The Alien Invasion: Non-native Species Invade the U.S.

What has the power to shut down electrical utilities, disrupt drinking water facilities and cause upwards of $5 billion in damage? An earthquake, hurricane...

What has the power to shut down electrical utilities, disrupt drinking water facilities and cause upwards of $5 billion in damage? An earthquake, hurricane or tornado could do the trick. But just as formidable is the zebra mussel, a thumbnail-size mollusk. Native to the Caspian and Black Seas, the zebra mussel hitched a ride to North America aboard a cargo ship sometime within the last two decades. Today, the zebra mussel has spread throughout lakes and waterways of the eastern United States and Canada, including the Great Lakes and Mississippi River basin. Clustering in massive colonies, zebra mussels clog water supply pipes of power plants, public water systems and industrial facilities. They also attach themselves to native freshwater mussels, interfering with the life cycle of our nation’s most threatened animal group and disrupting natural food chains. The zebra mussel is just one of many invasive species causing ecological and economic damage throughout the world.

The Alien Invasion

Many threats to the health of our natural ecosystems are obvious. Visible evidence includes water and air pollution, habitat loss and overexploitation of natural resources. But just as damaging is something less apparent — the invasion of non-native species.

According to the National Invasive Species Council, an invasive species is defined as one that is non-native to an ecosystem and whose introduction is likely to cause harm to the environment, economy or human health. Invasive species go by many names — non-native, alien, foreign, exotic, introduced and nonindigenous. They can be animals, plants or other organisms (e.g., microbes such as the AIDS virus). Human actions, intentional or accidental, are the primary means of invasive species introductions.

In the past, the Earth’s mountains and oceans provided natural barriers to the movements of many species. Early human migration was responsible for the first intentional introductions of non-native species to much of the world. In the United States, European birds and ornamental plants were brought to the New World to make colonists feel more at home. Non-native plants were imported to prevent soil erosion. Game fish were transplanted to lakes and rivers for sport fishing. The pet trade saw the introduction of exotic animals. Many non-native species arrived by accident via cargo, mail, ship, aircraft landing gear and the soles of people’s shoes.

Today’s increasing human population, expansion of global trade and ease of travel have resulted in an unprecedented number of alien species. About 4,000 non-native plant species and 2,300 non-native animal species live in the United States. Not all non-natives cause trouble, and there are benefits to importing species. Ninety-eight percent of the U.S. food supply comes from introduced species such as wheat, rice, domestic cattle and poultry. But just 79 invasive species have cost the U.S. economy $97 billion in damage. And while most effects are stated in terms of dollar costs to agriculture, forestry, ranching and industry, the biological damage to the nation’s natural forest, grassland and aquatic ecosystems is just as great.

Wreaking Havoc

In an ecosystem, most native plant and animal inhabitants are kept in check by natural biological processes such as competition, predation and disease. An introduced species, however, may not be subject to the biological controls of its new environment and may spread unchecked causing environmental, economic and public health problems.

Invasive species can eat, compete and mate with native species, reducing their numbers and genetic diversity. They can also introduce parasites that sicken and kill native species. Aside from habitat loss, invasive species are the second-most important threat to native species worldwide. In the United States, invasive species have contributed to the decline of 42 percent of all endangered and threatened animals and plants. And according to the American Fisheries Society, invasive species contributed to the extinctions of 27 North American freshwater fishes in the last 100 years.

The economic effects caused by invasive species are staggering. In North America, major Great Lakes water users such as municipalities and power plants spend $30 million every year on zebra mussel control. Invasive fish species such as the sea lamprey, ruffe and round goby have reduced native fish populations of lake trout, walleye, yellow perch and catfish — threatening a $4.5 billion sport and commercial fishing industry. And according to the U.S. Department of Agriculture, invasive species cause an estimated $7.4 billion in annual productivity losses of 64 crops.

Public health risks associated with invasive species include the introduction of diseases. The Asian tiger mosquito, introduced to the United States by accident, can transmit serious diseases such as dengue fever and some forms of encephalitis.

Aquatic Exotics

The effects of non-native species on the world’s waterways are tremendous. Invasive species pose one of the four greatest threats to the world’s oceans. Other threats are pollution, habitat loss and overexploitation of natural resources. And just like crops, cattle and poultry, some non-native species are introduced intentionally as food sources — the Japanese oyster, Manila clam and European mussel are raised commercially in the United States. While these foreign species may benefit humans, they can also establish wild populations that destroy native species and entire marine ecosystems.

Invasive aquatic plants such as purple loosestrife, Eurasian water milfoil and hydrilla spread quickly, crowding out native plants. Unchecked, these aggressive plants can impair water quality, navigation, and habitat for fish and wildlife.

Most invaders to oceans, lakes and rivers arrive as stowaways — attached to cargo ships or in the ballast water carried by these vessels. Ballast is any material used to weight or balance an object, and ships used to carry rocks, sand or metal for this purpose. Today, ships use water as ballast. When a ship is not carrying any cargo, it fills tanks with water to maintain its stability, balance and structural integrity. When a ship loads cargo, it discharges the water as needed. According to the International Maritime Organization, shipping accounts for the movement of 80 percent of the world’s commodities and transfers about 10 million to 12 million tons of ballast water across the globe each year. Thousands of marine species may be carried in ballast water in the form of microbes, small invertebrates, eggs and larvae. The majority of species do not survive from port to port. But all it takes are a few survivors to harm native species and their ecosystems. In southern Australia, Asian kelp is displacing the native seabed communities. In the Black Sea, the North American jellyfish has contributed to the collapse of entire commercial fisheries. And in several countries, introduced “red-tide” algae are absorbed by shellfish, which can cause paralysis and even death if consumed by humans.

Other invasive water species find their way to new environments through commercial and home aquariums. When aquarium plants, animals or wastewater are dumped into a storm drain, creek or ocean, the results can be disastrous. Caulerpa seaweed, native to tropical waters in the Caribbean and Indo-Pacific, is an attractive fast-growing plant popular in aquariums. When introduced to a new environment, however, this aggressive species grows as a dense smothering blanket, killing all native aquatic vegetation in its path. This invader has devastated the Mediterranean coast and has recently shown up in Australia and Southern California.

Island Intruders

Plants and animals whose natural range and population size are limited by geographic factors are particularly vulnerable to aggressive introduced species. Island ecosystems have evolved in isolation over millennia and have relatively fewer species than their mainland counterparts. Many island species lack defenses against mainland predators, competitors and microbes. On the Pacific island of Guam, the introduced brown tree snake has eliminated nine of 11 bird species. Native to the Solomon Islands, Papua New Guinea and northern Australia, the snake probably arrived on Guam via military transports in the 1940s. Since then, the snake has reached numbers of 12,000 per square mile in some areas and has no predators.

On the islands of Hawaii and Puerto Rico, the mongoose was intentionally introduced in the late 1800s to kill rats, which were accidentally introduced and destroyed sugar cane crops. The islands still have rats, and the mongooses are preying on native reptiles, amphibians and ground-nesting birds. The mongoose is responsible for the extinctions of at least 12 reptile and amphibian species in Puerto Rico and other nearby islands.

Large introduced mammals including feral goats, pigs, cats and dogs cause major destruction to island ecosystems because they have no natural predators. Cats and dogs prey on bird and rodent populations. Goats have eaten to extinction two plant species on the Channel Islands of California. And pigs can change healthy landscapes into wastelands by digging the soil to root for food. On the islands of Hawaii, the soil disturbance created by pigs provides breeding grounds for introduced mosquitoes that spread disease to native bird species.

The Natural Order

Every plant, animal and microbe plays an integral role in maintaining the health of its native environment. When an invasive species breaks one link in the ecological chain, it affects entire ecosystems. To the casual observer, an invasive species on its own can be a beautiful plant, a cute mammal, a tiny mollusk or a slithering reptile. But to the native animals, plants and humans that rely on healthy ecosystems, invasive species are destructive intruders.

Invasive Species Profiles

Zebra Mussel (Dreissena polymorpha)

Native range: Caspian and Black seas of the Middle East.

Invaded range: Eastern United States and Canada including the Great Lakes and Mississippi River basin.

Comments: The zebra mussel is believed to have arrived as a stowaway via the ballast water of a cargo ship in the late 1980s. Zebra mussels clog water intake pipes by attaching themselves in massive clusters. They also attach themselves to the shells of native freshwater mussels, interfering with their life cycles and disrupting entire ecosystems. Zebra mussel invasions of the Mississippi River are predicted to reduce native mollusk species by as much as 50 percent within 10 years. The zebra mussel has no predators or parasites in its non-native environment.

Green Crab

(Carcinus maenas)

Native range: European North Atlantic coast.

Invaded range: East Coast of North America since early 1800s; West Coast of North America since 1989.

Comments: The green crab was discovered in California’s San Francisco Bay in 1989. It may have arrived via shipments of lobster or live bait from the East Coast. Green crabs have voracious appetites and will eat native clams, oysters and crabs — threatening the ecosystems and shellfish industries of the Pacific Coast.

Hydrilla (Hydrilla verticillata)

Native range: Asia, Africa and Australia.

Invaded range: Southeastern United States, Texas, New Mexico, California and Washington.

Comments: An aquatic plant, hydrilla was introduced to Florida in the 1950s to be grown for the aquarium industry. The plant proliferated to other states, and by 1995 made its way to California and Washington. Hydrilla can grow 10 inches (25.4 cm) a day and spread from a few acres to several thousand acres in a few years. Covering the surface of lakes and rivers, hydrilla can outcompete native vegetation, reduce fish and other aquatic populations and eliminate open-water feeding areas for birds. It can also clog waterways making them inaccessible to swimmers and boaters. Hydrilla has invaded close to half of Florida’s public waters.

Caulerpa Seaweed (Caulerpa taxifolia)

Native range: Tropical waters of Florida and the Caribbean, and the Indo-Pacific.

Invaded range: Mediterranean Sea, Australia, Southern California.

Comments: Caulerpa seaweed is an attractive fast-growing plant popular in aquariums. It is believed that it was introduced to the Mediterranean around 1984 in wastewater from the Monaco Aquarium. This aggressive species grows as a dense smothering blanket, killing all native aquatic vegetation in its path. In 2000, it was discovered by divers in San Diego, most likely arriving via wastewater from a home aquarium. Because of its invasive nature, the state of California made it illegal to sell this plant in the aquarium trade.

Feral Pig (Sus scrofa)

Comments: Feral pigs are domestic animals that have escaped or have been released. Introduced to many parts of the world, feral pigs damage crops, transmit diseases and disrupt ecosystems by digging up native vegetation and spreading weeds. In addition to plants, their diet can include juvenile land tortoises, sea turtles, sea birds and reptiles. Some communities value feral pigs for hunting and food, so it is difficult to eliminate this invasive species.

Water Hyacinth (Eichhornia crassipes)

Native range: South America.

Invaded range: More than 50 countries on five continents.

Comments: Water hyacinth is one of the worst aquatic weeds in the world. It is a popular ornamental plant for ponds due to its large purple flowers. The plant grows quickly and can double its population size in 12 days. Water hyacinth covers the water’s surface, blocking sunlight and disrupting the ecology of native aquatic life. It also clogs waterways, limiting boat traffic, swimming and fishing.

Brown Tree Snake (Boiga irregularis)

Native range: Solomon Islands, Papua New Guinea, northern Australia.

Invaded range: Guam.

Comments: The brown tree snake probably arrived on the Pacific Island of Guam in the 1940s on military airplanes. It is responsible for eliminating nine of 11 native bird species. The snake also hurts the economy by crawling along electrical lines and causing power outages about every four days. The snake has made its way to Hawaii on six occasions via aircraft from Guam, and was found in cargo in Texas.

What You Can Do

The Nature Conservancy recommends the following to prevent and control invasive species:

– Know your own backyard. Learn to identify your region’s most threatening pests. Find out whom to contact to report new invasions, or to receive guidance on controlling pests on your property.

–  Landscape with native species or noninvasive ornamental plants appropriate to your region.

– Don’t release pets or aquarium plants and fish into the environment.

– Avoid disturbing natural areas; it increases their vulnerability to alien species invasions.

– Be careful not to send or receive potentially harmful plants or animals through the mail. Use mail-order services wisely.

– Don’t bring plants, fruits, soil or animals into the country from abroad, or to Hawaii from the mainland without having them inspected by quarantine officials. Fill out agricultural declaration forms completely and honestly.

– Clean boats and boating equipment before transporting them from one water body to another to avoid spreading aquatic pests such as zebra mussels or hydrilla. Leave behind unused bait and bucket water. Clean your boots and camping gear before setting out for other regions or countries, and again before returning home. On horse-packing trips, make sure that feed is certified weed-free.

– Spread the word. Educate yourself and others about the problem of alien species.

– Get involved. Join volunteer efforts to remove invasive species in natural areas, such as local preserves and state or national parks.


The problem of invasive species is so widespread that international efforts are under way to control and eliminate these destructive intruders. For more information, visit the following Web sites.

World Conservation Union Invasive Species Specialist Group:

National Invasive Species Council:

U.S. Geological Survey Nonindigenous Aquatic Species:

Aquatic Nuisance Species Task Force:

By Amy Gulick