When you think of invaders, you might think of extraterrestrial life or the unwanted creatures that occasionally scramble across your countertop. But what about the invaders that wreak more havoc than an average household nuisance? Invasive species -- or organisms that overtake and negatively affect existing plants, animals or humans after being introduced to a new environment -- cause an estimated $1.4 trillion in damages per year [source: Global Invasive Species Programme]. Often, an animal that evolved in one place can be invasive in another if it has enough food and few or no natural predators.
If you've seen kudzu's blanketing grasp on the southeastern U.S., the annoying presence of fire ants in your backyard or maybe chytrid fungus's devastating impact on amphibian populations, you're familiar with invasive species. These species are not only hard-hitting in the economic and biodiversity departments, they can also spread diseases and harm human health. So who's responsible for these invasions? In most cases humans are to blame, especially with increased travel, trade and exotic pet ownership in recent decades.
Of the thousands of invasive taxa to choose from, the five species on this list will surely penetrate your thoughts and -- who knows -- maybe your backyards, too.
See which animal spurred Australia to build a 1,139-mile (1,834-kilometer) long pest-exclusion fence on the next page.
This first invasive species has ravaged its new environment, all right -- so much so that the Australian government built a 1,139-mile (1,834-kilometer) fence in the early 1900s to prevent these critters from spreading [source: National Library of Australia News]. No need to blame an obscure or exotic species -- we know this bushy-tailed culprit well. Thomas Austin purposefully introduced the European rabbit (Oryctolagus cuniculus) to his estate in Geelong, Australia, in the 1850s. Somehow, that handful of rabbits escaped their enclosures and multiplied to 10 billion by 1926 [source: State Government of Victoria]. This furry invasion also inspired the commonly coined phrase "breeding like rabbits" [source: Introduced Species Summary Project].
As you can imagine, billions of rabbits require lots of space and food, and these guys have had no problem taking what's prime for the picking. Since they can eat a variety of plant matter, reproduce quickly and lack natural predators, European rabbits are still menaces to control. They have ruined grasslands used for grazing and competed with native species by digging burrowlike warrens that displace native creatures.
Over time, the government built more fences, but it couldn't cripple the rabbits' thirst for new territory. After years of construction, the rabbit fence currently stretches more than 2,000 miles (3,218 kilometers) today, and the Australian government is still looking for ways to better control rabbit populations. Researchers are now testing whether viruses or hormones can rein in populations in addition to the physical eradication methods in place.
Think these lagomorphs had an easy trip to Australia? This next invasive species hitched a ride across the globe using an unusual mode of transportation -- for a bivalve, that is.
4: The Zebra Mussel
Forget riding as a passenger on a nautical trip -- this species has found an easier way to catch a lift. Most speculate the zebra mussel (Dreissena polymorpha) found its way from Eastern Europe to the Great Lakes in the late 1980s by traveling through ship ballast water, or water stowed away for balance and released once docked again [source: McMahon, 1996]. This explanation makes sense, especially since these intrusive bivalves' larvae float well and are nearly invisible to the naked eye.
Though zebra mussels measure around 2 inches (5 centimeters) in length, it's their collective presence that makes them notable aquatic invaders. These shelled creatures can attach themselves to nearly any surface, including metals and the shells of other bivalves, and have a penchant for clogging water intake pipes. Industries that use water from the Great Lakes have spent millions of dollars removing zebra mussels from pipes. En masse, these mussels have a clogging effect and hinder water flow, which is problematic for companies that need water to cool off industrial equipment.
Zebra mussels have been an economic nuisance, but recent concerns have shifted to the species' impact on biodiversity, as it competes with native species for space and algae food. It would be nearly impossible to eradicate these mussels, but teaching people how to prevent their spread is increasingly common in many lake and river areas, even farther south of the Great Lakes where the species has been spotted.
Up next, see which accidentally introduced species is illegal to own in one area of the U.S.
3: Caulerpa Seaweed
This next invasive species was a benign aquarium decoration until its oceanic escape. Originally from the Gulf of Guinea, Caulerpa seaweed (Caulerpa taxifolia) was thought to have been released from an aquarium into the Mediterranean Sea by accident in the mid-1980s. The species soon earned the moniker "killer algae" after blanketing and smothering organisms on the ocean floor in the Mediterranean, in marine habitats near northern Africa and in coastal waters near the U.S. and southern Australia.
By growing thick, green fronds on top of other seaweeds, corals and sponges, Caulerpa seaweed undermines biodiversity and covers seafloors that many fish species use as nursery sites. This unruly alga spreads when improperly disposed of in the ocean or when seaweed cuttings are introduced to a new area. Not to worry, though -- apparently local septic systems thwart Caulerpa from spreading if some seaweed finds its way down the toilet [source: Wong].
In 2000, dense clumps of Caulerpa appeared off the coast of Southern California, but local officials nipped it before the seaweed could permanently establish itself. As a result, the San Diego, Calif., government made it illegal for anyone in the area to own or sell plants of the genus Caulerpa -- though a black market for this decorative plant has remained a problem in recent years [source: Johnson]. The seaweed has been controlled well in the U.S., but other areas, including the Mediterranean, aren't as fortunate.
You'll need a microscope to see this next airborne invader.
2: Soybean Rust
This invader is not only hard to detect, it's so small it can be carried miles away by wind without you even noticing it. This species, Phakopsora pachyrhizi, is an invasive fungus that causes a disease in soybean crops called "soybean rust." In addition to its recent discovery in the southern U.S. in 2004, this fungus is no stranger to soybeans in south Asia, southern Africa and parts of South America. Farmers usually know when the fungus has found a permanent home on their plants after seeing dusty, rustlike pustules on the undersides of diseased soybean leaves.
The fungus doesn't hamper soybean production altogether, but it can drastically decrease pod production, which can raise prices for many foods and commodities we depend on, such as certain meats and cooking oils. Since 79 percent of all edible oil consumed in the U.S. comes from soybean oil, it's hard to ignore the impact this species can have on local food supplies [source: EPA]. Soybean's high-protein content also makes it suitable to feed commercially produced poultry and swine. It's also the second largest economically profitable crop in the U.S. following corn, so farmers will likely duke it out with the fungus [source: United States Department of Agriculture]. Despite this fungus's wind-traveling powers, farmers can slow its spread by cleaning shoes and farming equipment after use.
Our last invasive species prefers the taste of blood over soybeans any day. Click on to read about the tiniest tiger in the world.
Soybean production can be reduced dramatically once soybean rust takes hold.
Image Credit: Hemera/Thinkstock
1: The Asian Tiger Mosquito
Often tucked away in bamboo and rubber tire shipments, female Asian tiger mosquitos (Aedes albopictus) are known vectors for the infectious disease dengue in Asia, but how harmful are they in their new homes in the Americas, Europe and Australia? It's not exactly clear, but researchers speculate the mosquito could be a possible carrier of West Nile virus and Japanese encephalitis -- both potentially fatal conditions. In addition to competing with other native species, the mosquito carries a few viruses harmful to humans and a handful that spread disease in other animals, including Eastern equine encephalitis in horses.
But what sets these guys -- or gals, shall we say -- apart from other mosquitoes are their unique appearances and behaviors. With black, shiny scales, striped legs and a vertical white stripe extending down the mosquito's back, you'll know when you see one of these invaders. Also, females of the species prefer to eat (suck blood) during the daytime rather than at dusk or night. Since this mosquito often feeds on more than one host in order to lay eggs, it can transmit diseases more easily, too. Preventing these mosquitoes from breeding is an important step in managing their populations. As with other mosquitoes, limiting standing water restricts where these tiny tigers can lay eggs.
Keep reading for more information on the organisms that populate our planet, invasive and otherwise.
The Asian tiger mosquito bites during the daytime rather than at night.
Image Credit: Getty Images
"Aedes albopictus (insect)." Oct. 27, 2007. Global Invasive Species Database. (March 25, 2010).http://www.issg.org/database/species/ecology.asp?si=109&fr=1&sts=
"Asian Soybean Rust Fast Facts." Unitedsoybean.org. (March 25, 2010). http://www.stopsoybeanrust.com/images/USBfast.pdf
Benson, A. J. & D. Raikow. "Dreissena polymorpha." Aug. 28, 2009. USGS Nonindigenous Aquatic Species Database, Gainesville, Fl. (March 25, 2010). http://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=5
"Caulerpa Taxifolia (alga)." April 18, 2006. Global Invasive Species Database. (April 2, 2010).http://www.issg.org/database/species/ecology.asp?si=115&fr=1&sts=&>
Chhun, Nokmenee. "Rosy wolfsnail (Euglandina rosea)." Nov. 19, 2002. Introduced Species Summary Project, Columbia University. (March 25, 2010).http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Euglandina_rosea.html
Dorrance, Anne; Lipps, Patrick; Mills, Dennis; & Vega-Sánchez, Miguel. "Soybean Rust." The Ohio State Extension Factsheet. (March 25, 2010).http://ohioline.osu.edu/ac-fact/0048.html
"European Wild Rabbit (Oryctolagus cuniculus) Australian Government Department of Heritage. (March 24, 2010).http://www.environment.gov.au/biodiversity/invasive/publications/pubs/rabbit.pdf
"Frequently Asked Questions about the Zebra Mussel." Dec. 22, 2009. Southeast Ecological Science Center. (March 25, 2010).http://fl.biology.usgs.gov/Nonindigenous_Species/Zebra_mussel_FAQs/zebra_mussel_faqs.html
Hayes, Andrew & Richardson, Barry. "Biological Control of the Rabbit in Australia: Lessons not Learned?" September 2001. TRENDS of Microbiology. Vol. 9, No. 9.
"How Soybeans are Used." North Carolina Soybean Producers Association. 2007. (March 25, 2010).http://www.ncsoy.org/ABOUT-SOYBEANS/Uses-of-Soybeans.aspx
"Information on Aedes albopictus." The Centers for Disease Control and Prevention. Nov. 7, 2005. (March 25, 2010).http://www.cdc.gov/ncidod/dvbid/arbor/albopic_new.htm
Johnson, Christina. "Caulerpa Update: Invasive "Killer" Seaweed Controlled. Feb. 2, 2005. Sea Grant, University of Southern California. (April 2, 2010).http://www-csgc.ucsd.edu/NEWSROOM/NEWSRELEASES/CaulerpaUpdate.html
Johnson, Christina. "Caulerpa Laws Difficult to Enforce Illegal Sales Continue." Aug. 3, 2006. Sea Grant, University of Southern California. (April 2, 2010).http://www-csgc.ucsd.edu/NEWSROOM/NEWSRELEASES/CaulerpaLaws_SGNewsList.html
Matthews, S. & Brandt, K. "Africa Invaded: The Growing Danger of Invasive Alien Species." Global Invasive Species Programme. 2004. (March 25, 2010).http://www.gisp.org/casestudies/showcasestudy.asp?id=69&MyMenuItem=casestudies&worldmap=&country=
McMahon, Robert. (1996). The Physiological Ecology of the Zebra Mussel, Dreissena polymorpha, in North America and Europe. American Zoologist, Vol. 36, 339-363.
Moore, Chester & Mitchell, Carl. (1997). "Aedes albopictus in the United States: Ten-Year Presence and Public Health Implications." Emerging Infectious Diseases, Vol. 3, No. 3. 329-334.http://www.cdc.gov/ncidod/eid/vol3no3/moore.htm
Noticewala, Sonal. "At Australia's Bunny Fence, Variable Cloudiness Prompts Climate Study." Aug. 14, 2007. The New York Times. (March 25, 2010).http://www.nytimes.com/2007/08/14/science/earth/14fenc.html?ex=1344744000&en=08233f5483387e1b&ei=5124&partner=permalink&exprod=permalink
Schepmans, Flavia. "European rabbit (Oryctolagus cuniculus) Introduced Species Summary Project, Columbia University. Feb. 17, 2003. (March 25, 2010). http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Oryctolagus_cuniculus.htm
"Soybean and Oil Crops." ERS/USDA Briefing Room. March 9, 2010. (April 2, 2010). http://www.ers.usda.gov/Briefing/SoybeansOilCrops/
Suiter, Dan. "Kudzu-eating Bug." Personal Interview. March 15, 2010.
"West Nile Virus and the Asian Tiger Mosquito." University of Maryland College of Agriculture and Natural Resources. Aug. 8, 2008. (March 25, 2010).http://extension.umd.edu/gardening/mastergardeners/local/Charles/westNileVirus.cfm
Woodfield, Rachel. "Noxious Seaweed Found in Southern California Coastal Waters." July 17, 2008. Southwest Regional Office of National Marine Fisheries Services. (March 25, 2010).http://swr.nmfs.noaa.gov/hcd/CAULERPA.htm
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