THROUGHOUT HISTORY, HUMANS travelling the world have brought plants and animals around with them—often intentionally, sometimes unwittingly. When these non-native species take root in new ecosystems and start causing trouble, they become known as invasive species.
Many species that are introduced to a region, such as US food crops including wheat, tomatoes, and rice, are not invasive. It is only when an alien species damages the surrounding ecosystem by crowding out other organisms and altering their habitat that it becomes invasive. Surrounding native species that have evolved to defend themselves primarily from other native predators can be ill equipped to handle attacks from new, unfamiliar enemies.
Invasive species examples
All kinds of plants and animals, including trees, fish, rodents, insects, and fungi, can become invasive. Some are initially introduced to new locations for a purpose, only to become pests.
For example, Asian carp—a common term for a group of related species including bighead, silver, grass, and black carp—were brought to the southern US to clean algae and vegetation from aquaculture ponds and sewage lagoons in the 1960s and '70s. Floods and other factors helped these large fish escape into the wild, where they quickly became a nuisance, eating up plankton other fish need as food. Other examples of invasive aquatic fish include certain types of tilapia such as blue and Mozambique, rainbow trout, and lionfish.
The mongoose was introduced to several islands in Hawaii and the West Indies to control the rodent populations on sugarcane plantations in the 1800s. But rodents weren't the only thing mongooses would eat: birds, reptiles, fruits, and other native species proved tasty to these transplants, and they are now ensconced, unwanted residents in the Hawaiian wild. A similar scenario happened with cane toads in Australia: Brought in to eat beetles on sugar cane plantations in the 1930s, they settled in and began to compete with native animals for food and shelter, while poisoning pets and people.
Introducing plants to new territory has also wrought unintended consequences. Kudzu, which was initially seen as a way to control soil erosion in the US South, proved to be a fast-growing invader that chokes out other plants and even harbours other invaders, such as soybean rust and the kudzu bug. And the P. juliflora mesquite, planted in Africa to prevent desertification, turned out to be a land-eating animal killer, blocking migration trails and watering holes for livestock.
Other species hitch rides uninvited to new destinations, attaching themselves to cargo ships and hiding in transported materials. The zebra mussel, which is native to Eastern Europe and Western Russia, made its way to Western Europe and the US as a stowaway in ships' ballast water. Now it crowds out other species and clogs water intakes everywhere from the US Great Lakes to canals in Britain. (British canals are also plagued by the American mink, first introduced via commercial farming.)
Similarly, the brown marmorated stink bug that has proliferated around the world, appalling households in the US and decimating crops in Eastern Europe, are believed to have spread via cargo ships and been bolstered by climate change, which is creating longer warm seasons for the bugs to reproduce. Climate change is also contributing to population explosions of native creatures, such as bark beetle species that have damaged forests in the US West, so that they begin to behave like invasive ones.
In some cases, invasive species including algae, mussels, and crustaceans can even ride across oceans on plastic, surviving for years on trash and reproducing where they land.
Invasive species solutions
Some of the methods used to control invasive species include manual clearing, hunting, and fishing. Invasive mesquite in Africa might be used as timber or charcoal, while cattle grazing, herbicides, and controlled burning have been used to tackle kudzu. Campaigns to eat aquatic invaders such as lionfish have their proponents, but critics worry they could be ineffective, or worse, popularise troublemakers.
Another concept, biological control, involves researching and cultivating natural enemies for invasive species. In the 1940s and '50s, two species of beetle were used to control St. John's Wort in California, where it was sickening livestock. But as the examples of the mongoose and cane toad illustrate, biological control can go awry.
While controlling invasive species often involves a menu of problematic choices, there have been successes. On Alaska's Hawadax Island, mass poisoning eradicated an infestation of rats. And in the early 2000s, Washington State used selective tree harvesting, pesticides, and public education to stop the spread of the citrus long-horned beetle after it was spotted in a shipment of bonsai trees from Korea.
Once invasive species are established, they are extremely difficult to eradicate, so the best solution is to prevent invasions in the first place. Pets of any kind should never be released into the wild. Travellers, boaters, and campers can also help by cleaning boots, vehicles, and equipment before and after visiting a destination.