"There is very little we can do about invasive species in Britain and this is because of their biology and because of the practical difficulties."

The National Invasive Species Council defines an invasive species as “a non-native species whose introduction does or is likely to cause economic or environmental harm or harm to human, animal, or plant health”. There are many invasive species in the British Isles, from mammals to trees, amphibians to insects, and some of these species do have an impact on the native wildlife.

Possibly the most well known invasive species in Britain is the Grey squirrel (Sciurus carolinensis). It was introduced into this country from America at the start of the 20^th century and since then has spread widely and become a pest (Mountford, 2006). It is well known that the introduction of Grey squirrels has had an effect on native Red squirrels (Sciurus vulgaris). Grey squirrels are more efficient at exploiting food resources due to their size and hardiness and ability to neutralise toxins in acorns. This can explain how Grey squirrels have displaced Red squirrels in oak dominated woodland. Grey squirrels can also put on more body mass in the autumn months than Red squirrels, enhancing their ability to survive through the winter. Grey squirrels have also been known to pilfer the food caches of Red squirrels in the spring, impairing the Red squirrels breeding success (Gurnell, et al. 2004).

Grey squirrels also cause serious damage by stripping bark from young deciduous trees particularly beech (Fagus sylvatica), sycamore (Acer pseudoplatanus) and oak (Quercus spp.). They peel off the outer bark to get to the sap underneath. Trees are killed outright when they are ring-barked low to the stem, and less extensive stripping can aid fungal infection and stunt growth (Putman, 1989).

Methods of control include spring trapping, which kills the animal, live catch trapping, warfarin poisoning and shooting. Once the animal has been caught they need to be destroyed as it is an offence to release a grey squirrel back into the wild (Anon, 2011). It is best to attempt control between April and July and in highly dense areas. Traps have been shown to be more successful than immunocontraception (Orueta & Ramos, 2001). Resident squirrels in a population can be trapped or poisoned within 5 weeks, but can re-colonise isolated woodland in 3 months and non-isolated woodland within 1 month (Mayle, et al. 2007).

Controlling the Grey squirrel is very difficult because their biology allows them to manipulate habitats easily and eradication can be difficult and they can re-colonise and area quickly. There are also practical difficulties involved such as funding and man power.

Another invasive mammal in Britain is the North American Mink (Mustela vison), which was brought over to the UK in the 1920’s for fur farming, which was banned in 2003 (Bonesi & Palazon, 2007). As the trade declined animals escaped or were deliberately released and the first wild breeding was in 1956 (Smith et al. No date). Since their establishment in the UK, American Mink have had an effect on many species including water voles (Arvicola terestris), which have decreased in numbers by 97% since 1900 (Bonesi & Palazon,  2007). They also have an effect on water birds such as coots (Fulica atra) and moorhens (Gallinula chloropus) (Ferreras & MacDonald, 2001) and breeding seabirds. Between 1989-95, feral North American Mink caused widespread breeding failures in Common Gulls (Larsus canus), Black-headed Gulls (Chroicocephalus ridibundus) and Common Terns (Sterna hirundo) on small islands along the western coast of Scotland (Craik, 1997).

A few projects have shown that it is possible to eradicate and control the species given that there is enough resources and man power. Culling projects in the Western Isles have shown that using dogs to locate den sites may be very important in removing the last individuals in a low-desity population (Bonesi & Palazon, 2007). Eradication on isolated islands is more successful as isolation slows mink dispersal making re-colonisation more difficult (Nordström & Korpimäki, 2004). The current 5 year programme off the west coast of Scotland is costing £1.65 million, funded by EU LIFE, and is to protect ground-nesting birds. Thus far it has been successful with more than 220 mink caught (Moore, 2003).

On mainland Britain it seems that competition with otters is involved in the regulation of mink numbers (Harris, 2002). A research programme conducted by WildCRU supports this and suggests that otters are the dominant competitor and mink numbers have declined as a result of an increase in otters (“Management of American Mink and Interactions with Native Mustelids”).

In the case of the American Mink it seems that control is possible with enough resources, in terms of funding, knowledge and man power, through culling with dogs and traps in more isolated areas, as well as by increasing otter numbers to compete with mink on the mainland.

Other invasive mammals in Britain include the Muntjac deer (Muntiacus reevesi) from south-east Asia, the Chinese water deer (Hydropotes inermis) and the Edible dormouse (Glis glis) from Europe.

An example of an invasive bird species in Britain is the Ring-necked parakeet, also known as Rose-ringed parakeet, (Psittacula krameri) which originates from Africa and Asia. They were introduced in the UK in 1969 and there was an estimated 1500 wild birds by 1996 and numbers are increasing by approximately 25% a year. There were an estimated 6000 birds by 2002 (Butler, 2003). They compete for nest sites with other native hole-nesting birds such as nuthatches (Sitta europaea), and as a result nuthatch numbers have declined (Strubbe & Matthysen, 2009). They are considered an agricultural pest in their native range and have caused damage to crops in the UK (Marchant, 2011).

According to a research project carried out by DEFRA in 2008-09, habitat modification is the most effective and environmentally friendly way to control numbers. However it is unpopular with growers as it requires expert knowledge, effort and materials. Really the only effective way of trying to control numbers is by shooting. Natural England has relaxed the protected status of these birds to allow farmers to poison and shoot only problem birds on their own land (Anon, 2009).

In the case of the Ring-necked parakeet, their biology and the practical difficulties do hinder the control of the species. They breed well and due to their protection laws, culling them is not possible unless they are causing major crop damage on private land. Most forms of eradication for this species require knowledge, effort, and equipment and can be expensive.

Other invasive birds species in Britain include the Ruddy duck (Oxyura jamaicensis) from North America, the Canada Goose (Branta Canadensis) and the Eagle Owl (Bubo bubo) from Europe.

In regards to plants, one of the best known invasive species is the Rhododendron (Rhododendron ponticum). It was introduced to Britain in 1763 for ornamental purposes (Cross, 1975). Rhododendron easily invade woodland and can displace many native shrubs and trees and they are very tolerant of shade. In Killarney, Ireland, Rhododendron has formed a mutual relationship with another invasive species, the Japanese Sika deer (Cervus Nippon). The sika deer overgraze, making room for the Rhododendron to establish, and in return the rhododendron provides shelter (Simberloff & Van Holle, 1999).

Rhododendron is able to grow and establish on most acid soils displacing native flora and altering entire ecosystems. They create a lot of leaf litter and create dark environments that are near sterile. Control is normally herbicide application, post-cut application and cutting. Herbicide application can be done with a knapsack or stem injection but is proving costly with mixed success. Failure results in increased labour and material costs (Tyler, et al. 2006). The most common disposal method after removal is burning, but once the Rhododendron has been eradicated there may be extra costs in restoration of the habitat (Dehnen-Schmutz, et al. 2004).

The Rhododendron, therefore, is difficult to eradicate due to its biology, and the ability to colonise an area easily and quickly. It also requires a lot of man power and cost to remove and destroy it.

Another very invasive species of plant in Britain is Floating Pennywort (Hydrocotyle ranunculoides). It was accidentally introduced to the UK in the 1980’s by the aquatic nursery trade and it can now be found in many locations across southern England and south Wales. It forms dense mats in slow-flowing water bodies and outcompetes native aquatics. It causes deoxygenation of the water which also affects fish and invertebrates in that body of water. It can also clog drainage systems and sluices which can then cause localised flooding. It is hard to control due to its rapid growth but can be removed however, once removed it requires on-going monitoring (Kelly, 2006).

A recently discovered invasive species in Britain is the ‘Killer Shrimp’ Deikerogammarus villosus. It is an invasive species of predatory shrimp that was found in Grafham Water in Cambridgeshire in September 2010. It preys on native species of invterbrates such as damselflies and water boatmen as well as young fish. It can alter the ecology of a habitat and can cause extinctions of native species (Anon, 2010).

They can survive for up to 6 days out of water so can be spread by over land transport. They also have rapid growth, early sexual maturity and a large reproductive capacity with females laying up to 200 eggs per clutch. They are high risk in terms of dispersal, establishment and ecological damage. They can change an environment by displacing species within an ecosystem which leads to changes in organic matter cycling such as eliminating amphipod and non-amphipod leaf shredders which will affect leaf-litter processing (MacNiel, et al. 2010).

The Environment Agency has begun the Check Clean Dry campaign. When you leave a body of water, check your clothing and equipment for organisms. If you find any, put them back into the body of water where they were found. Clean all equipment, footwear and clothing thoroughly. Dry all equipment and clothing properly and ensure you do not transfer water elsewhere (The Environment Agency, 2011). This campaign is to try and stop the spread of this predatory shrimp in the hope that it will not cause any long term damaging effects.

Other invasive plant species include Oxford ragwort (Senecio squalidus) from Sicily, Japanese knotweed (Fallopia japonica) and the Water fern (Salvinia minima) from Tropical America.

Not all invasive species in Britain cause damage or have harmful effects. These include the European pond terrapin (Emys orbicularis), marsh frog (Pelophylax ridibundus) from Eastern Europe and European tree frog (Hyla arborea). Non harmful trees and plants include the North American Red Oak (Quercus rubra), Cricket Bat Willow (Salix alba caerulea) from south-east Asia, Lombardy poplar (Populus nigra) from Italy, garden auricular (Primula auricular) from Europe, Heart-leaved dewplant (Aptenia cordifolia) from South Africa and the Siberian Iris (Iris siberica).

                Taking into account all of the example of invasive species I have mentioned, I partly agree with the statement “there is very little we can do about invasive species in Britain and this is because of their biology and because of the practical difficulties”. I partly agree because there are some things we can do about certain invasive species in the UK such as mink. They can be controlled by culling, trapping and hunting as well as more natural ways such as being out-competed by the native species of otter, so as otter numbers increase the mink numbers decrease, it will just take time. The Floating pennywort can also be controlled with constant management even though it can be costly in terms of time and resources.

Some species can be partially controlled such as the ‘killer shrimp’. Their movements can partially be controlled through care taken by people that have been involved with bodies of water where the species is present, but the species may never be able to be eradicated completely. I believe the same can be said for the Ring-necked parakeet. With habitat modification and extra effort put in by land owners to catch and shoot the parakeets their numbers could be kept under control.

I believe some species are not able to be controlled, such as the Grey squirrel because their numbers have increased so enormously and their biology enables them to exploit many niches and food resources, and their ability to outcompete and displace native species such as the Red squirrels.

References

Anon. (2009) Statement on Monk & Ring-necked Parakeets. Accessed: 16/11/11. Source: http://www.naturalengland.org.uk/about_us/news/2009/031009.aspx

Anon. (2010) ‘Alien ‘killer shrimp’ Found in UK’. BBC News online. 9^th September. Accessed: 17/11/11. Source: http://www.bbc.co.uk/news/science-environment-11246642

Anon. (2011). BASC Grey Squirrel Control. Accessed: 16/11/11. Source: http://www.naturalengland.org.uk/about_us/news/2009/031009.aspx

Bonesi, L. & Palazon, S. (2007) The American Mink in Europe: status, impacts & control. Biological conservation 134(4):470-483.

Butler, C.J. (2003) Population Biology of the Introduced Rose-Ringed Parakeet Psittacula krameri in the UK. Doctoral thesis, University of Oxford.

Craik, C. (1997) Long-term effects of North American Mink (Mustela vison) on seabirds in Western Scotland. Bird Study 44(3):303-309.

Dehnen-Schmutz, K., Perrings, C. & Williamson, M. (2004) Controlling Rhododendron ponticum in the British Isles: an economic analysis. Journal of Environmental Management 70(4):323-332.

Ferreras, P. & Macdonald, D.W. (2001) The impact of American Mink Mustela vison on water birds in the upper Thames. Journal of Applied Ecology 36(5):701-708.

Gurnell, J., Wauters, L.A., Lurz, P.W.W. & Tosi, G. (2004) Alien species and interspecific competition: effects of introduced eastern grey squirrels on red squirrel population dynamics. Journal of Animal Ecology 73(1):26-35.

Harris, S. (2002) Hunting With dogs: Hearing On The Evidence (9-11 September 2002). Session 1C: Managing and Controlling the Quarry Species (fox, deer, hare and mink) populations.

Kelly, A. (2006) Removal of invasive floating pennywort Hydrocotyle ranunculoides from Gillingham Marshes, Suffolk, England. Conservation Evidence 3:52-53.

MacNiel, C., Platvoet, D., Dick, J.T.A., Fielding, N., Constable, A., Hall, N., Aldridge, D., Renals, T. and Diamond, M. (2010) The Ponto-Caspian ‘killer shrimp’, Dikerogammarus villosus (Sowinsky, 1894), invades the British Isles. Aquatic Invasions 5(4):441-445.

Marchant, J., (2011) Ring-necked Parakeet, Psittacula krameri. Accessed: 16/11/11. Source: https://secure.fera.defra.gov.uk/nonnativespecies/factsheet/factsheet.cfm?speciesId=2886

Mayle, B., Ferryman, M. & Pepper, H. (2007) Controlling Grey Squirrel Damage to Woodlands. Forestry Commission Publication. Accessed: 16/11/11.  Source: http://www.forestry.gov.uk/pdf/fcpn004.pdf/$FILE/fcpn004.pdf

Moore, N.P., Roy, S.S. & Helyar, A. (2003) Mink (Mustela vison) eradication to protect ground-nesting birds in the Western Isles, Scotland, United Kingdom. New Zealand Journal of Ecology 30(4):443-452.

Mountford, E.P. (2006) Long-term patterns and impacts of grey squirrel debarking in Lady Park Wood young-growth stands (UK). Forest Ecology and Management 232(1-3):100-113.

Nordström, M. & Korpimäki, E. (2004) Effects of island isolation and feral mink removal on bird communities on small islands in the Baltic Sea. Journal of Animal Ecology 73(3):424-433.

Orueta, J.F. & Ramos, Y.A. (2001) Methods to Control and Eradicate Non-native Terrestrial Vertebrate Species. Council of Europe Publishing.

Putman, R.J. (1989) Mammals as Pests. Chapman and Hall Ltd.

Simberloff, D. & Van Holle, B. (1999) Positive interactions of nonindigenous species: invasional meltdown? Biological Invasion 1(1):21-32.

Smith, S., Booy, O., Wade, M. & White, V. (no date) Non-Native Species Secretariat publication. (Source: https://secure.fera.defra.gov.uk/nonnativespecies/index.cfm?sectionid=47

Strubbe, D. & Matthysen, E. (2009) Experimental evidence for nest-site competition between invasive ring-necked parakeets (Psittacula krameri) and native nuthatches (Sitta europaea). Biological Conservation 142(8):1588-1594.

The Environment Agency. (2011) Non-native invasive shrimp found in UK waters. Accessed: 17/11/11. Source: http://www.environment-agency.gov.uk/homeandleisure/wildlife/123281.aspx

Tyler, C., Pullin, A.S. & Stewart, G.B. (2006) Effectiveness of Management Interventions to Control Invasion by Rhododendron ponticum. Environmental Management 37(4):513-522.