Biological invasions

A biological invasion is an enormous increase in the numbers of a type of organism entering an ecosystem that the organism previously was not inhabiting. The “invading” organism may be an infectious virus, a bacterium, a plant, or an animal.

Background

Species introduced to an area from somewhere outside that area are referred to as alien or exotic species or as invaders. Because the exotic species is not native to the new area, it is often unsuccessful in establishing a viable population and disappears. The fossil record, as well as historical documentation, indicates that this is the fate of many exotic species as they move from their native habitats to invade new environments. Occasionally, however, an invading species finds the new environment to its liking; in this case the invader may become so successful in exploiting its new habitat that it can completely alter the ecological balance of an ecosystem, destroying and altering the local biological hierarchy. Because of this ability to alter ecosystems, exotic invaders are considered major agents in driving native species to extinction and are thought to be responsible for an estimated 50 percent of all known extinctions of land animals beginning in the year 1500.

Biological invasions by notorious species constitute a significant component of Earth’s history. In general, large-scale climatic changes and geological crises are at the origin of massive exchanges of flora and fauna. On a geologic timescale, invasions of species from one continent to another are true evolutionary processes, just as speciation and extinction are. On a smaller scale, physical barriers such as oceans, mountains, and deserts can be overcome by many organisms as their populations expand. Organisms can be carried by water in rivers or ocean currents, transported by wind, or carried by other species as they migrate seasonally or to escape environmental pressures. However, the geological and historical records of the Earth also show that specific biological invasions by exotic species have altered the course of world history. The extinction of genetically distinct populations is the least reversible of all global changes, and evidence suggests that biological invasions contribute substantially to an increase in the rate of extinction within ecosystems.

Humans have transplanted species throughout history to the point where most people are not aware of the distinction between native and exotic species living in their region. Recent increases in intercontinental invasion rates by exotic species, brought about primarily by human activity, create important ecological problems for the recipient lands. Among animals, the most notorious recent invaders of North America have been the house mouse and the Norway rat; others include the wild boar, donkey, horse, nutria, Pierid butterfly, house sparrow, starling, Africanized (“killer”) bee, tiger mosquito, and red fox. Feral swine and Asian carp are also problematic biological invaders in North America. One of the most destructive invaders is the house cat. More than seventy million domestic and feral cats live in the United States, and they are efficient at hunting small mammals and birds. Domestic cats are credited with killing twenty million birds annually in Great Britain.

It would seem logical to assume that invading species might add to the biodiversity of a region, but many invaders have the opposite effect. The new species are often opportunistic and successful predators that eliminate native species not adapted to their presence. For example, the brown tree snake was accidentally introduced to Guam during World War II as a stowaway on military cargo ships, and the snakes have eliminated most of the island’s birds. The snakes are credited with the extinction of one-third of the island’s native bird species, and the surviving bird population is so decimated that birds are rarely seen or heard. The invasion of the brown tree snake has unalterably reduced the biological diversity of Guam.

Ecosystem Alteration

The invasion of an ecosystem by an exotic species can effectively alter processes. An invading species does not simply consume or compete with native species but can actually change the rules of existence within the ecosystem by altering processes such as primary productivity, decomposition, hydrology, geomorphology, nutrient cycling, and natural disturbance regimes. Invading exotic species may also drive out native species by competing with them for resources. One of the exotic invaders of the North American continent is the zebra mussel, which came to the United States in 1986 in the ballast water of oceangoing vessels; it was carried from the Elbe or Rhine River in Europe and released into the water of the St. Clair River near Detroit, Michigan. The mussel larvae found biological conditions in the Great Lakes ideal. The mussel now exists in all the Great Lakes, and after the catastrophic flood of 1993, the mussels were sighted in the Mississippi River Basin. Mussel in certain locations of the Great Lakes is known to be astonishing—greater than 94,000 individuals per square meter. In 1990, the Detroit Edison power plant discovered a water intake pipe blocked by a mussel population density of 700,000 mussels per square meter. When they reach high population densities, the mussels are able to filter virtually all the larger plankton from the water. The planktonic of the Great Lakes, which supports Great Lakes fisheries, may decline so much that higher trophic species will be deprived of their vital plankton food sources. The mussels also cause a demise of native bivalves through competition for food and because they attach themselves to the shells of other bivalves. Asian carp are another example of a biological invader altering an existing ecosystem. The fish was brought to North America from China in the 1970s to clean up water in waste treatment plants. However, its introduction was too successful, and the Asian carp became prevalent in the Mississippi River. The fish is resilient and can out-compete other fish species for food and resources. Their tendency to jump out of the water has also caused numerous human injuries.

In 2010, Florida made it legal to own a Burmese python as a pet. However, irresponsible pet owners began releasing snakes, which can grow to be 20 feet long, into the Florida Everglades, where they thrived. More pythons wound up in the Everglades when they escaped during hurricanes and other storms. By 2024, the snakes had decimated the natural animal population there. By 2012, the populations of raccoons, opossums, and bobcats had decreased by about 90 percent. The enormous snakes are capable of swallowing a deer whole. In 2017, the Florida Fish and Wildlife Conservation Commission introduced the Python Elimination Program, which hires people to hunt Burmese pythons in the Everglades. However, the 4,000 pythons killed by the hunters were only a small fraction of those in the Everglades. Female Burmese pythons can lay 50 to 100 eggs per year, quickly increasing the population. Scientists estimated in 2024 that there are 100,000 to 300,000 Burmese pythons in the Everglades.

Forests

The invasion of native forests by non-native insects and microorganisms has been devastating on many continents. The white pine blister rust and the balsam woolly adelgid have invaded both commercial and preserved forestlands in North America. Both exotics were brought to North America in the late 1800s on nursery stock from Europe. The balsam woolly adelgid attacks fir trees and causes death within two to seven years by causing chemical damage and by feeding on the tree’s vascular tissue. The adelgid has killed nearly every adult cone-bearing fir tree in the southern Appalachian Mountains. The white pine blister rust attacks five-needle pines; in the western United States fewer than 10 pine trees in 100,000 are resistant, and since white pine seeds are an essential food source for bears and other animals, the loss of the trees has had severe consequences across the forest food chain.

Beginning in the 1800s, the deciduous forests of eastern North America were attacked numerous times by waves of invading exotic species and diseases. One of the most notable invaders is the gypsy moth, which consumes a variety of tree species. Other invaders of eastern forests have virtually eliminated the once-dominant American chestnut and the American elm. Other tree species that continue to decline because of new invaders include the American beech, mountain ash, white birch, butternut, sugar maple, flowering dogwood, and eastern hemlock. It is widely accepted that the invasion of exotic species is the single greatest threat to the diversity of deciduous forests in North America.

Effects on Humans, and Humans as Invaders

Some introduced exotic species are beneficial to humanity. It would be impossible to support the present world human population entirely on species native to their regions. However, many invading species degrade human health and wealth, and others affect the structure of ecosystems or the ability to maintain native biodiversity. Many invading species can act as vectors of disease: Examples include bubonic plague, vectored by rats; a host of diseases transmitted between human populations during first contacts, including smallpox, polio, influenza, and venereal infections; and malaria, dengue fever, Ross River fever, and eastern equine encephalitis, carried by mosquitoes. Mosquitoes alone are thought to account for half of all human deaths throughout history.

Humans, the ultimate biological invaders, have been responsible for the extinction of many species and will continue to be in the future. Like other animal invaders, humans tend to have a broad diet. Humans are also able to adapt culturally to diverse habitats, an ability that complements an ability to breed all year round. These attributes give humans a distinct advantage over less aggressive and less destructive species.

Bibliography

Burdick, Alan. Out of Eden: An Odyssey of Ecological Invasion. New York: Farrar, Straus and Giroux, 2005.

"The Burmese Python Problem: How 20ft Predators Are Wreaking Havoc on the Everglades." The Guardian, 13 Dec. 2024, www.theguardian.com/environment/2024/dec/13/the-burmese-python-problem-how-20ft-predators-are-wreaking-havoc-on-the-everglades. Accessed 26 Dec. 2024.

Cartwright, Frederick F., and Michael Biddiss. Disease and History. 2d ed. Stroud, England: Sutton, 2000.

Crosby, Alfred W. Ecological Imperialism: The Biological Expansion of Europe, 900-1900. 2d ed. New York: Cambridge University Press, 2004.

Duenas, Manuel-Angel. “The Threat of Invasive Species to IUCN-Listed Critically Endangered Species: A Systematic Review.” Global ecology and Conservation, vol. 26, 2021. ScienceDirect, www.sciencedirect.com/science/article/pii/S2351989421000263. Accessed 26 Dec. 2024.

Elton, Charles S. The Ecology of Invasions by Animals and Plants. London: Methuen, 1958. Reprint. Chicago: University of Chicago Press, 2000.

Hengeveld, Rob. Dynamics of Biological Invasions. New York: Chapman and Hall, 1989.

Little, Becky. 7 Invasive Species That Have Wreaked Havoc in the US, 24 Feb. 2020, www.history.com/news/invasive-species-list-mammals-birds-aquatic. Accessed 26 Dec. 2024.

Lockwood, Julie L., Martha F. Hoopes, and Michael P. Marchetti. Invasion Ecology. Malden, Mass.: Blackwell, 2007.

Mignano, Jenna. “7 of the Most Invasive Species in North America and Their Startling Effects.” Brightly.eco, 18 July 2022, brightly.eco/blog/invasive-species-in-north-america. Accessed 26 Dec. 2024.

Mooney, Harold A., and James A. Drake, eds. Ecology of Biological Invasions of North America and Hawaii. New York: Springer, 1986.

Mooney, Harold A., and Richard J. Hobbs, eds. Invasive Species in a Changing World. Washington, D.C.: Island Press, 2000.

Nentwig, Wolfgang, ed. Biological Invasions. New York: Springer, 2007.

Pimentel, David, ed. Biological Invasions: Economic and Environmental Costs of Alien Plant, Animal, and Microbe Species. Boca Raton, Fla.: CRC Press, 2002.