Non-Native Species

Non-Native Species detected within the Severn Estuary area

The Severn Estuary and the land and sea areas bordering it, like many other regions world-wide, are experiencing the arrival of new ‘non-native’ species of both animals and plants. Humans have introduced new (or alien) species to environments both inadvertently and deliberately for centuries, many of which have been benign or even contributed positively to Britain’s natural heritage1. However, whilst deliberate introductions are generally well managed, the increased global movement of people and goods through trade and tourism1,2, coupled with the effects of climate change, has resulted in increasing numbers of ‘uninvited’ non-native species now reaching our shores.

The routes by which non-native species may be introduced and/or expand the range of colonisation are many and varied. In the marine environment for example, species can be introduced as aquaculture escapees and by transportation in ship ballast and fouling on ship surfaces.

Invasive non-native species

The principal concern associated with the introduction and spread of non-native species is the potential for some to become invasive. Invasive non-native species are broadly defined as species that once introduced, have the ability to spread, causing damage to the environment, the economy or our health3. Invasive species upset the balance of the ecosystem as they may be bigger, faster growing or more aggressive than native species4. They may also have fewer or no natural predators to control their numbers. Native species are often unable to compete and fairly quickly the invasive species take over5.

It is widely accepted that the harm associated with invasive non-native species is second only to habitat destruction, as the main cause of biodiversity loss worldwide. There may often also be a significant economic impact such as the fouling of structures as seen in Cardiff Bay with Zebra mussels6 and obstruction of waterways as seen on parts of the Gwent Levels with Floating Pennywort (subject to an eradication programme in 2010)7 (See Fig. 1). A report published in 2011 has estimated the economic impacts of such species cost the UK economy, £1.7 billion per year2.

Species recorded within or near the Severn Estuary

There are many examples of both terrestrial and freshwater non-native plants and animals immediately inland of the Severn Estuary. These range from the widespread species such as mink and Japanese knotweed, to those with currently less extensive distribution, such as the Zebra mussel (See Fig. 2) and killer shrimp (see Fig. 3) in Cardiff Bay, the sunbleak fish in parts of Somerset and the highly invasive aquatic plant pennywort found at a number of coastal freshwater bodies.

Killer shrimp
Known as Dikerogammarus villosus, it has been given the nickname ‘killer’ not only as a result of its voracious appetite for a range of native species, particularly native shrimps and young fish, but also as it is known to sometimes leave its killed prey uneaten. Originally from the region around the Black and Caspian Seas, the Killer shrimp has spread across Western Europe since the late 1990s. Despite growing up to only 30mm long (larger than native freshwater shrimps), it has been identified as being the worst non-native invader by the Environment Agency. It has been present within Cardiff Bay for several years where it continues to be monitored. There is no evidence that the shrimp has spread to any nearby water bodies, a situation considered largely due to a successful ‘check-clean-and-dry’ campaign by Cardiff Harbour Authority.

Another non-native species of freshwater shrimp (Dikerogammerous haemobaphes), commonly known as the ‘Demon Shrimp’ has been detected at several sites within the River Severn catchment since 2011; the lowest site on the river being near Tewkesbury. As with the Killer shrimp, this species predates a wide variety of plants and small animals and competes with our native shrimp and fish for food. Furthermore, this invasive shrimp reproduces rapidly and can quickly exclude many native species from the water body.

A new addition to the list is the Asian Shore Crab with a single specimen reported at Aberthaw near Barry in early 2014, one of only two UK sites. The level of risk is currently unknown, but this species is likely to compete with native crab species and are potential predators of shellfish.

There have been relatively few non-native marine species recorded colonising the extremely high energy environment and turbid waters of the Severn Estuary – although numbers increase to the west of the Estuary along the more open coast. Those that have been documented are shown in Table 1.

Whilst some of the non-native species detected in the Estuary, such as the Slipper limpet (see Fig. 4), are considered as being potentially invasive, any significant impact has yet to be identified. Furthermore, the Australian barnacle (see Fig. 5) that will displace native barnacles in some habitats has colonised new areas in the Estuary6.

Unlike non-native freshwater fish species that tend to have been introduced either illegally or unintentionally by human activities, the presence of new marine fish species is likely to be due to more ‘natural’ circumstances. In recent years it has been seen that certain fish species are extending their northern range, possibly as a response to climate change; particularly changes to water temperature patterns. A number of these such as the grey triggerfish, a Mediterranean species, are now fairly common in waters west of the Severn Estuary during certain times of the year8. But as with other ‘off-course’ individuals, sightings within the Estuary are still rare.

Climate change

Climate change could have a substantial impact on biodiversity in the coming years – both by affecting the distribution of our native species, and by enabling some non-native species to become more common. Increasingly, there could be a rise in the currently benign non-native species that become invasive as the climate changes. Already we are seeing some evidence of animals occurring outside their usual or expected ranges. Recent research shows that the (generally northerly) expanding range of some species including marine molluscs, plants, migratory birds and fish are consistent with patterns of climate change seen in the UK over the past 30 years1.

A distinction can be made between the spread of non-native species by human intervention and the inevitable changes to species composition that naturally occur in a dynamic environment such as natural colonisers, which may expand or shift their native ranges in response to climate change (though it can be argued that the migration of species due to climate change is a natural response to a man-made problem)5.

  1. The Invasive Non-Native Species Framework Strategy for Great Britain. Defra 2008
  2. Third Sector GB Invasive Non-Native Species & Biodiversity Conference (7 June 2011)
  3. Invasive non-native species
  4. Environment Agency. Invasive Species
  5. Environment Agency response to Natural England’s consultation on draft Policy on Invasive Non-Native species. May 2009
  6. Presentation by Paul Brazier, Countryside Council for Wales at Marine Non-Native Species workshop, Menai Bridge, Wales 1st March 2010
  7. Becky Davies, EAW (pers. comm. July 2011)
  8. Peter Gough, EAW (pers. comm. July 2011)
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