Invasive Species

Introduction

Invasive species, also called invasive alien (non-native) species, are plants, animals, or microbes that damage ecosystems in which they do not occur naturally. The introduction of a new species into an ecosystem may be deliberate or accidental: When the introduction is delib-rate, some benefit is expected, such as the elimination of an existing pest. In most cases, introduced species cannot survive or spread widely in an alien environment and so do little or no damage; for example, in the United States the common goldfish is a widely distributed introduced species, but rarely achieves high population densities and so does not displace native species.

However, some species that are introduced into environments where they did not evolve are suddenly freed from predators or other factors that kept its numbers in check. In these cases the species becomes invasive, that, is, tends to spread widely, breed in large numbers, and displace native species. So many species are being transported across ecosystem borders by humans today that the few that become invasive are causing immense damage. Losses due to invasive species are estimated at $400 billion per year worldwide, about 5% of global gross domestic product (total economic output).

Historical Background and Scientific Foundations

Human cultures have been colonizing the globe in stages ever since a group of modern humans first emigrated from Africa about 60,000 years ago. Settlers have often carried crop plants, microorganisms, food animals, parasites, and other creatures with them, deliberately or accidentally. In some cases, these species have multiplied greatly, destroying native species by filling up their habitat, preying on them, eating up their food, and the like. Invasive alien species have caused the extinction of thousands of native species throughout history.

For example, the common rat accompanied human settlers of the Pacific islands as they spread from island to island, arriving in the western Pacific about 40,000 years ago and in the remote islands of the southern and eastern Pacific a little over 1,000 years ago. The rat has established itself on hundreds of Pacific islands and caused the extinction of hundreds of native species, especially birds (for example, in New Zealand). It continues to be a threat to native species in many places. For example, in the Galapagos Islands in the early 2000s, rats were endangering the last nesting sites of the indigenous mangrove finch, with fewer than 100 finches surviving as of 2006. At that time, rats and feral cats (wild cats descended from pets) were also threatening a subspecies of marine iguana found only on one island of the group, Isabela.

During the last few centuries, the spread of nonnative species accelerated due to the increasing number of ships traversing the world. Species were sometimes deliberately introduced to islands, as when whalers and pirates deliberately stocked the Galápagos with pigs, goats, and other animals in the 1700s to be hunted for food on future trips. After centuries of fairly harmless residence, the goats of Isabela in the Galápagos began to multiply in the late 1980s, possibly because changing weather patterns increased their food supply. The goats stripped the vegetation, causing erosion and threatening to starve out the giant, land-dwelling Galápagos tortoises.

In the last century, the rate of species invasion has increased still further as globalization results in increased trade, air travel, and increased shipping volumes and variety of products being shipped to every continent. Invasive species, along with habitat destruction, are now one of the largest threats to global biodiversity. Islands and aquatic environments are particularly vulnerable; for

WORDS TO KNOW

BIODIVERSITY: Literally, “life diversity”: the wide range of plants and animals that exist within any given geographical region.

ECOSYSTEM: The community of individuals and the physical components of the environment in a certain area.

GLOBALIZATION: The integration of national and local systems into a global economy through increased trade, manufacturing, communications, and migration.

INDIGENOUS SPECIES: A species that is native to its region, but may occur in other regions as well.

example, the deliberate introduction of the Nile perch to Lake Victoria in Africa in the 1950s, to serve as a food fish, led to the extinction of about 200 native species of cichlid fish. Zebra mussels, a type of shellfish, began to spread from their native Russian lakes in the 1700s and had colonized most of Europe and Britain by the mid-twentieth century; in 1988 they first appeared in American fresh waters and rapidly spread throughout the ecosystems of the Great Lakes. Besides causing ecological damage, they are also an expensive nuisance to industry, clogging the intake pipes of power plants and water-treatment plants.

Although species have been introduced by humans for thousands of years, the scientific study of invasive species, the field termed invasion ecology, began with the publication in 1958 of a single textbook, The Ecology of Invasions by Animals and Plants by Charles S. Elton (1900–1991). Elton, a British ecologist, was the first scientist to analyze the effect of human actions on species invasions. (Ecology is the study of the interactions of living things with each other and their environments.) Among Elton’s early insights that have been confirmed by later work was the claim that simpler food webs such as those found on islands and in temperate climates are likely to be more vulnerable to invasion than complex food webs, since the latter contain a wider variety of predators that might constrain an introduced species. On this theory, more-diverse ecosystems are more stable in the face of challenge by species introduction.

Impacts and Issues

Invasive alien species can alter entire ecosystems. Invasive insects, by threatening native species, can impact species higher up the food chain, such as birds and fish, as well as plants that may be pollinated or dispersed by the

native insects. Invasive species also cause economic harm: Weedlike invasive plants may interfere with farming, insects may destroy entire forests, invasive marine plants and animals may destroy entire fisheries.

Nor are these effects merely hypothetical. In the United States in 2008, invasive species were directly helping to reduce the populations of 42% of the country’s endangered and threatened species, and were causing between $120 billion and $140 billion of economic loss every year. In Brazil, South African lovegrass has spoiled about 10% of southern grazing lands for the purposes of raising cattle, causing about $30 million of loss to the industry every year.

In Africa, alien species were arriving in luggage, aircraft, imported soil and plants, and by other means, including deliberate introduction (as in the case of the Nile perch, now 80% of the food-fish catch in Lake Victoria). A 2003 report by the World Conservation Union on the effect of alien invasive species in Africa’s wetlands found that economic damage was in the billions and that the effect on human health was severe. For example, the red water fern (azolla), an Asian plant, has invaded some African rivers and lakes. The plant can double its biomass every two to three days, carpeting the surface with mats that harbor mosquitoes and snails. The mosquitoes carry malaria, which kills 1 to 3 million people worldwide every year, while the snails carry bilharzia (schistosomiasis), a rarely fatal but debilitating (weakening) disease that afflicts about 300 million people. Another invasive species, the water hyacinth, which was brought from South America as an ornamental plant, can double its biomass in 12 days. Water-covering plants of this type deprive the waters beneath them of light and oxygen, killing off some fish species. The Louisiana crayfish, an import from the United States, attacks native species of plants, crustaceans, and snails, and its burrows damage earthen dams.

In some cases, measures can be taken to manage or even eliminate particular invasive species. For example, in the Galapagos Islands starting in 2001, the government of Chile and international science organizations undertook a six-year effort to root out invasive species of plants and animals. Botanists armed with herbicides(chemicals that kill unwanted plants) have fought back invasions by such plants as blackberry, quinine bush, and passionfruit in order to preserve stands of rare native trees. Rangers shot 140,000 goats on the island of Isabela; sharpshooters in helicopters, tracking dogs, and sterile female goats that would waste the mating efforts of males were all used. Pigs were purged from Santiago Island and wild donkeys from both Isabela and Santiago. Eradication efforts are ongoing in Galapagos National Park. Although the islands are more vulnerable to invasion initially, they are also more amenable to eradication, because re-colonization by eradicated species is not as likely.

See Also Ecosystem Diversity; Human Impacts

BIBLIOGRAPHY

Books

McNeely, J. A., ed. The Great Reshuffling: Human Dimensions of Invasive Alien Species. Cambridge, MA: World Conservation Union, 2001.

Mooney, H. A., and R. J. Hobbs, eds. Invasive Species in a Changing World. Washington, DC: Island Press, 2000.

Perrings, C., et al., eds. The Economics of Biological Invasions. Northampton, MA: Edwar Elgar, 2000.

Periodicals

Guo, Jerry. “The Galapagos Islands Kiss Their Goat Problem Goodbye.” Science 313 (2006): 1567.

Raghu, S., et al. “Adding Biofuels to the Invasive Species Fire?” Science3131 (2006): 1742.

Stokstad, Erik. “Feared Quagga Mussel Turns Up in Western United States.” Science 315 (2007): 453.

Withgott, Jay. “Are Invasive Species Born Bad?” Science 305 (2004): 1,100-1,101.

Web Sites

Charles Darwin Research Station. “Invasive Rat Eradication.” http://www.darwinfoundation.org/files/species/pdf/rats-en.pdf (accessed May 5, 2008).

Global Invasive Species Program. “Global Strategy on Invasive Alien Species.” http://www.gisp.org/publications/brochures/globalstrategy.pdf (accessed May 5, 2008).

U.S. Department of Agriculture. “National Invasive Species Information Center.” http://www.invasivespeciesinfo.gov/ (accessed May 5, 2008).

Larry Gilman