye, and so it is important that if you get clothing or equipment wet while using a waterbody, you should clean, check and then dry it before exposing it to another waterbody.

Some of the INNS that can be found in the Don Catchment:

Floating Pennywort (Hydrocotyle ranunculoides) – See the non-native species secretariat ID Sheet

Where is it from and why is it here?

Floating Pennywort is an aquatic plant from the Americas. It was brought to the UK in the 1980s to be used in the aquatic nursery trade, but it inevitably escaped and is currently spreading around the country. This plant is fairly new to the region, but unfortunately it can now be found on the Don below Rotherham.

Why is it a problem?

The growth rate of Floating Pennywort is extraordinary; up to 20cm per day. If unchecked it can quickly smother waterways, blocking navigation, shading waterways and starving them of oxygen, and clogging drainage, potentially exacerbating flooding.

What can I do about the spread of this species?

Firstly the most important thing is not to inadvertently spread this species. All it takes are small fragments of this plant to be transferred by boat or fishing equipment into a waterway for a new colony to become established. Therefore it is essential that boats and equipment are cleaned if they have been potentially exposed to pennywort.

Secondly, the Environment Agency can control colonies of pennywort. The sooner they are discovered, the quicker the EA can act, and the easier and cheaper the colonies are to eradicate. If you find floating pennywort, please report your sighting to the EA on 0113 213 4840.

Signal Crayfish (Pacifastacus leniusculus)
Where is it from and why is it here?

North America – An escapee from stock farmed for eating

Why is it a problem?
The Signal Crayfish is spreading rapidly across the UK. Perhaps its most negative impact is the effect it has on the endangered native White-clawed Crayfish (Austropotamobius pallipes). The two species cannot coexist as Signal Crayfish are bigger, more aggressive and carry a disease (crayfish plague) that is fatal for White-clawed Crayfish. Combined these factors result in signals rapidly replacing White-clawed crayfish. In the Don Catchment, only a few colonies of White-clawed Crayfish remain, while Signal Crayfish are thriving and still spreading.

Signals also have other negative effects on river ecosystems. Amongst other things signals consume fish eggs, and so may have a negative impact on fish stocks. They become very abundant in British rivers, and the deep burrows they dig are thought to reduce the stability of river banks.

What can I do about the spread of this species?

Firstly users of water courses must be careful not to inadvertently spread this species. Before moving from one waterbody to another, people must disinfect all objects that come have come in contact with water, e.g. by drying fishing equipment in the sun.

Unfortunately there are no effective measures to eliminate the spread of Signals. They can breed faster than they can be harvested, and shelter in their deep burrows to sit out other stresses such the draining of waterways. Signals are good to eat, and there is online advice on how to harvest them (just don’t expect this to effect the Signal Crayfish population).

However there is still reason for hope. White-clawed Crayfish might be able to survive behind barriers such as reservoirs which block the spread of Signals, in what are termed arc sites. Understanding where Signals are in the catchment is important when planning where such arc sites might be located and if they can or need to be protected, so sending in sightings of this species (or the White-clawed Crayfish) is important. If you find a either a Signal Crayfish or a White-clawed Crayfish, please send details of date and location (with a grid reference would be preferable) to Sheffield City Ecology Unit who are monitoring the distribution of these species, (we would also be grateful if you could inform the trust of your sightings). Additionally, records may be sent to South Yorkshire Econet. Note that if you wish to harvest Signal Crayfish that you must first apply for a license.


American Mink (Neovison vison)
Where is it from and why is it here?

North America – An escapee from stock farmed for fur

Why is it a problem?
Mink are voracious predators that have adapted well to life in Britain. Many British species such as the charismatic European Water Vole (Arvicola amphibius) have evolved mechanisms to avoid British predators, but are poorly adapted to protect themselves against mink. This has lead to a huge decline in Water Vole abundance in recent years. Mink have spread to many places in the Don Catchment including the centre of Sheffield, where it used to be possible to see Water Vole swimming in the canals.

What can I do about the spread of this species?

Please send any records of sightings of this species to the Sheffield City Ecology Unit or to South Yorkshire Econet.


Himalayan Balsam (Impatiens glandulifera)
Where is it from and why is it here?

Introduced in an age of exploration, Himalayan Balsam became a popular garden plant due to its large attractive flowers, its exotic origin, and its ability to thrive in British gardens. However, as with all invaders, it thrived too well, and began to spread along the river networks of the country. Today dense stands of this plant are a common sight along the rivers of the Don Catchment, and across much of the UK.

Why is it a problem?
Himalayan Balsam does so well in many riparian locations that it out competes most other plant species. While it is surely a pleasant sight in its native montane range, in the UK it’s so prolific along river banks that it often makes it difficult to see the river.

What can I do about the spread of this species?

The seeds of this species are spread readily by water, so only concerted efforts starting at the upstream end of a catchment can be effective. Himalayan Balsam is so abundant that to eradicate it from the Don Catchment would take a monumental effort. South Yorkshire Econet want records of this species to monitor its distribution.

More about Himalayan balsam here


Japanese Knotweed (Fallopia japonica)
Where is it from and why is it here?

A plant with a truly fearsome reputation, Japanese Knotweed is one of the most troublesome invasives in the UK. As its name suggests it is originally from East Asia, but was introduced like Himalayan Balsam into the Americas and Europe as an ornamental plant before the dangers of this activity were fully understood. It is tolerant of a wide range of environmental conditions, spreads rapidly, and is extremely difficult to eradicate, leading to the IUCN classify it as one of the world’s worst 100 invasive species.

Why is it a problem?
In its native range, Japanese Knotweed is controlled naturally by other species that have coevolved with it, such as insect herbivores that feed on it. In the UK there is little that suppresses this species, and so it can rapidly outcompete native species. Amazingly in the UK, all the plants are female, but despite this it spreads very successfully vegetatively, either from small fragments or from its root network. Its growth is so powerful that it can push through tarmac, or even through the floor of buildings, making it a developer’s nightmare.

What can I do about the spread of this species?

A very cautious approach must be taken to tackle this species. Activities that risk distributing even the tiniest of plant fragments such as shredding is counterproductive as it can seed new areas. The root network of Japanese Knotweed can be very extensive, sometimes reaching up to seven meters. If even a tiny amount is left in the ground then new plants can grow from this. Careful application of herbicide can be used effectively but advice must be sought. Advice from the Environment Agency can be found here. Records of this species are being collected by South Yorkshire Econet.

More about Japanese Knotweed here

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