Alien species are species that have managed to establish breeding populations in new geographic regions, often far from their native ecosystems. Along with habitat destruction (as when farmers clear woodlots to plant crops), pollution, and overexploitation of resources (for example, when fisheries are exhausted by overfishing, or forests disappear from clear-cut logging), alien species are responsible for much of the ecosystem disrup-tion—and extinction of native species—that is occurring in the present "Sixth Extinction." Nearly 100 percent of the importation of alien species (sometimes also called "exotic species") in recent history is the result of introduction by humans—either deliberately or accidentally.
Species change their geographic ranges under natural circumstances as well. The annual migrations of many bird species, as well as monarch butterflies and African wildebeest (whose migrations follow the rainfall), show that many species can populate very different geographic regions during different times of the year. More subtly, in the process known as habitat tracking (q.v.), climate and other kinds of environmental change can induce species to shift the boundaries of their habitat. For example, during the Pleistocene ice ages, when large sheets of continental glacial ice extended southward from the Arctic in both Eurasia and North America, the ranges of native species contracted southward as well, moving back to the north when the glaciers melted back during the warmer interglacial periods.
On an even larger scale, plate tectonic movements and other long-term geological processes have also changed the geographic distribution of species. Horses, for example, originated in Europe but spread to North America, where most of their later evolution
occurred. And, in perhaps the most famous example, when plate tectonic processes finally culminated in a complete land bridge between North and South America, when the Isthmus of Panama emerged approximately 2.5 million years ago, South American species started moving northward while North American species moved into South America. Apparently this "faunal interchange" was sufficiently abrupt that the mixing of species from the two regions resulted in extinction of some of the local species.
Such instances of faunal interchange leading to extinction are, however, relatively rare; little extinction occurs during normal habitat tracking, presumably because entire ecosystems are more or less transplanted, especially in instances of global climate change. But there is little doubt that when species are individually introduced around the globe, they sometimes cause ecological havoc—up to and including the loss of substantial numbers of local, native species. Perhaps the most notorious example of an alien species is the brown tree snake—a case that is discussed in the entry Birds of Guam and the Brown Tree Snake.
Our own species, Homo sapiens, responsible for spreading alien species around the globe, was the very first of the alien species. Our cultural adaptations allowed us to leave our native African tropical habitat and invade foreign terrains with very different climates and plant and animal species. (Our ancestors, Homo ergaster or Homo erectus, had also been sufficiently well advanced culturally that they had left Africa in numbers, almost a million years earlier.) The migration of Homo sapiens out of Africa into Eurasia—and ultimately into the Americas and across the entire world—was itself ecologically unusual. It is thought that our invasion of local ecosystems—where we encountered game animals that had never been hunted by humans—led directly to extinctions, especially of the larger game mammals. Most anthropologists now agree that perhaps the first victim of our spread around the world was the Neanderthal species—a distinct species of humans living in Ice Age Europe who, unable to cope competitively with anatomically modern Homo sapiens, became extinct a few thousand years after our arrival in Europe a little after 40,000 years ago.
Some scientists have pointed out that (in addition to overhunting) newly arrived humans surely must have brought with them other species—hence starting the process of introducing alien species around the globe. In addition to the possibility that humans brought pets with them as they invaded new territories, some scientists think that they and the animals with them also transmitted pathogens—disease-causing viruses and bacteria. These scientists think that, much as the measles and other diseases brought by early European explorers and settlers to the New World proved devastating to native Americans who had no immunity (because they had never been exposed prior to European contact), diseases brought by people and the animals that came with them might have had devastating effects on many native species. And it is true that many smaller species of, for example, the Ice Age biota of North Amer-ica—in addition to the mastodons, mammoths, wooly rhinos, and other large and hunted mammalian species—became extinct when humans arrived in North America in significant numbers around 12,500 years ago.
There can be no question, however, that the bulk of the explosion in the introduction of alien species around the globe came during the age of exploration, the age of colonization, and now, in the postindustrial world of globalization. The Norway rat (Rattus norvegicus) has been hitching rides on ships at least since the days of Christopher Columbus. And rocks used as ballast for ships that have off-loaded their cargo are often dumped in foreign lands— apparently the main reason why so many species of European plants got to the United States, as their seeds were intermixed with the crushed stone. Although some of these plants may have decorative flowers, they do interfere in many places with native flowers. Indicative of their status as invaders, however, is the fact that in the eastern United States, at least, most wild-growing native European plants occur along roads and railway lines (transported originally to these already-disturbed sites by the recycling of ballast rock from ships); this is a good indication that not all invading species are able to take over undisturbed habitat.
Indeed, it is not always possible to determine why some invading alien species are wildly successful while others fail to become established at all. Periodic attempts, for example, to introduce the East Coast horseshoe crab, Limu-lus polyphemus, to San Francisco Bay have repeatedly failed (horseshoe crabs are collected to be ground into fertilizer and pig food—a practice that has gotten so out of hand that horseshoe crabs, once very abundant, are now endangered in such places as New Jersey's coastline). This failure is difficult to understand, inasmuch as horseshoe crabs are ecologically very generalized: they can withstand great fluctuations in temperature and salinity, they are not choosy about what they eat, they have few natural enemies (except man!), and they are even notoriously successful at surviving in the heavily polluted waters in the East. Yet they cannot survive in San Francisco Bay—and that remains a mystery.
Thus some alien species—perhaps most— are doomed not to survive. Others (such as the European plants already mentioned) can survive only in human-disturbed habitats, sometimes because they are already "commensal" (adapted to live in close association) with human beings. Perhaps because cities sprang up in Europe long before they did in the United States and Canada, bird species commensal with humans in Europe, such as house sparrows, starlings, and pigeons, have thrived in the United States—and, indeed, in many settled areas throughout the globe. More recently, North American human commensal species, such as the Carolina gray squirrel, have become common in Europe (and even in Cape Town, South Africa); Canada geese have recently become so common in Europe that they are considered pests.
European colonists, longing for home, often deliberately transported familiar species from their homelands—often with disastrous consequences for native species. For example, most of New Zealand's native bird species are nowhere to be found near human settlements, where many of the birds in evidence are species transplanted from Europe. But, no matter whether deliberately or accidentally introduced, it is the alien species that unexpectedly thrive—often beyond the level seen in their native homes— that cause the most serious damage.
A very recent example of a wildly successful alien species is the European zebra mussel, now spreading throughout the Great Lakes and adjacent freshwaters in southern Canada and the eastern and central United States. This region was covered by glaciers as recently as 12,000 to 18,000 years ago, so the native freshwater mussels have only recently colonized these regions themselves. Because the shells of freshwater mussels at one time were extensively used to make buttons, and because of pollution from industrialization, native North American mussels were already in serious trouble—with at least thirteen species becoming extinct—in 1930. Now the arrival of the European zebra mussel poses an additional threat.
European zebra mussels also directly threaten human economic life. They grow so rapidly that they are continually fouling boats and wharves—and, more important, clogging the intake and outlet pipes of factories, water treatment facilities, and power plants. They have become a costly economic nuisance.
It is generally assumed that some alien species flourish in nonnative surroundings because of an absence of natural enemies—be they predators or disease-causing microbes. Sometimes, however, it is difficult to pinpoint what the reasons are, and sometimes attempts to redress the balance using "biological warfare," by introducing native predators, backfire. A recent outbreak of a lethal form of conjunctivitis (an eye disease) has drastically cut back the number of house finches in the eastern United States. House finches (Carpodacus mexicanus) were brought to Long Island from the West Coast several decades ago, and their numbers have exploded up and down the East Coast and westward to the Mississippi River, displacing their close relatives the purple finch (Carpodacus purpureus) and other eastern bird species. It remains to be seen, but this particular, and apparently successful, invading species now has a fight on its hands. Only time will tell whether it will adapt to this disease and rebound, continue to exist in the East in diminishing numbers, or disappear completely from the regions it has so recently invaded.
See also: Birds of Guam and the Brown Tree Snake;
Extinction, Direct Causes of; Habitat Tracking;
Human Evolution; Sixth Extinction
Eldredge, Niles. 1998. Life in the Balance: Humanity and the Biodiversity Crisis. Princeton: Princeton University Press; Reaka-Kudla, Marjorie L., Don E. Wilson, and Edward O. Wilson, eds. 1996. Biodiversity II. Understanding and Protecting Our Biological Resources. Washington, DC: Joseph Henry; Wilson, Edward O. 1993. The Diversity of Life. Cambridge: Harvard University Press.
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