Invasive Species: When Introductions Go Wrong

$423 Billion Per Year—and That Is Probably an Undercount

A 2021 study by Christophe Diagne and colleagues, published in Nature, estimated the total global economic cost of biological invasions at $423 billion annually in 2017 dollars—a figure that had at least quadrupled in real terms every decade since 1970 as global trade and travel accelerated the movement of species across barriers they would never have crossed naturally. The estimate almost certainly underestimates true costs because the majority of invasive species impacts—ecological damage to biodiversity, ecosystem service losses, unpaid labor costs of management—are never monetized. The number that can be quantified is already staggering; the number that cannot be is larger still.

Some introductions were accidents. Others were decisions that seemed reasonable at the time.

The Brown Tree Snake: Guam's Silent Catastrophe

Boiga irregularis, the brown tree snake, arrived in Guam in cargo from the Admiralty Islands following World War II—most likely stowing away in military equipment between 1945 and 1952. Guam's forest birds, having evolved on an island with no native land snakes, had no behavioral defenses against predation. The consequences were catastrophic: of 12 native forest bird species that were present when the snake arrived, all 12 are now extinct in the wild on Guam. The Guam rail and Guam flycatcher exist only in captive breeding programs. The bridled white-eye and Guam broadbill have been declared extinct with no surviving individuals anywhere.

The snake's effects cascaded further: with frugivorous birds gone, seed dispersal collapsed, and the forest's capacity to regenerate native trees declined. Spider populations exploded in the absence of bird predation, leading to webs that enveloped vegetation and made outdoor spaces uncomfortable. Power outages became frequent as snakes climbed utility poles; by some estimates, the brown tree snake has caused more than 1,400 power outages on Guam since the 1970s, costing millions of dollars annually in economic disruption. Guam's tourist industry suffers from the absence of birdsong—a detail that illustrates how deeply ecological and economic losses intertwine.

Cane Toads in Australia: A Solution That Became a Problem

In 1935, approximately 3,000 cane toads (Rhinella marina) were imported to Queensland, Australia, to control cane beetles devastating sugarcane crops. The reasoning seemed sound: cane toads are large, prolific, and in their native range in Central and South America, they prey on insects including beetles. The reasoning was wrong. The cane toads had no interest in adult beetles on tall sugarcane stalks and ate everything else—native insects, small mammals, lizards, frogs, and carrion. Their parotoid glands secrete a potent cardiac glycoside toxin that kills predators that attempt to eat them, including freshwater crocodiles, goannas, quolls, and snakes that evolved without any exposure to toad toxins and have no avoidance behavior.

Cane toad numbers in Australia are now estimated at over 200 million. The species has expanded from Queensland across the Northern Territory, New South Wales, and into Western Australia. In areas where cane toads have recently arrived, native predator populations crash before beginning a partial recovery as individual animals that survive toad encounters learn avoidance or populations with innate toad-avoidance behaviors are selected for—an evolutionary arms race playing out in real time.

Kudzu: The Vine That Ate the South

Kudzu (Pueraria montana) was introduced to the United States from Japan at the 1876 Centennial Exposition in Philadelphia and was actively promoted by the U.S. Soil Conservation Service from the 1930s through the 1950s as an erosion-control plant, with farmers paid to plant it. By the time the USDA listed kudzu as a weed in 1953, it was already established across millions of acres in the southeastern United States. Kudzu grows up to 30 cm per day in warm conditions and can climb and smother virtually any surface—fence posts, utility poles, abandoned buildings, and forest trees—blocking sunlight and killing everything beneath it. The vine now covers an estimated 2–7 million acres across the southeastern U.S., though recent research suggests the problem, while real, may have been overstated in some accounts. It is classified as a Federal Noxious Weed.

Lionfish in the Atlantic: An Aquarium Release with Consequences

Red lionfish (Pterois volitans) and common lionfish (P. miles), native to the Indo-Pacific, were first documented in Atlantic waters off Florida in 1985, almost certainly released by aquarium hobbyists. Lionfish are spectacularly adapted predators with venomous spines that deter most predators (Atlantic fish have no evolved avoidance response), broad thermal tolerance, and an insatiable appetite. Studies in the Bahamas found that lionfish reduced native fish recruitment by 79% on experimental reefs. Their gut contents reveal consumption of over 70 species of fish and invertebrates, including many commercially and ecologically important species.

Lionfish have spread throughout the Caribbean, Gulf of Mexico, and Atlantic coast to Rhode Island and into the Mediterranean. No established biological control exists. Management focuses on human consumption (lionfish taste excellent and have become a promoted restaurant dish across the region) and organized culling dives, but neither has demonstrably reduced population densities at meaningful scales.

Case Study Comparison

Zebra Mussels and the Great Lakes

Zebra mussels (Dreissena polymorpha), native to the Caspian and Black Sea regions, arrived in the Great Lakes in 1988, discharged in the ballast water of a transatlantic cargo ship. They filtered phytoplankton from the water column at rates that fundamentally altered lake food webs, causing the collapse of native plankton communities and the fish populations that depended on them. Their dense colonization of pipes, water intake structures, boat hulls, and native mussel shells has caused an estimated $5 billion in damage and control costs since arrival. They subsequently spread to the Mississippi River system via recreational boating.

Nile Perch in Lake Victoria

Nile perch (Lates niloticus) were deliberately introduced into Lake Victoria in the 1950s and 1960s to create a commercial fishery. The result—sometimes called one of the worst ecological disasters in history—was the collapse of Lake Victoria's extraordinary endemic cichlid fauna, one of the most spectacular examples of adaptive radiation on Earth, with over 500 species evolved in the lake. Nile perch predation, combined with eutrophication and fishing pressure, drove an estimated 200 cichlid species to extinction or the brink thereof within decades, in what biologists describe as the largest vertebrate mass extinction in recorded history attributable to a single cause.

European Starlings: Shakespeare's Fault?

All approximately 200 million European starlings (Sturnus vulgaris) in North America descend from 100 birds released in Central Park in 1890 by Eugene Schieffelin, a member of the American Acclimatization Society who reportedly sought to introduce all birds mentioned in Shakespeare's works. Starlings outcompete native cavity-nesting birds (eastern bluebirds, red-headed woodpeckers, tree swallows), damage agricultural crops, and create aviation hazards. The story of the Shakespeare connection is occasionally debated by historians, but Schieffelin's role in the release is documented; his motivation for that specific group of birds is somewhat less certain.

Prevention, Eradication, and Management

Invasion biologists recognize a hierarchy of responses, strongly preferring prevention over eradication and eradication over management. Prevention—border biosecurity, ballast water treatment, screening of ornamental and pet trade species—is orders of magnitude cheaper than managing established invasions. Early detection and rapid response can eliminate new introductions before populations establish and spread. Once an invasive species is well-established, eradication becomes exponentially more difficult and expensive with time; management (controlling populations to acceptable damage levels) may become the only practical option, often requiring permanent ongoing investment with no prospect of elimination.