invasive species, any nonnativespecies that significantly modifies or disrupts theecosystems it colonizes. Such species may arrive in new areas through naturalmigration, but they are often introduced by the activities of other species. Human activities, such as those involved in global commerce and thepet trade, are considered to be the most common ways invasiveplants,animals, microbes, and other organisms are transported to newhabitats.

Most introduced species do not survive extended periods in new habitats, because they do not possess the evolutionaryadaptations to adjust to the challenges posed by their new surroundings. Some introduced species may become invasive when they possess a built-in competitive advantage overindigenous species in invaded areas. Under these circumstances, new arrivals can establish breedingpopulations and thrive, especially if the ecosystem lacks natural predators capable of keeping them in check. The ecological disruption that tends to follow such invasions often reduces theecosystem’sbiodiversity and causes economic harm to people who depend on the ecosystem’s biological resources. Invasivepredators may be so adept at capturingprey that prey populations decline over time, and many prey species are eliminated from affected ecosystems. Other invasive species, in contrast, may prevent native species from obtaining food, living space, or other resources. Over time, invading species can effectively replace native ones, often forcing the localizedextinction of many native species. Invasive plants and animals may also serve asdiseasevectors that spreadparasites and pathogens that may further disrupt invaded areas.

A global problem

Since the dawn oflife onEarth, species have migrated and colonized new areas. In some cases, migrating species were unable to establish sustainable populations in new habitats and quickly died out. In other cases, they either were incorporated into the existing structure of the ecosystem or were responsible for modifying nativefood chains by outcompeting native competitors or decimating native prey. One of the most significant species invasions in Earth’s history took place during thePliocene Epoch (5.3 million to 2.6 million years ago) after theformation of anisthmus connecting North and South America. Numerous predator species migrating fromNorth America toSouth America are thought to have contributed to the extinction of many of South America’s mammalian species.

Since their emergence,modern humans (Homo sapiens) have played an ever-increasing role in species invasions. As a result of their colonization of all but the most extreme of Earth’s ecosystems and their tendency to transform naturalenvironments into agricultural and urban landscapes, modern humans are among the most successful invasive species. However, humans also contribute substantially to the introduction of different species to new areas. Tens of thousands of years ago, migratory bands of humans were accompanied by parasites, pathogens, and domesticated animals. With the rise of civilization, many exotic plants and animals were brought from distant lands to broaden the palettes of consumers or serve as curiosities ingardens andcircuses.

Although the collection and transport of exotic species dates to ancient times, written records of their ecological effects extend back only a few centuries. One of the best-known historical examples of such species is the Norway, or brown,rat (Rattus norvegicus). Thisrodent, which is believed to have originated in northeasternChina, spread throughout the islands of thePacific Ocean. Since the rat’s accidental introduction during thevoyages of exploration between the late 18th and 19th centuries, populations have established themselves on numerous Pacific islands, includingHawaii andNew Zealand, where they prey on many nativebirds, smallreptiles, andamphibians. Some other introductions during this time, however, were deliberate:dogs,cats,pigs, and other domesticated animals were taken to new lands, and there they caused the extinction of many other species, including thedodo (Raphus cucullatus) fromMauritius by 1681.

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Although invasive species occur on all continents,Australia andOceania have been particularly hard-hit. The first wave of invasive species arrived in Australia and the islands of the Pacific with European explorers in the form offeral cats and various rat species.European wild rabbits (Oryctolagus cuniculus), which originally inhabited southern Europe andNorth Africa, were deliberately introduced into Australia in 1827 to serve as a familiar elements for settlers in a new land, and the rabbits multiplied significantly. Over time, they degraded grazing lands by stripping the bark from native trees and shrubs and consuming their seeds and leaves. Thered fox (Vulpes vulpes), a small predator found across much of the Northern Hemisphere, wreaked havoc onmarsupials and native rodents since its introduction in the 1850s. Ironically, thered fox was brought to Australia to help control the aforementioned European wild rabbits. Thevoracious canetoad (Bufo marinus), whose native range spans from northern South America to southernTexas, is a poisonous species with few natural predators. It was introduced into Australia in the 1930s from Hawaii to reduce the effects of beetles onsugarcane plantations. Cane toads are responsible for a variety of ills, such aspopulation declines in native prey species (bees and other small animals), population drops in amphibian species that compete with them, and the poisoning of species that consume them. A large number of invasive plants have also been introduced to Australia.Giant sensitive tree (Mimosa pigra) may have been introduced by the Darwin Botanic Garden sometime before the 1890s; upalatable to most wildlife, it formsvast thickets and disrupts nativewetland ecosystems.Cherry guava (Psidium cattleianum),Surinam cherry (Eugenia uniflora), Arabian coffee (Coffea arabica),lantana (Lantana camara), and theice cream bean (Inga edulis) are all invasive species that were brought as food or ornamental plants and escaped cultivation.

OnGuam,Saipan, and several other Pacific islands, thebrown tree snake (Boiga irregularis), a native of Australia and Indonesia, has caused the extinction of several birds, reptiles, and amphibians and two of Guam’s three nativebat species since its accidental introduction to these islands in the 1950s. Although the snake may have been brought to the islands to control native rodent populations, it is more likely that the original invaders werestowaways aboardmilitary aircraft and cargo ships.

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North American ecosystems have been greatly affected by invasive species over the last two centuries. During the 19th and 20th centuries theGreat Lakes region was altered by thesealamprey (Petromyzon marinus), a primitivefish indigenous to the coastal waters of the North Atlantic and westernMediterranean Sea. Thesea lamprey uses a specially modified sucker to latch onto a game fish and drain itsblood. It is thought that the development of theErie,Welland, andSt. Lawrence canal systems allowed the fish to migrate into the Great Lakes. In the 1980s the introduction of thezebra mussel (Dreissena polymorpha), a filter-feedingmollusk, created further ecological and economic disruption. This species is native to thewatersheds supplying theBlack,Aral, andCaspian seas. Many traveled in the ballast water in oceangoing ships, and they were subsequently released when this water was dumped into the Great Lakes. Large numbers of zebra mussels have been shown to clog water-intake pipes and remove much of thealgae from the aquatic ecosystems they inhabit.

Introduced into theUnited States from Eurasia in the 1970s to help control algae on catfish farms in the Deep South,Asian carp—most notablybighead carp (Hypophthalmichthys nobilis) andsilver carp (H. molitrix)—escaped into theMississippi River system during flooding episodes in the early 1990s. After establishing self-sustaining populations in the lower Mississippi River, they began to move northward. Although breeding populations have been restricted to the Mississippi Riverwatershed, they could, if they enter the Great Lakes ecosystem, seriously disrupt the food chains of the major lakes and adjoining rivers. Compared with other species of Asian carp, these two pose the greatest danger. They consume large amounts of algae andzooplankton, eating as much as 40 percent of their body weight per day. They are fierce competitors that often push aside native fish to obtain food, and their populations grow rapidly, accounting for 90 percent of thebiomass in some stretches of the Mississippi and Illinois rivers.

By 2010 theBurmesepython (Python bivittatus), a native ofSoutheast Asia, was challenging theAmericanalligator (Alligator mississippiensis) for dominance in the wetlands of southern Florida. Released into the Florida landscape afterHurricane Andrew damaged pet stores in 1992, as well as by change-of-heart pet owners, Burmese pythons soon established breeding populations in the state. Growing to nearly 6 metres (20 feet) long, these giant constrictor snakes became significant predators in the area. The python’s penchant for consuming the Key Largo wood rat (Neotoma floridana) and thewood stork (Mycteria americana) have caused both species to decline locally.

Parts of the United States are covered bykudzu (Pueraria montana, varietylobata), a fast-growing vine native to southern and easternAsia. Kudzu was introduced into North America for erosion control and decorative purposes in the late 19th century; however, it deprives native plants ofsunlight. Similarly,Japanese knotweed (Fallopia japonica) has proven extremely difficult toeradicate in eastern North America and can form dense monocultures that crowd out native plants. In addition, a large section of the United States is plagued by thered importedfire ant (Solenopsis invicta), an aggressive swarming and biting species native to South America. The species may have arrived in the United States in shipments of soil and other landscaping materials.

Some introduced species have a global distribution. Most notable examples in this category are disease-causingmicrobes. Early European colonists of the New World and the Pacific introduced organisms that cause thecommon cold,smallpox,sexually transmitted diseases, and other illnesses to lands whose people had no resistance to them. Beginning in the late 1960s, a strain ofhuman immunodeficiency virus (HIV), which causesAIDS, was first carried by infected humans from Africa toHaiti. Later AIDS would spread to populations across the globe. Global trade and pet trafficking are often blamed for accidental disease outbreaks among other species, such as theworldwide spread ofamphibian chytridiomycosis infrogs and other amphibians and possibly even avian influenza (bird flu) andWest Nile virus among various organisms.