Immigration’s a hot topic – and it applies to non-native plants, animals and insects, all over the world | Tim Blackburn

5 hours ago 1

Britain would be a wasteland if it weren’t for immigration. Fifteen thousand years ago, most of the country was buried a kilometre deep in ice – not ideal conditions for life. That all changed as we moved out of the last ice age into the current, milder climate phase. The ice sheets retreated, leaving an empty landscape for anything with the wherewithal to seize the opportunity and move in. Tens of thousands of species did, mainly heading north from the European continent to which Britain was then joined. The result was a native biota where almost every species is an immigrant. Our ancestors were among them.

Immigration is a natural process, but it’s one that has been fundamentally changed thanks to humanity’s wanderlust. As we’ve moved around the world we have taken many other species along with us – some deliberately, some accidentally – to areas they couldn’t have reached without our assistance. These include many of the most familiar denizens of the British countryside. Grey squirrel, ring-necked parakeet, horse chestnut, rhododendron – none of these would be in Britain if they hadn’t been brought by people. They are what ecologists call aliens. Anywhere people live you’ll also find aliens.

Invasive alien species (IAS) are immigrants, but of a sort never before experienced in nature. Most natural colonists come from nearby. Now, pretty much anything can get anywhere from anywhere. Japanese knotweed, giant rhubarb (Gunnera), Himalayan balsam and Canadian pondweed are aliens living in Britain. Natural colonists need to have the traits necessary to make the journey to a new environment, but aliens don’t. Rats didn’t have the strokes to reach the Isles of Scilly, but humans lent a hand. Likewise, stoats to Orkney. The midge Eretmoptera murphyi made it from South Georgia to the Antarctic peninsula, 500 miles away, despite being flightless.

Humans have been moving species for thousands of years, but the rate at which aliens are arriving is speeding up. For most of the last 500-odd years, a new alien species has appeared somewhere roughly every other month or so, or around seven new aliens per year worldwide. The rate of arrival started to climb in the middle of the 18th century, as trade routes between the European colonial powers and their possessions were regularised. It has continued to climb ever since. By the end of the 20th century, more than one new alien species was appearing somewhere every day, on average. More alien bird species arrived in the 17 years between 1983 to 2000 than there had been in the 400 years between 1500 and 1900.

The rate at which aliens are arriving is accelerating, and so is the rate at which the arrivals spread. Take the 1,000-plus alien insect species expanding across Europe. For most of the last two centuries, they spread at an average of 3.3km a year – not especially fast, but moving. That speed has slowly picked up though – to 12.1km/yr for insects first seen between 1970 and 1989, and a speedy 37km/yr for insects appearing after 1989.

The reasons for the great alien acceleration are pretty obvious. Humans have always travelled, but in just a few centuries we’ve gone from wind-driven carracks to diesel-powered container ships, and now planes. We’re moving further, faster and with greater capacity to carry cargo with us. Trade deals eliminate borders and reduce the opportunities to intercept aliens, but that trade also supplies a demand for novel species. At least 1,000 species of alien mammal, 2,600 species of bird and 2,200 species of reptile were traded on the international market between 1975 and 2021, most probably as exotic pets. The internet facilitates this. It’s easy to find websites where you can buy 60 different species of spider, 50 of scorpion and dozens of other types of invertebrate. The Royal Horticultural Society’s Plant Finder lists more than 70,000 plant varieties available for sale in Britain, the vast majority of which will be alien given our native flora is only around 1,750 species. Any plant that comes with soil is probably bringing in invertebrates and microbes too. Even a Colombian tree frog, in one well-publicised recent case.

You’ve probably been told that biodiversity is good, so why not just welcome all these aliens? The short answer is that aliens have impacts. Aliens drive extinctions of naive natives – they’ve been fully or partly responsible for more extinctions in the last 500 years than any other cause, even habitat destruction. Aliens affect human health – think malaria or yellow fever in the Americas – and are estimated to wipe at least $423bn off global economies. That might not sound like a lot, but it is nearly half of what developing countries have been arguing they need annually by 2030 to combat the devastating effects of climate breakdown. It’s true that most aliens don’t have (or have not yet had) impacts, but we often don’t know which ones will until it is too late.

We have a moral duty to address the environmental problems we’ve caused, and there is plenty we can do to stem the tide of alien species. Strict biosecurity regulations reduced the rate of alien plant arrivals in New Zealand to almost zero, and have greatly reduced the arrival of new aliens in the North American Great Lakes. Public awareness of threats helps – the earlier a problem species is noticed, the easier it is to deal with. Vigilance by beekeepers and the wider public have (so far) prevented the establishment of the bee-killing Asian hornet in the UK. While some might balk at the implications – lethal control of alien species will often be required – looking the other way is simply making a choice about where deaths fall. Asian hornets or honeybees. Scilly’s rats or the seabird chicks and eggs they consume.