Invasive species are non-native organisms that pose a threat to an ecosystem, to the environment, to the economy, or to human health. They may be animals, plants, or microorganisms that usurp the habitats of native life forms, causing them to decline in population or to disappear from their natural environment. These organisms are introduced either accidentally or intentionally by human beings or their activities. Not all introduced species are invasive; however, an organism that is beneficial in one place may become a nuisance in another. Species described as “introduced” are not considered a threat to their new environment, whereas invasive species are regarded as pests.
Factors that Make a Species Invasive
In most cases, invasive species are very competitive, highly adaptive, and extremely successful at reproducing. Factors relating to the new environment, however, are also important. For example, an organism may have been held in check in its place of origin by predators; if its new environment lacks predators, there may be nothing to stop it spreading uncontrollably. A predatory animal in its natural environment may be part of a stable ecosystem, as prey animals have adapted to deal with it. In a new environment, where potential prey lack these adaptations, it may threaten other species with extinction.
How New Species Are Introduced
Organisms can spread outside their native habitats through international trade and travel. Insects, fungi, and microorganisms can arrive on imported fruit and vegetables, on garden and houseplants, and in soil carried with these items. In some cases, imported garden plants themselves have become invasive. People can unwittingly carry microorganisms and even plant seeds from one country to another. Ships can carry a whole host of potentially harmful life forms, from marine organisms clinging to the sides or in ballast water to small mammals, such as rats and mice, inside the ship itself.
Many invasive species have been introduced deliberately. Animals kept as pets can, if allowed to breed in the wild outside their original environment, become a major threat to an ecosystem. Animals and plants have sometimes been brought to new environments for agricultural or other commercial purposes, only to become a major pest. Some organisms have been imported in an attempt to control other invasive species.
Examples of Invasive Species
The domestic cat, native to northeast Africa, was introduced from Egypt to countries worldwide thousands of years ago. Although cats are much-loved house pets in many places, they are also highly effective predators of small mammals and birds. These qualities led to their introduction into many areas to control rodents, but also to them threatening other species with extinction. In New Zealand, for example, cats have had a major impact on native mammals and birds, which evolved in the absence of predatory mammals and so have no defenses against them. Domestic cats may also interbreed with native wild cats, with the potential result that the native species is gradually replaced by hybrid animals.
Imported animals may also threaten plant populations. Grazing animals, such as goats and sheep, have been introduced to many areas and allowed to roam free, with devastating effects on the local flora. Due to heavy grazing, many rare plants are threatened with extinction and forests are unable to regenerate, as seedlings and small saplings are quickly consumed.
Many plant species have become a serious problem after arriving in a new area. A dramatic example was the introduction of the “prickly pear” cactus into Australia in the early 20th century. It was intended to be a cheap and natural form of fencing and to help establish a dye industry, but it quickly spread out of control, forming dense and impenetrable thickets over large patches of agricultural land. After several failed attempts, it was eventually brought under control by the introduction, from the plant’s natural habitat, of a moth whose caterpillar eats the cactus. This moth has itself become an agricultural pest, after spreading to some parts of North America.
Pampas Grass, originally from South America, is a large ornamental plant often found in gardens, and it is sometimes planted in other areas to help stabilize soil. In some parts of the world, however, it has become a problem, competing with native and agricultural plants, and sometimes posing a fire hazard, due to the accumulation of dry, dead leaves.
Microorganisms and fungi, which can be transported in the form of invisible spores, pose a more insidious threat. Many plant diseases have entered new areas with devastating effects, carried there by humans and goods in ways that are often difficult to determine. Diseases caused by fungi and viruses can decimate crops, and in some cases, whole trees can be killed by a rapidly spreading fungal infection.
The Cost of Invasive Species
It is estimated that half of all the species on the endangered species list — 57% of animals and 39% of plants — are declining at least in part due to invasive species. They cost the United States an estimated $128 billion US dollars (USD) annually and $400 billion USD worldwide due to economic losses such as damage to crops. Much of this money is also spent on the prevention of invasions, education about the effects of these organisms, and control measures.
Prevention and Control
Efforts to prevent invasive species from becoming established in new areas focus on tighter import controls, checks on imported goods, and, where practical, subjecting goods and materials to treatment with insecticides or sterilization procedures. Some countries have banned the import of certain plant and animal species. Control of species that have already become established can be difficult. The methods used can include pesticides for plants and insects, physical removal of large plants, culling of animal pests, and the introduction of natural predators for plants and small animals.