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I wrote a while ago about some of the bird species that have colonised the UK over the past century and a half. Many of these got here under their own steam, with the most recent arrivals able to survive here as a result of climate change. So far, the feeling is that most of these birds are not causing a problem for our native species. There are a number of plants and animals, however, that humans have introduced to this country, both deliberately and accidentally, that have had a detrimental effect on our native flora and fauna. These are officially called ‘non-native invasive species’. Below I’ll look at this often controversial topic, a few of the plants and animals involved and the impact they have on native wildlife.
Millennia of Movement
Any discussion of native versus invasive wildlife is, of course, fraught with difficulty. Arguments about what is or isn’t native rage about a number of well-established UK species. And as an island nation, we inevitably need to import some plants and animals. From the first farmers bringing cultivated grasses to grow from the Continent to the Normans importing rabbits, some of our most familiar species arrived via these imports. Our island status also means there has been lots of human movement to and from our shores since the end of the last Ice Age. And many of these migrations inevitably brought some little stowaways with them. These include more obvious accidental arrivals such as the brown rat. This was almost certainly a stowaway from Russian ships in the 1700s. It also applies to less obvious species, though. Plants such as common field-speedwell, for example, probably arrived in the UK mixed up with crop seeds in the 1800s.
Officially, a non-native species is any brought here by humans, either by design or accidentally, since the end of the last Ice Age. Those that came before the Middle Ages are generally described as naturalised. We describe them as invasive if they have a negative impact on the native environment. They might outcompete local species for food or bring new diseases. Some newcomers are predators, and native species are often ill-adapted to deal with them. Others cause physical damage to habitats through burrowing or destructive root systems. As well as upsetting carefully balanced ecosystems, many also add to existing pressures from habitat loss and climate change. There is often also an economic impact because efforts to remove invasive species can cost millions of pounds. However, there is not always a consensus on whether something is detrimental. Some biologists also argue that invasive species can benefit ecosystems. As well as providing resilience to pressures from climate change, they say that trying to eradicate them is a costly distraction from more beneficial conservation measures.
The Age of Ornament
Some of the most damaging, and costly in a financial sense, invasives are plants. During the 18th and 19th centuries, collectors from the UK roamed far and wide across the globe, bringing thousands of specimens home with them. Landowners started to plant the most attractive or useful ones in great numbers on their properties. Those that did well soon spread beyond the estate wall. One of the most famous of these escapees is Rhododendron ponticum. Native to parts of mainland Europe and Asia, its thick, glossy leaves and fast-growing nature made it attractive in the 1800s as gamebird cover. But it very quickly crowds out native plant species, seriously reducing biodiversity. It is also extremely difficult to get rid of. The plant has had a major impact on temperate rainforests in particular, both here and in Ireland. Some places, such as Killarney National Park in Ireland, have spent millions in an effort to get rid of it.
A number of other plants introduced to the UK as ornamentals have gone on to cause problems in the wider countryside. Perhaps the most notorious is Japanese knotweed. Introduced in the 1800s from its native Asia, it forms dense thickets quickly, again crowding out native species. It is another difficult species to remove as new plants can grow from small remnants of root or stem. Having it on a property can reduce a house’s valuation. Himalayan balsam and giant hogweed also quickly outcompete native plants. Both spread quickly via waterborne seed dispersal, releasing thousands of seeds into the burns and rivers they grow along. Giant hogweed has an additional impact. When the plant’s sap is on human skin and reacts to sunlight, it can cause severe blisters.
While the plants above were all brought to the UK deliberately, sometimes our imports harbour unintentional visitors along with them. These aren’t just a result of Victorian collectors, either. Today’s live plant import trade is worth millions. These imported plants, and the soil brought with them, also bring thousands of extremely damaging organisms along as hitchhikers. This includes the harlequin ladybird. Introduced to mainland Europe from Asia to control aphids, it likely came here via plant imports in the early 2000s. It has spread rapidly, killing native ladybirds, butterflies and moths as it goes. The box-tree moth is another Asian species arriving accidentally via plant imports. It can destroy box hedges and is also spreading rapidly. Other invasive species brought in on imported plants include oak processionary moth, horse chestnut leaf miner and ash dieback fungus.
Jettisoned ballast water from ships is another source of stowaways. Ballast is vital in the shipping industry to help maintain a vessel’s stability as it uses up fuel and offloads cargo. But ships also need to release this water again when refuelling and reloading. Often this means emptying water into one harbour from another halfway around the world. Inevitably, when the water is originally taken on board, local organisms are sucked up too, only to be dumped elsewhere later. This can involve a vast range of species, from viruses to bacteria and molluscs to fish. In the UK, the freshwater zebra mussel is one such ballast water stowaway, although it also arrives on ship hulls. This mollusc has spread across Europe and to the Americas from its native Russia and Ukraine. It alters ecosystems by consuming large amounts of plankton, a vital food for many native species. They also cause physical damage to piers, locks and jetties. Other such stowaways include the carpet sea squirt and Chinese mitten crab.
No article about invasive species would be complete without covering the grey squirrel. Between 1876 and 1930, this North American species was introduced to large estates as an ornamental species on at least 30 different occasions. Their most devastating impact is on our native red squirrel. Not only do they outcompete reds for food, but they also carry the squirrel pox virus. While greys are just carriers of the disease and are immune, it kills red squirrels. As a result, reds have gone from most of England and Wales barring the Isle of Wight, Brownsea Island, Anglesey, Formby, Cumbria and Northumberland. Scotland remains its only stronghold. Grey squirrels also eat birds eggs and damage trees by stripping bark. One glimmer of hope is the resurgence of pine martens in the UK. Studies show that where pine marten populations exist, grey squirrels decline as they aren’t as good at escaping predation by them as reds.
Some invasive species are native to some parts of the UK but cause issues elsewhere. Many of our islands’ ecosystems have not had to adapt to living alongside the same suite of predators as mainland birds and animals. But when new predators do arrive, the results can be devastating. One such case was the deliberate introduction of hedgehogs to some of the Hebrides, Orkney, Shetland and the Scillies by gardeners hoping to combat slugs and snails. Unfortunately, hedgehogs are opportunistic feeders. As a result of turning to bird eggs, they have had a huge impact on ground-nesting birds on the islands. Conservationists on some islands have employed culls (and then trapping and relocation following a public outcry) to eradicate them. The situation also highlights the complicated nature of conservation. Hedgehogs are red-listed on the mainland due to huge declines but classed as invasive on our islands.
Commercial motives have been behind the introductions of some species that went on to become invasive. The first American mink fur farm was established in the UK in 1929. By 1956, a combination of escapes and deliberate releases by animal rights activists meant this small but effective predator was breeding in the wild here. These semi-aquatic mustelids are now common across the UK, although exact numbers are hard to determine. The already vulnerable water vole is the biggest casualty of its arrival. Although water voles were already declining by the 1950s due to habitat loss and pollution, populations plummeted once mink became established. Minks also hunt frogs, game birds, fish, ducks and other water birds such as moorhens and coots. Signal crayfish are another commercially introduced species that now cause huge issues. They outcompete our native, white-clawed crayfish, carry crayfish plague, kill a range of other fish, frog and invertebrate species and dig damaging burrows into riverbanks.
The Game Bird Question
My final invasive species have only recently been legally classed as a danger to native wildlife. This is despite the fact that they are non-native and the UK releases at least 40 million of them into the wild every year. Their status as either wildlife or livestock also changes depending on who you ask and the circumstances involved. Pheasants have been popular gamebirds since the 19th century, although the Normans probably first introduced them here. The smaller red-legged partridge arrived in the late 1700s. Incredibly, although no one would consider releasing any of the other invasive species covered above into the countryside now, the only recent changes to the law regarding pheasant and red-legged partridge releases have been minor. Shooting businesses now have to apply for a special licence to release birds within 500 metres of designated areas such as Special Protection Areas.
Otherwise, there are no real restrictions on where and how many birds a shoot releases. This is despite the threat of bird flu, close association with raptor persecution and competition with native grey partridges. Part of the issue centres on its being livestock in some circumstances and wildlife in others. Ultimately, though, because of the money involved, it is very unlikely that gamebird releases will be reduced in any meaningful way any time soon. Pheasants, as well as red-legged partridges, are therefore invasive species that many are quite happy to welcome and treat very differently from the others covered here.
Ecosystems consist of a complex web of delicately balanced relationships between multiple organisms. A change in conditions, such as the arrival of new species, can have a profound impact on these balances. They impact biodiversity, as well as costing us billions. Islands are particularly vulnerable. On islands, native species are geographically isolated for long periods and so have not adapted to cope with any novel forms of competition or predation. The UK is fortunate that our island status is relatively recent, and we are not far from continental Europe. This means that we haven’t seen the same level of extinctions seen in island groups such as New Zealand and Hawaii where all manner of species from cats to possums have killed off a number of native birds and animals. But our native flora and fauna is still vulnerable. Water voles, red squirrels, and elm trees are just a few of the native organisms lost completely in parts of the UK due to the impact of invasives.
At the same time, some of the language we use, including the word invasive itself, unfairly shifts the blame for any conflicts onto the introduced organisms themselves. We call them pests or vermin and assign ‘evil’ motives to them. They are just trying to survive in a new environment, however, having arrived there unwittingly via our actions. As such, this makes many of the decisions conservationists have to make concerning culls extremely difficult. Sadly, in an age dominated by continued human movement and climate change, we are going to have to make these decisions increasingly often.
Dan Eatherley’s book delves into millennia of arrivals to the UK, showing how closely their presence here reflects our human history. He also makes quite clear that, while some of our most troublesome invasives are more recent arrivals, organisms have been coming to our island nation for a very long time. In actual fact, this makes it hard to determine what exactly is native and what isn’t.
Fred Pearce’s book, meanwhile, explores the more recent argument that invasive species might just be beneficial to ecosystems facing an onslaught of habitat loss and climate change.
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