NOVA Home Find out what's coming up on air Listing of previous NOVA Web sites NOVA's history Subscribe to the NOVA bulletin Lesson plans and more for teachers NOVA RSS feeds Tell us what you think Program transcripts Buy NOVA videos or DVDs Watch NOVA programs online Answers to frequently asked questions
Deep Sea Invasion

Matching Aliens With Impacts
by Rick Groleau and Rebecca Deusser

Deep Sea Invasion homepage

Next to the loss of habitat, alien invasion—the introduction and spread of species not indigenous to a location—is the leading cause of species endangerment and extinction.

Here, read about the 10 plants and animals that have significantly altered habitats following their introduction.

Cane toad (Bufo marinus)

Cane toad


The cane toad competes with native amphibians and is also highly toxic. It excretes toxins as a way to protect itself against predators, and it may sicken or kill small animals (including cats and dogs) that bite it. Native to the southern U.S., Central America, and northern South America, these animals have spread to Australia and some islands in the South Pacific.

Cane toads were deliberately introduced to Queensland, Australia, in 1935 to control two beetle pests of sugarcane. With no natural predators, the toads quickly spread. They were not effective at controlling the beetles.

Crazy ant (Anoplolepis gracilipes)

Crazy ant


The crazy ant, which originated in either Africa or Asia (scientists are not sure which), is found in many cities in the tropics, in parts of the U.S., and on many tropical islands from Hawaii to Zanzibar. On Christmas Island in the Indian Ocean, its kind live in "supercolonies" and prey on red crabs. Crazy ants have killed as many as three million crabs in one 18-month period there.

The crazy ant, which gets its name from its rapid and erratic movement, also preys on insects, reptiles, birds, and small mammals. Hordes of crazy ants have even been known to attack and kill pigs and dogs. In the fall it eats the sugary droppings of sap-sucking insects such as aphids and mealybugs, which it protects from predators. Left unchecked, these insects increase in numbers, resulting in damage to the rainforest canopy.

Feral pig (Sus scrofa)

Feral pig


Ancestors of the feral pig made their way back to the wild, either by escape or release. Found in many parts of the world, the feral pig damages crops by trampling and "rooting." It attacks and feeds on livestock (mainly lambs) and causes the spread of weeds. It also spreads diseases such as leptospirosis and foot-and-mouth disease. Feral pigs feed primarily on plants but also on earthworms, insects, amphibians, reptiles, and birds.

Miconia (Miconia calvescens)



This plant from Central and South America was brought to Tahiti in 1937 and planted in a botanical garden. Fruit-eating birds have spread its seed, and now miconia covers more than half the island. It has become the dominant canopy tree in Tahiti, shading out many native plants. Unlike the island's indigenous trees, it has a shallow root system, which has led to landslides where the trees have grown on hillsides. It thrives in the tropical climate and spreads quickly. In its native habitat of Central and South America, fungi and insects control its spread.

Purple loosestrife (Lythrum salicaria)

Purple loosestrife


This plant from Europe and Asia came to North America in the early 1800s on the wool of European sheep and within ships that used soil as ballast. It was also brought as an ornamental plant and as a source of nectar for bees kept by beekeepers. By the 1830s the plant was common along the east coast of the U.S. Purple loosestrife is now found throughout most of the U.S. and Canada. It spreads rapidly and invades wetlands by displacing native plants, creating an environment that provides little food or shelter for native wildlife.

Small Indian mongoose (Herpestes auropunctatus)

Small Indian mongoose


This predator, which is native to areas from Iran to Indonesia, was introduced to several locations in the 1800s to control rats, another invasive species. It was first brought to Puerto Rico in 1877 to control the black rat. Since then it has caused the extinction of seven species of reptiles and amphibians on the island. The small Indian mongoose also spreads diseases such as leptospirosis. It can make its home in a range of habitats, from forests to open plains.

The small Indian mongoose is now found in Costa Rica, Hawaii, Fiji, the islands of the Caribbean, and other places around the world.

Mosquito fish (Gambusia affinis)

Mosquito fish


The mosquito fish, a guppy-like fish that grows up to three inches long, may look harmless, but it has become a pest around the world. Native to the eastern and southern U.S., it was originally introduced in many areas to control mosquitoes, but it also feeds on the eggs of many commercial and rare fish. Moreover, it is an aggressive little fish that attacks other fish, big and small alike. Although the mosquito fish is an invasive species, using it to control mosquito larvae in ornamental ponds, unused swimming pools, and animal troughs is still considered acceptable.

Strawberry guava (Psidium cattleianum)

Strawberry guava


The strawberry guava, which is native to the Atlantic coast of Brazil, has now spread to Polynesia, Mauritius (an island in the Indian Ocean), Hawaii, Puerto Rico, and other tropical locations around the world.

In Hawaii, the feral pig, another invasive species, assists in the spread of strawberry guava by eating its fruit and dispersing its seed. The plant was brought to Hawaii (and elsewhere) for its edible fruit but soon after its introduction escaped from gardens. A shrub or small tree that can reach 15 feet in height, the plant not only shades out underlying native vegetation, it also sends out feeder roots that inhibit native plants' growth. In Hawaii, strawberry guava is considered an especially invasive plant pest.

Zebra mussel (Dreissena polymorpha)

Zebra mussel


This species arrived in the lower Great Lakes of the U.S. either attached to the hulls or loose in the ballast water of oceangoing ships. Zebra mussels can filter nearly a quart of water a day. As they do so they feed on plankton, consuming food that native shellfish rely on for their survival. They clog the piping systems of industrial plants and power utilities and can interfere with locks and dams on rivers. Zebra mussels also damage boat hulls and engines.

Native to the Caspian Sea in eastern Europe, the zebra mussel first appeared in Britain in 1824. Since then it has spread to Ireland, Denmark, Sweden, Finland, Italy, and much of western Europe. It first appeared in the Great Lakes in 1988. Interestingly, since its introduction to Lake Erie, the lake has become noticeably clearer, allowing for deeper sunlight penetration and an increase in aquatic plant life.

Water hyacinth (Eichhornia crassipes)



Native to South America, water hyacinth is one of the worst invasive species known. It spreads at extremely fast rates, in favorable conditions doubling its numbers in as little as 12 days. The plant, which floats on the water's surface, can cover large areas, thus limiting boat traffic, swimming, and fishing. By blanketing the surface, the plant also prevents sunlight from entering the water. Moreover, when it dies, the decaying plant matter robs the water of oxygen. The reduced amount of sunlight and oxygen in the water disrupts plant and animal life, adversely affecting the habitat's biodiversity.

Back to top

Interactive version Matching Aliens With Impacts

Matching Aliens With Impacts
Requires Flash

Deep Sea Invasion Web Site Content
Chronology of an Invasion

Chronology of an Invasion
Follow the alarming spread of Caulerpa taxifolia around the world.

The Impact of Invasive Species

The Impact of Invasive Species
Alexandre Meinesz on the degrees of menace posed by alien invaders.

Battling Introduced Wildlife

Battling Introduced Wildlife
Daniel Simberloff on strategies for combating weed-like species.

Matching Aliens With Impacts

Matching Aliens With Impacts
Match 10 successful invaders to the damage they've caused.

Rick Groleau is managing editor of NOVA online and Rebecca Deusser is the site's intern.

Send feedback Image credits
NOVA Home Find out what's coming up on air Listing of previous NOVA Web sites NOVA's history Subscribe to the NOVA bulletin Lesson plans and more for teachers NOVA RSS feeds Tell us what you think Program transcripts Buy NOVA videos or DVDs Watch NOVA programs online Answers to frequently asked questions