by Catherine Keogan
Invasive species are more than just visitors to a foreign land: they take over an ecosystem wholly unprepared to keep them in check. New conditions allow them to thrive in ways that weren’t possible in their native setting — whether a lack of predators, milder climate, or prey that have no defences against an alien invader — and disrupt the balance of nature, driving some species into extinction. Here are some of the worst invasive species in North America.
Asian tiger mosquito (“Aedes albopictus”)
While originally from the tropics, the Asian tiger mosquito has proven adaptable to a wide range of conditions. International shipping allowed it to spread from its native Southeast Asia to every other continent in the world, and climate change appears to be extending its range. The mosquito is especially pernicious because it can carry more than 20 human pathogens and parasites, including yellow fever, West Nile virus, Zika virus, dengue and chikungunya.
Cane toad (“Rhinella marina”)
The cane toad, native to South and Central America, was widely introduced to control sugarcane pests and is now invasive almost everywhere it was planted, including Hawaii and Florida. Famed for its rampant destruction in Australia, the giant toad is poisonous, omnivorous — even willing to scavenge garbage and carrion — and a prolific reproducer.
Lionfish (“Pterois volitans”)
As few as three lionfish, genetic research shows, bred an unstoppable dynasty in the Atlantic that now extends from Rhode Island to Belize. It’s the first marine species to invade coral reefs. With no natural predators — and venomous spines to ward off pretenders — it simply out-eats everything around it, disrupting not only reefs but commercial fishing as well. Lionfish secrets include jaws that can unhinge, blowing jets of water to confuse prey, and a stomach that can swell 30 times its normal size.
Zebra mussel (“Dreissena polymorpha”)
Zebra mussels are sharp, tiny mollusks native to Eastern Europe. First spotted in the Great Lakes in 1988, they had carpeted Lake Erie within a year, and have now invaded lakes and rivers across North America, depleting plankton food sources for other aquatic species, and growing in thick mats that damage boat engines and block intakes for drinking water and power plants.
Water hyacinth (“Eichhornia crassipes”)
Water hyacinths, with their easy growth and attractive clusters of violet, iris-like flowers, were introduced from South America as ornamentals in the late 19th century and quickly began establishing in the wild in ponds and rivers around North America and the globe to become the most damaging aquatic weed in the world. They double every two weeks, forming thick mats that suffocate water bodies and clog intake pipes and irrigation ditches.
Cat (“Felis Catus”)
Domesticated some 4,000 years ago in Egypt, our feline companions have travelled with us to every continent to become one of the most aggressive wildlife killers in North America. Up to 350 million birds in Canada are killed by feral and outdoor cats each year; in the U.S., they kill as many as four billion birds and 20 billion mammals annually, making them the largest human-influenced source of mortality for birds and mammals in the country.
Argentine ant (“Linepithema humile”)
Argentine ants are one of the world’s most invasive species, established on every continent in the world except Antarctica. They are relentless home invaders that outlive poison bait traps by having numerous queens, farm garden pests, and kill native insects, disrupting delicate food webs. In places like North America, the ants are so closely related (originating from a few hitchhikers) that they essentially form vast supercolonies of cooperative cousins.
Gypsy moth (“Lymantria dispar”)
The European gypsy moth caterpillar has an outsized appetite for the leaves of more than 500 tree and shrub species, harming trees and destroying valuable crops. Since 1970 gypsy moths have defoliated an estimated 30 million hectares (74 million acres) of forest in the United States alone. Like tent caterpillars, they experience periodic population explosions, swarming in such numbers that they can completely denude and even kill entire trees.
Nutria (“Myocastor coypus”)
The nutria (a.k.a. coypu or swamp rat) is the stuff of nightmares. Weighing up to nine kilograms (20 pounds), it looks like a cross between a beaver, rat and platypus. Nutria can swim, burrow, breed year-round, survive independently at just five days old, and eat 25% of their body weight daily. Originally imported from South America for their fur, they have devastated wetlands across the U.S. and Canada, and some states are now paying bounties for tails and even suggesting recipes to control the rodent’s numbers.
Common carp (“Cyprinus carpio”)
The Eurasian common carp is one of the most widely introduced game fish in the world, weighing up to 60 kg (130 lbs), but its versatility, quick growth and destructive impacts on freshwater ecosystems have earned it a spot in the top 100 most invasive species on the Global Invasive Species Database. Honourable mention goes to Asian carps, which have worked their way up the Mississippi and now threaten the Great Lakes.
Dutch elm disease (“Ophiostoma ulmi sensu lato”)
In the 19th century, a fungal disease imported from Europe swept across North America and wiped out nearly all the elm trees on the continent. Spread by bark beetles, Dutch elm has proven incredibly difficult to eradicate and efforts to breed resistant native cultivars have had mixed results, although Asian hybrids have proven hardier against the disease.
Kudzu vine (“Pueraria montana” var. “lobata”)
The kudzu vine is famous as a monster that swallows entire landscapes in the American South. While its destructive abilities have been somewhat exaggerated, the Asian vine can grow 18 metres (60 feet) in a single season, smothering vegetation and destroying infrastructure. Despite its near-mythical reputation as a southern scourge, the rapacious vine is now making its way north into Canada — spread largely through intentional plantings.
Northern snakehead (“Channa argus”)
Considering that an air-breathing fish that could walk essentially gave rise to us landlubbers, it’s not surprising the northern snakehead is such a successful invader. The Eurasian predator — dubbed “frankenfish” for its amphibious powers and sharp teeth all the way down its throat — have spread through mid-Atlantic states and fears grow that it will eventually reach the Great Lakes system. One of the secrets of its success: both parents actively care for their young after they hatch.
Starling (“Sturnus vulgaris”)
Blame Shakespeare. One of his devotees decided to introduce birds mentioned in the bard’s works to North America in 1890; a century later, 200 million of the flocking birds destroy crops, spread disease and highjack other birds’ nests. Their sole saving grace may be the incredible science that underpins their murmurations.
Black rat (“Rattus rattus”)
Black rats are the poster child of the invasive scourge. Along with their Norwegian rat cousins, they are one of the most successful species on the planet. They are smart, omnivorous and incredibly adaptable. They also transmit disease, destroy 20% of all crops worldwide, contaminate food stores, eradicate entire species (especially on islands) and cause untold millions in damage with their ability to wriggle into and chew through almost anything.
Emerald ash borer (“Agrilus planipennis”)
This tiny, jewelled beetle is responsible for killing hundreds of millions of ash trees in the U.S. and Canada in just the last two decades, and its range is spreading. The Asian native’s larvae burrow their way through the bark, cutting off the flow of nutrients and almost invariably killing the tree. Once they move into an area, ash populations are virtually wiped out within just a few years, threatening the bird and insect species that depend on them and risking a cascade of devastating effects.
Purple loosestrife (“Lythrum salicaria”)
Homesick colonists brought over purple loosestrife for their gardens in the early 19th century; today, 190,000 hectares (470,000 acres) of North American wetlands from coast to coast are choking with it. A familiar roadside ornament, loosestrife also spreads through ditches and agricultural channels, causing millions in damages annually.
Wild boar (“Sus scrofa”)
The wild boar, or razorback, is one of the most destructive invasive species in the United States (and the world); even the Canadian prairies and colder northern provinces are beginning to build up a worrying population. They cause US$1.5 billion in damages to U.S. crops each year, and in heavily infested areas, collisions with vehicles — including at least one fighter jet — are a problem. True wild boars in North America are rare; most are hybrids with feral domestic pigs.
Himalayan blackberry (“Rubus discolor”)
Himalayan blackberries are found in British Columbia and across the U.S., including Hawaii. Imported for their bountiful, sweet berries, they form impenetrable thickets of 10-metre-long (33 feet) thorny canes. Even the leaves are prickly. Incredibly difficult to eradicate (although goats appear to be effective controls), the thickets shade out native seedlings, damage riparian areas, create fire hazards, spread plant pathogens and pose a major nuisance to wildlife and property owners.
House sparrow (“Passer domesticus”)
The seemingly inoffensive brown house sparrow is, in fact, one of the most successful species in the world with an estimated population of more than a billion. House sparrows are prolific breeders and compete fiercely with other birds for their nests — quite willing to slaughter fledglings and even nesting females to claim ownership — but the true secret to their success is their long association with humans.
