The news these days is full of stories about species taking over where they shouldn’t be. Some have been here for decades (kuzdu, fire ants, Africanized “killer” bees), while others are newer concerns (emerald ash borers, lionfish, Asian giant hornets). Non-native flowers, trees, insects, fish, snakes, rodents, birds, and more all have the potential to seriously disrupt or even destroy a local species — even an entire local ecosystem, as pulling out one domino causes others to fall.
This is not quite a purely modern concern, but the balance has tipped very sharply in the last century or so, for a very simple reason: before the advent of steamships and other means of traveling more quickly, it was simply too difficult for many species to leap the gap between their native range and some new, hospitable location. Some things still managed it, of course, but the barriers to entry were much higher.
For an illustration of why, consider the measures the British had to take in order to smuggle tea plants and tea seeds out of China, with an eye toward establishing plantations in India and breaking the Chinese monopoly on tea. Getting the material out of the regions where it grew to the coast, then on board a ship and across the sea, then back overland to the plants’ new home, was a journey of months, with highly variable conditions the whole way. Plants in pots didn’t thrive, and on board the ship, where are you going to put them? In the hold, sheltered but away from the sunlight they require? On deck, to be pummeled by unfamiliar weather and the activity of sailors all around? The salt-laden air alone could kill them; it certainly contributed to killing seeds, which arrived dead and failed to germinate upon planting.
Animals didn’t necessarily fare much better. Humans don’t do so great with long stretches at sea, and we’re a remarkably robust species, capable of adapting to almost any environment. (Some popular discourse even classes us as invasive, though scientists usually don’t adopt that terminology.) Large animals take up a great deal of cargo space and, like plants, sicken in darkness and confinement; tiny critters hitching an unofficial ride stand a high chance of dying off after too long in an inhospitable environment. Even rats, nearly as adaptable as humans, may not survive a long voyage — not when there’s a ship’s cat dedicated to hunting them down, or starving humans eating anything they can catch.
So in pre-modern times, it was relatively rare for plants and animals to invade without deliberate and sustained effort on the part of humans. It’s no surprise that we mostly made that effort for species vital to our subsistence: crops and livestock, like wheat, barley, chickens, and cows.
As that list implies, I’m looking particularly at what we brought from the Old World of Eurasia-Africa to the New World of the Americas — but it wasn’t a one-way street. Although the Columbian Exchange properly refers to more than just the swap of plants and animals (there was also a huge movement of everything from precious metals to ideas), for our purposes here, it’s the living material that matters. Difficult as it is to imagine Italian food without tomatoes, Ireland and Eastern Europe without potatoes, southern Asia without chili peppers, all those crops originated in the Americas. Fewer animals made the leap in that direction, though, and had a smaller impact when they did; on that front, the exchange was vastly more lopsided.
Some species make only a limited mark in their new home. They face fair competition from local equivalents or are vulnerable to local predators and grazers; where humans don’t keep supporting them, they may die out. Others, though . . . The introduction of horses to North America fundamentally transformed the culture of the Plains tribes in particular, giving them a degree of mobility in hunting and warfare unthinkable in prior centuries. But of course diseases were even more transformative: not animals per se, but similarly alien life-forms that spread through their new environment like wildfire.
The first big shift in our ability to spread species around the globe came with the advent of what at the time were called Wardian cases, after their inventor, Nathaniel Bagshaw Ward; nowadays we call them terraria. In the mid-nineteenth century, he found that many plants can thrive when planted inside a sealed glass case, needing only sunlight and the moisture already inside to sustain them. Sheltered from the destructive salt air of the ocean, the contents of a closed terrarium can travel across the world and arrive ready to settle into their new home. Suddenly it became possible to plant British gardens in Australia with the familiar flowers and hedges of home.
Ornamental use is one of the drivers of invasive plant introduction: we like how something looks, so we add it to our landscaping, and the next thing you know it’s popping up everywhere else, too. Which is not to say that all non-native species used ornamentally wind up invading; most of them don’t. And there are other reasons we introduce plants to new locations, of course. The infamous kudzu vine was supposed to stop soil erosion during the Dust Bowl, with the government literally paying people to sow it . . . which in hindsight was a terrible idea, though actually various species of privet have been more destructive in the United States than kudzu.
When it comes to animals, the story is different. We bring some creatures in as pets, then wind up with wild populations when some of them escape and breed, but many of the most destructive examples today came in completely by accident. Zebra mussels, for example, may have hitched a ride in the ballast water of ships before being flushed out into new waterways; another theory says they rode on anchor chains, surviving out of the water long enough to be dropped back in. Insects may be present inside shipping crates. In the past they would have died on the long journey to their destination, but the speed of transit today means they’re still alive and well and ready to breed when we open the box.
The core problem with all invasive species, whether plants or animals, is that they’re wildly out of balance with the environment into which they’re introduced. No local herbivore grazes on the grass; no local predator considers the insect food. They’re immune to local fungi and other diseases. Attempting to introduce a second species to control the first, as with the myxoma virus and rabbits in Australia, rarely works and may produce new problems. Paula Robinson’s 1988 SF short story “Can You Spare an Elephant?” starts with mushroom spores accidentally introduced to a bio habitat; the beetles added to eat the overgrowth of mushrooms require lizards to eat the beetles which still haven’t fixed the mushroom problem, and meanwhile the birds are out of control because they’ve been feasting on the beetles, plus some pet hamsters escape and start breeding . . .
Which isn’t to say we don’t have ways of addressing the problem. Although the Australian “Emu War,” which dispatched artillery units to try and curb the crop-eating emu population, was not terribly effective, offering bounties to individuals who killed emus was more successful. A newer spin on this is “invasivorism,” deliberately incorporating invasive species into cuisine as a means of curbing their population. Sustained efforts to eradicate a species in a given area can make a real difference, especially when paired with the re-introduction and cultivation of native species that had been driven out. And on the preventative side, we’ve begun more carefully inspecting, quarantining, and decontaminating everything from cargo to the water used in firefighting, to give invaders fewer opportunities to establish a beachhead.
It’s a long road to walk, though, and we’ve only just begun. Nor can we walk it all the way back: for better or for worse — for better and for worse — any world in which there’s long-distance travel, especially at speed, will see species exchanged both deliberately and by accident.