Introduced Species

Nature Gallery (Global Trends)

Introduced Animal Species

Tens of thousands of years-only an instant in the long history of life on Earth - is all it has taken for human beings to spread out over the planet. During the last few centuries the pace of movement has steadily increased: by choice or by force, people from all corners of the world have resettled in other locations.

In the course of transporting themselves and their goods, people have brought many other species along with them. Some they deliberately transplant from their native habitats to new and distant homes. Others they transport unknowingly. The result is a biological exchange so enormous that there are few environments on Earth without some permanent residents that have been delivered there by people.

Domestic animals and crop plants, because of their obvious utility, have been taken everywhere. Extensive tracts of land in the tropical and temperate zones, where forests, savannahs, grasslands, and deserts once stood, have been utilized by humans and converted into settlements, pastures for domestic animals, and fields and plantations for crops. People and their livestock now outnumber all other land mammals of comparable size.

Gardeners have transplanted flowering herbs, shrubs, and trees prized for their ornamental qualities. Foresters have established tree farms filled with foreign species. Hunters have released game birds and mammals into new habitats, and trappers have introduced non-native fur bearers. Lakes and rivers have been stocked with exotic species to provide food and sport. Nostalgic settlers, wishing to correct what they perceive as nature's oversight in their adopted homes, have introduced new species from the old country. Additionally people have unintentionally introduced cockroaches, house mice, and countless other species of pest as stowaways.

Many of these living imports failed to become established, but populations of other species, no longer constrained within the ecosystems in which they evolved, have multiplied to plague proportions. Throughout the world, these adaptable, opportunistic species, both plant and animal, have disrupted ecosystems and displaced native species to such an extent that many scientists consider introduced species to be as serious a threat to the preservation of biodiversity as outright habitat destruction. Few places on Earth lack evidence of serious disturbance.

Invited Guests

The case of the European rabbit in Australia has achieved classic status in the field of conservation biology, an example not only of an apparently innocuous act gone wrong but also of the cascade of ecological problems that often follows the release of alien species. An English immigrant to Australia who longed for the sport of shooting rabbits set two dozen of the animals loose on his Victoria estate in 1859. The rabbits bred heavily and, finding themselves in hospitable country with few predators, soon overran a great track of temperate Australia.

Within a few decades, tens of millions of rabbits had stripped the countryside of much of its vegetation, riddled the ground with their burrows, bankrupted wheat farmers and sheep ranchers, and deprived native animals of food, water, and shelter. In desperation, people spread poison and killed many rabbits, which also had the effect of killing many native animals. The rabbit population soon rebounded whereas that of the native species did not. Another alien-the European red fox-was released to prey on the rabbits, but it turned its attention instead to small mammals, reptiles, and ground-nesting birds. Not until 1950 did Australia begin to bring its scourge of rabbits under control. A South American virus that causes myxomatosis, a disease fatal to rabbits, was introduced, but only after incalculable harm had been done.

Another example of this phenomenon was the prickly pear, an American cactus introduced as a landscape ornament. It soon covered millions of hectares of Australian grasslands and pastures, rendering them useless to livestock and wildlife alike. Another alien-this time the cactus-eating caterpillar of a species of moth-had to be imported to stem the spread of the prolific prickly pear.

But species imported for the biological control of other species may become every bit as damaging as the pests they were brought in to suppress. Seeking relief from the ravages of cane beetles and other harmful insects, growers released the giant South American marine toad in Queensland, the Philippines, Taiwan, Hawaii, Florida, Cuba, and other areas where sugar cane is raised. The toad took over. Prolific, voracious, and toxic, it spread from the cane fields to infest the countryside, towns, and gardens nearby. It displaced native amphibians. It consumed both pests and beneficial insects indiscriminately, as well as any other animals it could catch. Dogs and other predators that tried to eat the toad received instead a mouthful of poison.

Another notorious example of an introduced species gone wild is the common starling, which was intentionally freed in New York's Central Park for reasons that must seem incomprehensible to people today who find this aggressive pest less than charming. The birds bred and spread with such speed that they reached the Pacific coast of North America in little more than 50 years.

In common with many pests, the starling originated in Eurasia. The majority of the traffic in introduced species has been one-way, from the continental mainlands of the eastern hemisphere to the islands and continents of the rest of the world. Ecologists believe the long history of human occupation and ecosystem disruption in the Old World permitted those species best able to exploit such situations to spread into all available habitats. When these opportunist species were later carried to comparatively intact environments on islands and in the Americas, they quickly colonized newly disturbed habitats into which few native species were prepared by evolution to expand.

Stowaways

Many species have dispersed across the world on trains, boats and aeroplanes. Seeds, spores, eggs, larvae, and the adults of plants, fungi, insects, snails, and other small animals travel readily in cargoes of fruit and vegetables, grain, timber, soil, and other internationally-traded items. Modern container traffic promotes this form of transport, and inspectors at ports of entry cannot intercept all the stowaways.

Human vehicles may assist in the spread of larger animals as well. The brown tree snake, for example, was accidentally carried from New Guinea to Guam on a military cargo ship at the end of World War II. As the snake rapidly overspread the island, the native birds went into a catastrophic decline. Not everyone agrees that the snake is the sole cause of the disaster, but it has certainly played a part. Because the brown tree snake is adept at hiding itself in aircraft landing gear, cargo holds, and containers, many people fear that it will inevitably reach Hawaii, where it may deal the final blow to already insecure indigenous birds.

All over the world, thousands of ships, some of them with ballast tanks that can hold millions of litres, routinely pump water in at one port and dump it at another. Any living things that have survived the journey are flushed out, and some of these begin to breed in their new environments. The once thriving fisheries of the Black Sea no longer exist, ever since a species of comb jellyfish arrived from North America in the ballast tanks of a ship. The alien jellyfish proliferated so rapidly that it now accounts for an astonishing 95 per cent of the sea’s biomass.

Rapid transport by ship and aeroplane also facilitates the spread of viruses, fungi, bacteria, and other disease-causing entities. Although rarely thought of as introduced aliens, such pathogens are exactly that. Rinderpest, a viral disease affecting hoofed mammals, devastated domestic cattle, buffalo, antelopes, giraffes, and other species of African ungulates when it was carried from India at the end of the 19th century. A fungus from Japan destroyed the chestnut, a species of major ecological and economic importance in the forests of eastern North America. Additionally, a parasitic microbe introduced to the United States during the 1950s causes whirling disease, which is wiping out rainbow trout and related fish in the streams of the Rocky Mountains.

Human history is littered with examples of populations that suffered after exposure to foreign germs to which they had never developed immunity. As many as two-thirds of the original inhabitants of the Americas ultimately perished after contracting smallpox, measles, and other diseases carried by European explorers and colonists.

Vulnerable Island Life

Many island species are endemic: they are restricted to the individual island or archipelago they inhabit. Having evolved in isolation, these island populations often lack the ability to cope with the intense pressures-from competitors, predators, parasites, and diseases-that prevail in the more interconnected parts of the world, especially the eastern hemisphere. If a local population of a mainland species should become extinct, it can often be revived by immigrants that survive elsewhere in the species' range. However, the disappearance of an island species is final: no pool of reserve individuals is left to rebuild the population.

For these reasons, island life is particularly sensitive to disturbance and prone to extinction. The collective land area of islands, even when including Australia, is much smaller than that of the continents. Nevertheless, about half of all species and more than half of all races known to have become extinct in historic times were island endemics. In many cases, introduced species were the principal cause of their demise.

Maori settlers brought the Polynesian rat and the dog to New Zealand. Together these newcomers set about preying on the native birds, from giant moas on down. Centuries later, European settlers brought other species of rats and more dogs, plus sheep, goats, cattle, pigs, cats, and mice, many of which established feral populations. In addition, the Europeans formed acclimatization societies dedicated to populating the islands with the deer, rabbits, weasels, wallabies, possums, and other species that nature had inconveniently neglected to provide.

The indigenous vegetation and wildlife suffered heavily from the combined assault of people and the animals they brought. Almost half the plant species growing in New Zealand today are naturalized exotics, more than 40 species of bird have become extinct since the first human contact, and the islands' unique ecosystems have been utterly transformed.

In the remote archipelago of Hawaii, the original lowland forests have been largely cleared for agriculture, and the remnants invaded by foreign species that have escaped from gardens, parks, and plantations. For every two native Hawaiian plant species there is now one established alien species, and new candidates for naturalized status continue to come to light.

The relentless grazing of feral pigs, goats, and sheep destroy Hawaiian habitats from sea level to the uplands. Exotic mosquitoes spread avian malaria and other diseases to native birds. Native insects-of supreme ecological importance in pollination and decomposition-fall prey to Argentine ants, North American wasps, and other aliens. Giant snails from Africa, imported to serve as food for humans, escaped and became major pests to both indigenous and cultivated plants. Predatory snails from America, released to control them, instead developed an appetite for native Hawaiian snails and have already annihilated a number of species. The Indian mongoose, introduced in 1883 to control the introduced rats that infested the fields of introduced sugar cane, proved much more efficient at exterminating native birds, a lamentable talent it had already demonstrated in Jamaica and other Caribbean islands.

Towards Fauna Uniformity

Ecosystems are complexes of interdependent species within the physical environment they occupy. The interactions among these species are often subtle, and the integration of their roles is tight. Only when the complex unravels are many of the intricacies revealed. Unfortunately, ecosystem breakdown is a frequent outcome when people introduce alien species. A comparative handful of generalist plant and animal pests replaces a host of specialized, co-evolved native species, and global biodiversity—the collective variety of life that exists both within ecosystems and between them—declines.

Continental species and ecosystems are generally more resilient than those of islands. Populations tend to include a greater number of individuals, and species tend to occupy larger ranges. A pool of potential immigrants may therefore be available to restore a local population that has become extinct. Nevertheless, mainland ecosystems are by no means invulnerable to species introduction. The Eurasian house sparrow is now established as a serious pest in temperate regions of Africa, Australia, and the Americas, where it does considerable damage to crops and competes so successfully for food and nesting sites that native birds suffer.

Wilfully introduced, the nutria, a semiaquatic rodent native to southern South America, escaped from fur farms and is now feral in North America and Eurasia, where it displaces native fur bearers, damages marshes and rice paddies, and undermines dykes and levees with its burrows. Another exception to the rule that most aggressive pests come from the Old World, the North American muskrat doubled the area of its already wide range following its arrival in Eurasia, where it was introduced in the early 20th century.

Many species are comprised of two or more geographic races or subspecies that are adapted to the particular conditions of the places they inhabit. People further simplify and homogenize the world biota when they transplant individuals of one group into the range of another. This happens especially with game species and fur bearers. In the genetically blurred hybrid populations that result—from salmon, trout, and freshwater bass to ring-necked pheasants, wild boar, and red deer—the specialized qualities of form and behaviour that adapted the originals to their environments may be irretrievably lost.

Not only native species but even naturalized exotics maybe genetically compromised by competitive species. Domesticated European honey bees, for example, were deliberately carried to the Americas, where they soon became invaluable as pollinators of both cultivated and wild plants. These bees, having evolved in the temperate zone, were genetically programmed to set aside ample supplies of honey as winter food—a characteristic prized by people. However, the bees often failed to thrive in the year-round heat and humidity of the American tropics.

To correct this problem, Brazilian beekeepers imported African honey bees to breed with the local variety, but the experiment backfired. The highly aggressive African bees overwhelmed resident bees and proved unmanageable for beekeepers. Coming from the winter-free tropics, they lacked the characteristic of hoarding honey, and passed on this undesirable characteristic to their hybrid offspring. These Africanized bees—the notorious killer bees—have since spread from Brazil throughout the tropics and subtropics of the New World, replacing the far more docile and productive honey bees of European descent.

Towards Flora Uniformity

Naturalized plants are often regarded as native by people who cannot remember a time without them. Indeed, whether they grow in a city, suburb, or countryside, few neighbourhoods lack some introduced species. Many of these are of economic or ornamental value. Such apparently quintessential elements of the Mediterranean flora as the olive, fig, lemon, and tomato all originated elsewhere.

Many other foreign plants are weeds that thrive in places disturbed by human activity. Dandelion, shepherd's purse, chickweed, and nettle, for example, invade lawns, old fields, roadsides, and building sites in America, Australia, and New Zealand. As with animals, Eurasian plants dominate the list of global weeds, and many of these penetrate the tropics.

Other, much more pernicious foreign species aggressively invade native ecosystems: the rubber vine from Madagascar smothers forests and woodlands in Queensland, Australia. Exotic pines, acacias, and other weeds from the Mediterranean region threatens the fynbos, the extraordinarily diverse vegetation unique to the area of the Cape Province in South Africa. The Japanese honeysuckle overwhelms native plants in the woods of eastern North America, and Scotch broom has the same effect in the prairies, meadows, and open forests of the Pacific Northwest.

Tropical and subtropical regions have become such a mass of plant species that a universal flora-simplified and uniform-threatens to supplant local biodiversity. Only with painstaking weeding by hand has a diligent group of volunteers managed to restore a few examples of native plant communities in Bermuda, which has otherwise been overtaken by introduced varieties. The Everglades of the United States and other ecosystems of south Florida have been invaded by no fewer than 400 species of exotics. Especially aggressive are casuarina or Australian pine, Brazilian pepper, and melaleuca or cajeput, a particular danger to wetlands because of the speed at which it soaks up water through its roots and transpires it through its leaves.

Aquatic Aliens

Invasive water weeds choke rivers and lakes worldwide. The blooms of the water hyacinth in South America belie the notorious ability of this floating plant to block the light to underwater species, drainage ditches and irrigation channels, and impede navigation throughout the tropics. It was originally introduced as an ornamental variety.

Many plants that are neutral enough in their home waters become vicious weeds when sent abroad. A North American aquatic species called elodea obstructs waterways in Europe. Hydrilla and water milfoil—natives of the eastern hemisphere—now plague lakes, reservoirs, and streams in Canada and the United States. Cordgrass is a valuable keystone species in its native salt marshes along the Atlantic coast of North America. But introduced in Great Britain, New Zealand, and Pacific North America, it alters mudflats and wetlands and ousts native plants that local wildlife depends on for food and shelter.

Owing to the widespread practice of ballast-water discharge from ships, estuaries have become the meeting places for a varied mix of brackish- and salt-water species. San Francisco Bay in California has the unenviable distinction of being the most bio-polluted estuary in the world. The bay is now home to catfish and bass from eastern North America, European green crabs and Chinese mitten crabs, isopods from Australia and shrimps from Korea, western Atlantic horse mussels and western Pacific littleneck clams. Several hundred additional alien species—algae, sponges, worms, mollusks, crustaceans, and fish—have also taken up residence. Scientists believe a new species is added every three months. The future of this ecological mix is unpredictable, but it is certain that many San Francisco Bay natives will go the way of the extinct thicktail chub and the endangered delta smelt, two fish that could not cope with the new influx of species.

Lakes are the aquatic equivalent of islands in the sea and, like islands, they often contain endemic species that are particularly susceptible to competitors and predators brought in from the outside. Among lake faunas that have been disturbed, that of Lake Victoria has sustained the greatest loss. This largest of the great lakes of Africa was the site of a remarkable and rapid expansion of species. In the space of less than 15,000 years, a handful of ancestral forms gave rise to more than 300 species of cichlid fish unique to the lake.

The recent discovery of the breathtaking speed of this event produced great excitement. Unfortunately, scientists realized at the same time that this remarkable diversity was becoming extinct even more quickly. Siltation and artificial enrichment of the lake from agricultural activity are partly to blame, and the Nile perch, a voracious fish released several decades ago, is eating the remaining fish that have survived the changes to the lake environment. In historic times, no mass extinction of vertebrate species can compare to this: almost 200 of Lake Victoria’s endemic fish may already have vanished, and most of the rest are threatened.

Another set of great lakes, those of North America, has likewise endured assaults from exotic species. Construction of locks and canals permitted the sea lamprey to penetrate the lakes beyond the rapids and falls that had once barred its way. The invasion of the lamprey, a primitive, superficially eel-like creature that attaches itself to a fish and drains its vital fluids, decimated populations of lake trout, whitefish, walleyes, and chub. With so many of its predators eliminated, the alewife—another invasive species—exploded in numbers and soon came to dominate the lakes. Continuing this ecological cycle, people introduced yet more foreign species—hybrid trout and several species of Pacific salmon—to control the alewife and restore the once great sport and commercial fisheries.

Then in 1988, decades of effort to rehabilitate the Great Lakes were set back by the accidental importation of the zebra mussel. This tiny mollusk, a native of the Caspian and Black seas, arrived as a passenger in a ship’s ballast tanks and established itself extremely rapidly. Mussels by the millions have already attached themselves to underwater surfaces, fouling boats and piers and clogging the intake and outflow pipes of factories and power plants.

By aggressively excluding other species and monopolizing the supply of plant plankton that constitutes the base of the food chain, the mussels threaten to further destabilize the already precarious lake ecosystems. In less than a decade, they have overwhelmed the basins of the Great Lakes and Mississippi River, where they are pushing the remnants of the world’s richest freshwater mollusk fauna to extinction. Economists estimate that the zebra mussel will have done US$5 billion worth of damage within 15 years of its introduction.

Does It Matter?

Despite these accounts of the results of human meddling in nature, people continue to ask whether it matters, since the history of life on earth is an endless account of species moving from one place to another. Numberless species have already become extinct, and it is probable, even certain, that many extinctions resulted from the failure to compete with superior immigrants. Perhaps the human-assisted movement of a species should not be regarded as contrary to nature. After all, there is little difference compared to the dispersal of plant seeds in the mud clinging to a bird’s feet; or with an animal floating to a distant shore on a raft of vegetation torn loose in a storm; or even with the spread of any species across a bridge of land exposed when sea levels drop.

Species translocations and extinctions have, of course, always occurred, and there is no qualitative difference between those that humans bring about and any others. The distinction—and it is significant—concerns the rate at which these events occur. People now move other species around the globe at such an enormously accelerated pace that ecosystems have little or no time to reach a new equilibrium. The tragic legacy of the ascendancy of modern humans will be an unbalanced and biologically impoverished world.

Increasingly, concerned voices offer moral, aesthetic, and economic arguments against the introduction of foreign species. Education campaigns encourage people, businesses, and governments to prevent the inadvertent transport of foreign species, to control existing exotics, to restore degraded ecosystems, and to undertake any future introductions with the utmost care. Human welfare, after all, depends on preserving biodiversity. Through their mixing of species and disruption of ecosystems, human beings imperil themselves.