Invasive Species

When a species ends up in a new environment, it may or may not survive. If it does survive, it may do too well.

In most cases, a species fails to survive in a new environment. Usually conditions do not favor its survival. But in a few cases, a species manages to thrive. It may not have natural predators in the new environment. Or it may kill native species that have no defenses against it. Or it may multiply so fast that it simply crowds out native species. All these things can produce environmental destruction and species extinction.

An introduced species that overgrows in its new habitat is called "invasive." Invasive species include all manner of organisms, from microbes to mammals.

Native species sometimes evade natural controls. They multiply and become nuisance species. One example is the Canada goose. It changed its migratory patterns to thrive in developed areas. It now crowds out other goose varieties. And it fouls parks and lawns with droppings. Another nuisance species is the white-tailed deer. It often wanders beyond forested areas to graze in suburban gardens.

Invasive Insects

Some invasive species spread to new habitats gradually. The Africanized honeybee presents a well-known example. In the 1950s, scientists hoped to improve beekeeping in South America. The tropical conditions there did not suit common honeybees. These bees originally came from Europe.

Scientists decided to try importing honeybees from Africa. While these bees were being studied, some of them escaped.

The new bees began breeding with the European bees. The result was a hybridized, "Africanized" honeybee. These bees were more fierce than the European bees. They attacked people and livestock with little provocation. They acquired the nickname "killer bees."

Africanized honeybees have spread through South and Central America. They have spread into the southern United States.

Africanized honeybees displace populations of native bees. This causes widespread harm. The invading bees are more difficult to manage. Planned pollination becomes more complicated. And honey production may fall.

Many invasive insects destroy native plants. The emerald ash borer (EAB) is native to Asia. It crossed the Pacific during the 1990s. Most likely it arrived aboard cargo in ships or airplanes.

The EAB was first discovered in Michigan in 2002. By 2014, the EAB had spread to 22 states and two Canadian provinces. The EAB has killed tens of millions of ash trees.

Other Invasive Animals

The Galápagos Islands have long inspired naturalists. These islands have many unique species and are isolated from other landforms. As a result, the islands constitute something of an open-air laboratory. Yet even this remote sanctuary faces the threat of invasive species.

Harmful invaders include large mammals and tiny insects. For example, donkeys and goats threaten to eliminate plants that sustain native animals such as cactus finches, mockingbirds, and land iguanas. And fire ants brought to the islands have killed tortoise and bird hatchlings.

Island species prove highly vulnerable to invaders. For instance, the Hawaiian Islands are home to many flightless birds. These birds survived for millions of years in the absence of natural predators. But they proved defenseless against introduced animals such as dogs and cats.

As of 2013, 29 Hawaiian bird species were known to have been eliminated since 1778. (In 1778, the first Europeans visited the islands.) Besides invasive species, factors contributing to the extinctions included hunting and habitat destruction.

In the last 200 years, Australians have imported many destructive species. Many of these species are ferals. These are animals that became wild after they were introduced for domestic purposes. Ferals in Australia include the rabbit, fox, and cat. Without natural predators, these animals multiply unchecked. The rabbits overgraze plant life. The foxes prey on vulnerable marsupials. And the cats kill ground birds.

Another notable feral in Australia is the cane toad. It was introduced to control the native cane beetle. The cane toad did little to suppress the cane beetle. It spread diseases harmful to native species. And it poisoned native predators that were unable to tolerate the cane toad's toxic secretions.

Invasive species have long plagued the Great Lakes and its tributaries. Recent examples include the zebra mussel and the fishhook water flea. Asian and European ships introduced these pests when they dumped their ballast water. Both species consume plankton. Plankton forms the base of aquatic food chains. So its depletion endangers many native fishes.

Invasive Plants

Invasive plants can wreak havoc on entire ecosystems. Gardeners introduced Asia’s water hyacinth into Florida in the 1880s. It spread throughout the waterways of the Southeast. It clogs streams and rivers. Another harmful invader plant is purple loosestrife. It is present in every state except Florida. Yet another harmful invader plant is kudzu. It is present throughout the Southeast.

Plants have traveled across oceans. Seeds of common ragweed were carried from North America to France during World War I. The weed has spread through Europe. It is viewed as a health threat. Its pollen causes hay fever and worsens asthma. In Great Britain, the sale of floating pennyroyal and water primrose is forbidden. These aquatic plants grow in dense mats. They crowd out native species and choke rivers. South America also has invasive plants. In Chile the major problems are Mediterranean plants. They were brought by Spanish explorers. The sweet briar rose is widespread. There is little hope of its complete removal. Botanists are more optimistic about rooting out the Portuguese broom. It has just begun to take hold.

Impact of Wider Travel and Trade

The carrying of nonnative species has increased in all directions. The cause? Worldwide travel and trade.

The giant hogweed was carried to Canada from Asia. Gardeners planted it in their backyards. Now it is crowding out native species.

The American bullfrog was imported to four continents. People kept them as pets and to control insects. It is now considered an invasive species in France. In South America, the bullfrog is pushing out the native Pizarro's frog.

The Louisiana crayfish was sent to fish farms in China. It escaped. Now it infests streams throughout the country.

The brown tree snake of Australia hid in cargo shipped to Guam. There it encountered no native predators and abundant food sources. The snake has since wiped out the bird and rodent populations on the island.

And the list goes on. European sand dune grass threatens native grasses in South Africa. Asian carp lurks in the rivers of the American midwest. In Ireland the New Zealand flatworm preys on native earthworms. Farmers rely on the native earthworm to improve soil quality. Burmese pythons now colonize the Florida Everglades.

Scale of the Problem

Alien species are found almost everywhere on Earth. In some cases they become invasive. They change ecosystems and interfere with the success of native species. Sometimes they prey on native species. Sometimes they compete with them for nutrients and sunlight. The consequences of invasive species are far reaching. They harm human health by spreading disease. They harm economies, too. Billions of dollars are lost each year to damage in agriculture. Most of all, the environment suffers. Invasive species harm native species. This leads to unhealthy ecosystems. Next to habitat destruction, invasive species are the main cause of extinction on Earth.

The disruptions caused by invaders have given rise to a field of study. It is called invasive ecology. The disruptions also have prompted educational programs. These programs are aimed at the public and inspectors at airports and seaports. They explain how to curb the spread of invasive species.

SOURCE: The New Book of Knowledge

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