The Age of Invasives
By Dav Bridger, Ranger
As the world becomes ever more interconnected, species start to appear far from the places they naturally occur.
Native species are those that are regarded as occurring naturally in a specific geographical area, such as Britain. Non-natives are those that have been accidentally or deliberately introduced by people, perhaps for food and agriculture, or as garden plants, or for exotic collections.
Of course, many familiar non-native species have become harmoniously naturalised in the UK, and are generally thought of as British flora and fauna in our landscape and culture – think Common Poppy, Brown Hare and Little Owl.
However, when non-native species proliferate and cause harm to human activity, the economy, or the environment they become classed as invasive.
Here are three official invasives (and one that's causing significant issues) found in Sussex, and why they pose problems for our wider wildlife.
Harlequin Ladybird
Native to eastern Asia, the Harlequin Ladybird (or ‘lady beetle’, pictured above) is one of the most invasive insects on the planet. They were introduced in Europe to feed on and control aphids, and made their way to the UK in 2004. However, they don’t stop at eating aphids - the eggs of other ladybirds, as well as those of butterflies and moths, are also targeted. As such, they outcompete the UK’s own native ladybirds for food, which may be contributing to the decline of our native Two-spot Ladybird. The Harlequin can be hard to identify, as they can have over 100 different colour patterns with different numbers of spots on their wing cases. However, they are never smaller than 5mm long, and have distinctive orangey-brown legs.
Rhododendron ponticum
A favourite among gardeners and a staple of country estates, this shrub is evergreen and can grow enormous, whilst producing swathes of vibrant clusters of flowers. Introduced to the UK by the Victorians in the late 18th Century for its ornamental value, this Rhododendron species is native to Asia but now widespread within Britain. Its size and dense canopy outcompetes native plants for space and light, and it spreads quickly. Under this thick canopy, the plant's own leaf litter will acidify and degrade the soil, making conditions more favourable for Rhododendron ponticum establishment and growth. Additionally, this species can be host to disease, in particular a water-mould that can spread to over 150 other plant species, including Oak trees, causing ‘Sudden Oak Death’ or SOD.
Fallow Deer
📷 Dave Kilbey
The UK is currently home to six species of deer, but only two, Red Deer and Roe Deer, are native. Of the other four species, the Fallow Deer, with its characteristic speckled flanks, has been naturalised in the UK the longest. First brought over by from the eastern Mediterranean by the Romans, genetic analysis has revealed that this population became extinct in the UK after the Roman Empire fell. Much like the Pheasant, it was then the Normans in the 11th Century that again introduced Fallow Deer to the UK. These animals were kept in deer parks as game for hunting, but escapees soon became established in woodlands.
Without any large natural predators, numbers of Fallow Deer have grown, and damaged the health of our woodlands. They graze emerging shoots and young saplings, preventing young trees and shrubs from coming through, which creates woodland with little vegetation and biodiversity between the ground and the treetops. Buck (male) Fallow Deer also rub their antlers up against trees in the early autumn, stripping them of their protective bark, and making them liable to stunted growth and disease.
New Zealand Pigmyweed
📷 Fran Southgate
The picturesque idyll of our ponds, streams and wetlands is plagued by this invasive succulent, since its introduction to the UK in the early 1900s as an oxygenating plant for ponds. As its name suggests, it is native to Australia and New Zealand, and has been banned from sale in the UK since 2014. It forms a dense carpet across the sediment, outcompeting other plants and covering areas of mud that wading birds need to feed on.
Just a single fragment, so long as it contains a node, can regrow and produce a new plant. This makes controlling pigmyweed very hard - not only can animals transport fragments in their fur and feathers, but humans can too, on equipment, clothing and footwear. So, while it may look lush, green and quaint in our freshwater environments, it’s no exaggeration to say that New Zealand Pigmyweed is choking our waterways and harming our biodiversity.
Comments
Very interesting article. However Rhodedendrons cannot have been introduced by the Victorians in the late 18th century as Queen Victoria didn’t come to the throne until 1837. Presumably you meant to say Georgians.
10 Apr 2025 10:56:00
Sussex Wildlife Trust:
Hi Jan. Rhododendron ponticum is typically the species you see most often (with the purple flowers) and was first introduced to the UK via Gibraltar in the 1760s but it wasn't until the 1880s-1890s that it was commonplace and sold as a garden plant to the masses. So, although introduced as a non-native by the Georgians, it wasn't quite invasive until it became widespread by the Victorians.
As an angler for 65 years i have never seen mink take a fish. But the introducing of Otters which are out and out killers over fish large and small . A 30 pound carp cost about £3000 and has taken more than 15 years to grow .An otter will just drag it up the bank take 1 bite and leave it to die. Otters also eat water birds and water voles. The countryside has changed a great deal in the last 30 years . And otters should not be part of it . Will you bring back wolves to the wild i think not .All you people worry about is what you can see non the surface and do not care what goes on underwater because you do not see it
10 Apr 2025 11:09:00
Sussex Wildlife Trust:
Hi Grahame. Otters are a native species to the UK, and a protected species. Native prey/predator relationships are natural, with native prey species having evolved and adapted to live alongside predators such as Otters. For instance, Water Vole can escape into burrows to avoid Otter predation and keep the population level balanced. The non-native, invasive Mink, however, are slim enough to follow Water Voles down their burrows and decimate whole populations. Otters do predate fish, as you mention, and so do American Mink. Preferred native prey species for Otters are native freshwater fish, such as Brown Trout, and Eels, both of which have suffered significant decline due to a multitude of human influences, leaving Otters and other predators to hunt other species.
It seems so counterintuitive to so many that eradicating so many species of plant and animal species in Britain is necessary, but this isn’t their native home & eradication is the only way to ensure the survival of native species that are in danger of becoming extinct.
10 Apr 2025 11:18:00
Thanks for this. Aware these four just a few of the invasive species we have here. Is there anything we can do when out and about to help reduce the damage of such species?
10 Apr 2025 11:27:00
Sussex Wildlife Trust:
Hi Barnaby. You can log invasive species by iRecord: https://irecord.org.uk/ - including photos will help with identification verification. The Non-Native Species Secretariat (NNSS) can access the iRecord data and it can be used to help monitor the spread and establishment of invasive species. There's some that they are particularly interested in, such as the Asian Hornet: https://www.nonnativespecies.org/non-native-species/recording.
So as someone that cares, how can one help with this,? does one report when seeing. Ie on iRecord and upload to Sussex Biodiversity Records?
10 Apr 2025 17:20:00
Sussex Wildlife Trust:
Hi Jane. You can log invasive species by iRecord: https://irecord.org.uk/ - including photos will help with identification verification. The Non-Native Species Secretariat (NNSS) can access the iRecord data and it can be used to help monitor the spread and establishment of invasive species. There's some that they are particularly interested in, such as the Asian Hornet: https://www.nonnativespecies.org/non-native-species/recording.
Can we also highlight that Three Cornered Leek is not wild garlic and should not be growing outside gardens nor should Few Flowered Leek. Both spread like mad.
11 Apr 2025 08:55:00
This is interesting and not the species I expected! Surely things like Rhodedendron and Fallow Deer can be relativly easily dealt with?
11 Apr 2025 10:04:00
Sussex Wildlife Trust:
Hi Gareth. Rhododendron is a challenge as there is so much of it! The shrubs can be huge, and have to be cut as low as possible but can still come back. Smaller plants have to be dug up with as much root material as possible, with a mattock or tree popper. It's definitely possible to remove it but there is just so much of it about. We've done lots of work on Rhododendron on our heathland sites like Stedham and Graffham East, thanks in part to our volunteers! More info available here should you be interested: https://sussexwildlifetrust.or... As for Fallow Deer, without apex predators such as Lynx or Wolf, human control is often the option that takes place. Sussex Wildlife Trust are not involved with any culling programmes for Fallow Deer.
Came into forestry from the BTCV and spent 40 years in the woods, mostly ASNW. Never seen them look so bare, hollowed out by the deer , great to see you highlighting the problem. In the absence of predators we have to cull, it is our responsibility to all the wildlife that is suffering as the result of our past stupidities.
15 Apr 2025 10:58:00
I do wonder whether we’ve got it quite right with invasive species. With climate change, some native species are likely to disappear anyway, depleting our wildlife further. Wouldn’t it be better to allow some non-native species which can flourish in changed climatic conditions to establish themselves so that we still have a rich diversity of species within these islands?
16 Apr 2025 10:24:00
Sussex Wildlife Trust:
Hi Richard. It's an interesting question - the topic of invasive species is never a straightforward one! There worth noting the subtle difference between non-native (introduced) and invasive (causes harm to the ecosystem). Some can be both. You are right that many non-native species can slot in quite well to an ecosystem, sometimes to a benefit. However, some non-native species which are also invasive can cause damage at such a rate that it is deemed beneficial to control them.
Very useful summery of the main threats, thank you.
18 Apr 2025 10:55:00