Hertfordshire’s alien invaders: 10 non-native species

PUBLISHED: 13:46 01 October 2019 | UPDATED: 13:54 01 October 2019

Signal crayfish (photo: Linda Pitkin/2020 Vision)

Signal crayfish (photo: Linda Pitkin/2020 Vision)

Linda Pitkin/2020 Vision

Herts & Midddlesex Wildlife Trust’s non-native invasive species officer, Martin Ketcher, outlines a rogues’ gallery of introduced animals having an impact on Herts

Non-native invasive species are conservatively estimated by the Non Native Species Secretariat to cost the British economy £1.7bn every year. These aliens can pose a risk to native wildlife and illustrate how delicate our ecosystem is. Here are 10 that have impacted Herts to a lesser or greater degree.

- American mink

An escapee from fur farms in the 1950s and 60s, the American mink is one of the biggest threats to our native water vole - now the fastest-declining mammal in the UK. Minks are fierce predators, feeding on anything small enough to catch, including ground-nesting birds and water voles, with devastating impacts on biodiversity. They are good swimmers and can easily enter the water vole's underground burrows to take their young.

Grey squirrel (photo: Dgwildlife/Getty Images/iStockphoto)Grey squirrel (photo: Dgwildlife/Getty Images/iStockphoto)

- Grey squirrel

One of our most familiar mammals and one that many people feel positive towards as they provide easy wildlife encounters, the grey squirrel is native to North America and introduced to Britain in the 1870s as a fashionable addition to large country estates. It took to our climate and spread quickly. By 1937 it had become illegal to import and keep them.

The grey squirrel outcompetes the native red squirrel because of its breeding cycle - a female grey can have two litters a year - and because it can carry squirrel pox to which reds are highly vulnerable. The grey squirrel has driven red squirrels out of Hertfordshire and most parts of England with only a few isolated populations left in the north of the country and on the Isle of Wight.

American mink (photo: SLS Ye Gauthier/iStock/Getty Images Plus)American mink (photo: SLS Ye Gauthier/iStock/Getty Images Plus)

- Harlequin ladybird

There are nearly 50 different native ladybird species in Britain, 26 of which look like the classic round, spotty ladybird so familiar to us. This makes the harlequin a rather inconspicuous invader - it's difficult to distinguish it from the natives.

It is the UK's fastest-invading non-native species. Only introduced in 2004 as pest control for aphids, it has since spread all over the south east of England.

They're here to stay as the harlequin ladybird has become fully established and it is difficult to control populations without harming other wildlife. While they do compete with native ladybirds for food - and readily turn to eating them - native insects may now have adapted to prey on harlequins, helping to keep numbers in check.

Harlequin ladybird (photo: Amy Lewis)Harlequin ladybird (photo: Amy Lewis)

- Canada goose

The Canada goose is our largest goose and maybe the most familiar as they are common across most of England. As the name suggest, they are non-native, having been introduced from America around 300 years ago. Canada geese happily nest on park lakes, flooded gravel pits and reservoirs. Not particularly averse to human settlement they can cause nuisance and pollution in urban areas. In the wild, Canada geese can have an impact on the nesting behaviour of native wildfowl, as they outcompete them for breeding grounds.

Canada geese and goslings in Tring (photo: Josh Kubale)Canada geese and goslings in Tring (photo: Josh Kubale)

- Japanese knotweed

If you have ever had to deal with Japanese knotweed as a homeowner or prospective buyer you will know the problems associated with this plant. it was brought to the country in the 19th century as an ornamental plant and potentially to stabilise railway banks. Growing faster than the prolific Himalayan balsam, Japanese knotweed can crack tarmac, block drains, undermine foundations and invade homes. With no natural enemies in the UK, it is incredibly difficult (and expensive) to get rid of. It grows faster than Himalayan balsam and simply cutting won't cut it, as even tiny amounts of roots can regrow into a full plant.

Japanese knotweed (photo: Tim Hill)Japanese knotweed (photo: Tim Hill)

- Muntjac deer

Not much bigger than your average dog, the non-native muntjac deer is a common sight in Hertfordshire woodlands, parklands and even gardens. It quickly gained a stronghold across south east England after it was introduced to Woburn Park in Bedfordshire at the start of the 20th century.

Native to south east China, muntjacs are notorious browsers, eating the shoots of shrubs as well as woodland herbs and brambles. In high density they can cause significant damage to local flora. Like other deer species in the country, muntjacs are thriving due to a lack of natural predators such as lynx, wolf and bear that would have controlled populations hundreds of years ago.

Muntjac deer (photo: MikeLane45/Getty Images/iStockphoto)Muntjac deer (photo: MikeLane45/Getty Images/iStockphoto)

- Ring-necked parakeet

Among the muted plumage of many of our native birds, the ring-necked parakeet is probably the most exotic invader. It is native to central and South Africa and Asia, but its origins in our area are a bit more mystical - rumour has it that birds were released from film studios in Middlesex after filming The African Queen in the 1950s. Fact is that it is now thriving in London and throughout the Home Counties, seemingly preferring the urban jungle. While it has been spreading fast, its threat to native wildlife hasn't been established. Conservation organisations are carefully monitoring numbers and its impact.

Ring-necked parakeet (photo: JBLumix/Getty Images/iStockphoto)Ring-necked parakeet (photo: JBLumix/Getty Images/iStockphoto)

- Himalayan balsam

As its name suggests, Himalayan balsam is native to that mountain range. With its pretty pink, nectar-rich flowers it was introduced to England in 1839 as a garden plant. It escaped from gardens within decades, quickly becoming one of the country's most invasive weeds. It poses a serious threat to riverbank flora and is spreading on to road verges and into damp woodlands. Wherever Himalayan balsam can get a foothold, it outcompetes native flora by overshadowing and smothering it. An annual, it grows up to 10 feet in a year, before dying back in winter, leaving riverbanks bare. Exploding pods catapult its seeds up to 22 feet away, which then stick to fur, shoes and even car tyres, increasing the spread.

It is a legal requirement for landowners to tackle Himalayan balsam on their land. Every summer Herts and Middlesex Wildlife Trust organises 'balsam bashing parties' to try to free its nature reserves from the invader. The trust has also partnered with Tring-based Puddingstone Distillery, which produces the world's first Himalayan balsam-infused pink gin. All parts of the plant are edible, so we might just eat and drink our way out of the problem.

Himalayan balsam (photo: Josh Kubale)Himalayan balsam (photo: Josh Kubale)

- Spanish bluebell

Almost half the world's bluebells are found in the UK, but our native is being outcompeted by a Spanish invader. The Spanish bluebell is a garden escapee from the 19th century and now widespread across the country. Compared to its delicate English cousin, the Spanish is much more robust and can spread quicker. English bluebells can take years to become established. There are now many hybrids between these two, so an unambiguous identification may prove difficult. Generally, our English have bell-shaped flowers with a more drooping stem. Spanish bluebells in your garden are not necessarily a reason to worry - unless you live close to woodlands - but it's important to keep an eye on them so that they don't spread uncontrollably.

Spanish bluebells (photo: Richard Burkmar)Spanish bluebells (photo: Richard Burkmar)

- Signal crayfish

After the crayfish plague had ravaged native crayfish populations in the 1970s, the government introduced the signal crayfish as seafood, not knowing that this species itself is a carrier for that very crayfish plague. Our native white-clawed crayfish has suffered a huge decline since, being outcompeted for food and habitat as well as not being immune to the crayfish plague. The signal crayfish builds burrows in riverbanks, causing erosion and leading to destabilised riverbanks, an increased flood risk and silted waters. So far, control attempts have been unsuccessful, so the strategy is to control populations by making sure the crayfish plague isn't being introduced to new waterways.

Latest from the Hertfordshire Life