by accident – rather than getting there ‘naturally’ by walking, flying, swimming or wafting on the wind. In Britain, this often means anything brought here after rising sea levels cut us off from the European continent sometime between 7,000 and 9,500 years ago at the end of the last Ice Age. But what then do we call extinct fauna and flora that we have since reintroduced? The western capercaillie and European beaver, both around long before we became an island, were wiped out less than 300 years ago by hunting. According to the above interpretation, we couldn’t treat them as ‘non-native’, yet both occur in twenty-first-century Britain thanks to human intervention. (The capercaillie was reintroduced to Scotland in the late 1830s, and successful releases of beaver have occurred at several locations across the UK over the last decade or so.)
At least with our current population of capercaillies and beavers we are sure how and why they’re here. But, with many other species, it can be tricky to know when they first arrived, and whether people were involved. The sycamore, first recorded growing in the wild in Britain in 1632, is often regarded as introduced. Some say it was brought over in the fifteenth or sixteenth century, others suggest an earlier, possibly Roman, introduction, but either way we’re looking at a non-native. Or are we? The problem is that sycamores, indigenous to central Europe, are fast-growing, fast-spreading trees well suited to Britain’s temperate climate. While people have planted most of our sycamore stands, we can’t exclude the possibility that sycamore seeds may have also taken root naturally from time to time having been blown across from the continent. If true, our sycamore population might comprise both natives and non-natives.
A question mark also hangs over the white-clawed crayfish. Considered our sole indigenous freshwater crayfish, and the focus of intensive conservation activity, some experts now suspect the crustacean was introduced for food in the thirteenth century. Even with things that we’re 100 per cent sure are foreign, pinning how and when they arrived is a challenge.
Advances in DNA sequencing and analysis techniques are now shedding light on these mysteries. For instance, the presence on the Orkney Islands, off the north coast of Scotland, of a vole found nowhere else in Britain is something of a conundrum. The Orkney vole, as it’s known, is an endemic subspecies of the common vole, a variety found on the European continent. So, did the Orkney vole scamper there naturally on a temporary land bridge from Europe before the last Ice Age, 20,000 years ago, and somehow manage to weather the chill while its British mainland relatives died out? Or, as seems more probable and has long been suspected, is the vole a far more recent introduction by people? Genetic studies seem to support the latter hypothesis, with Orkney voles shown to be more related to those in southwest France and Spain than to geographically closer populations. The voles are now thought to have arrived with humans on Orkney during the Neolithic period some 5,000 years ago in ships directly from the continent. The rodents may have stowed away in consignments of livestock fodder, or were intentionally brought as food items, pets or even for religious reasons.
Another difficulty with the notion of an invasive species as currently defined are those organisms, unarguably native, which behave in a manner that can only be described as, well, ‘invasive’. Take hedgehogs. On the British mainland they couldn’t be more popular; and faced by a catalogue of threats including habitat loss and agricultural pesticides, not to mention the risk of being flattened by motor vehicles, they’re the focus of a nationwide preservation society. Yet on islands, Mrs Tiggy-Winkle goes berserk.
In 1974, someone on South Uist in Scotland’s Outer Hebrides released half a dozen hedgehogs into their garden to keep down the slugs and snails. Within a couple of decades, the introduced mammal’s population had swollen to 5,000 individuals, and had spread across causeways to the nearby islands of Benbecula and North Uist. The Hebridean hedgehogs eschewed garden pests in favour of the chicks and eggs of dunlins, ringed plovers, redshanks, lapwings and other shoreline birds, and are implicated in a 50 per cent decline in their numbers. Since 2001, nearly £3 million has been spent on removing the invaders. At first, they were culled with lethal injection, but tactics changed when animal rights groups kicked up a stink, so right now the hedgehogs are trapped alive and released on the mainland. In New Zealand, where introduced European hedgehogs have also run riot, the authorities have been less squeamish, eradicating them from the 86-hectare Quail Island.
Natives don’t just act up on islands. Foxes and carrion crows, both indigenous to the UK, can obliterate nesting bird colonies. Red and roe deer can be every bit as destructive to woodlands and agricultural crops as their introduced counterparts: muntjac, fallow and sika deer. Common ragwort, bracken, brambles and nettles – natives all – often spread out of control, suppressing other plants, and are sometimes regarded as weeds. Even beech trees, indigenous to England, upset Scottish conservationists when their seedlings throw shade north of the border. A further complication is that a non-native might misbehave in one location but elsewhere – typically back home – cause no trouble at all, indeed may even be endangered. So, it’s seldom fair to tar the entire species with the invasive brush. The hedgehog was one example but there are plenty more, such as the rhododendron, which spreads aggressively here but not in New Zealand, or the Japanese knotweed, whose penetrating roots are blamed for weakening buildings across Europe but which in Asia is surprisingly scarce.
Professor Helen Roy, a leading British expert on biological invasions based at the Centre for Ecology and Hydrology in Oxfordshire, suggests that perhaps the only time when an understanding of whether a troublesome organism is native or not truly matters is when an incursion is still recent, or hasn’t yet occurred, since it offers a possibility of intercepting it. Most of the time, though, the focus might be better spent on understanding, managing and, where practical, reducing any negative impacts of the troublemaker in question.
Despite these complexities, the term ‘invasive species’, as a label for fast-spreading, harmful non-native organisms, seems embedded in the common psyche. So how many do we have now in Britain? This is difficult to answer but thinking about it in terms of the invasion process can help. Scientists recognise several key hurdles any aspiring invader must clear. First, the organism needs an ‘invasion pathway’; in other words it must get itself transported to a new region by humans. We may intentionally facilitate this, as with organisms brought for agriculture, hunting, horticulture, aquaculture, as biological control agents and for countless other reasons. But plenty of things use us to move around without our consent; think of plankton suspended in the ballast water of ocean-going vessels, the legions of wood-boring beetles holed up in internationally traded furniture, the seeds and spores peppering the mud of a tourist’s hiking boots, the soil-borne invertebrates hitching a ride in plant pots. Crucially, the globetrotter must survive transit. This is no mean feat; the journey might take weeks during which the stowaway may be subjected to extremes of temperature, lack of food or moisture, and other privations. And that’s even before the prospective invader has to contend with strict quarantine measures imposed by vigilant customs officials.
Before moving on, an important clarification is perhaps required. Should a new species reach our shores from its region of origin with no direct human intervention – be it by flying, rafting on an ocean current, blowing in the wind, catching a ride on a (non-human) animal, or in some other fashion – then this organism is classed as a ‘natural colonist’, and not counted in the statistics. A classic example would be the Eurasian collared dove, which started spreading west from Asia in the nineteenth century and was first recorded breeding in Norfolk in 1955. The distinction between natural colonist and human-mediated invader is not always as clear-cut. For instance, how to treat all those weird and wonderful new species set to colonise Britain – both on land and off our coasts – as the climate begins to warm? We may not directly be involved in their spread, but since human activities are driving global climate change, we are not innocent of this process. We might be happy to welcome recent natural colonists such as the tree bumblebee and small red-eyed damselfly, but are liable to baulk at accepting a malarial mosquito.
Importantly, though, an element of the natural about part of a new organism’s journey movement won’t earn it a pass as a natural colonist, if humans are known to have played a key role somewhere along the line.
