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
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
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.
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.
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
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
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.
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
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
The Natural Order
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
Zebra Mussel (Dreissena polymorpha)
Native range: Caspian and Black seas of the
Invaded range: Eastern United States and Canada
including the Great Lakes and Mississippi River
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.
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.
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
Caulerpa Seaweed (Caulerpa taxifolia)
Native range: Tropical waters of Florida and the
Caribbean, and the Indo-Pacific.
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
Water Hyacinth (Eichhornia crassipes)
Native range: South America.
Invaded range: More than 50 countries on five
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,
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:
t 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.
t Landscape with native species or noninvasive
ornamental plants appropriate to your region.
t Don’t release pets or aquarium plants and fish
into the environment.
t Avoid disturbing natural areas; it increases
their vulnerability to alien species invasions.
t Be careful not to send or receive potentially
harmful plants or animals through the mail. Use
mail-order services wisely.
t 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.
t 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.
t Spread the word. Educate yourself and others
about the problem of alien species.
t 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