The impact of invasive species

Invasive species are among the leading threats to the native wildlife of most countries, with approximately 42 percent of endangered species at risk from them. Invasive species can be any kind of organism - for example, a mammal, amphibian, fish, insect or plant - that is not native to an ecosystem. Often they reproduce and spread with great speed. Contrary to popular belief, a plant or animal classified as an invasive species does not necessarily originate in another country. For example, lake trout are native to the Great Lakes of North America, but are considered an invasive species in Yellowstone Lake in Wyoming because they compete with native cutthroat trout for habitat.

When a new species is introduced into an ecosystem, native wildlife may struggle to compete with it for food and other resources. Invasive species can change the food web in an ecosystem by destroying or replacing native food sources, while providing little or no food for local wildlife. In extreme cases, the invader may prey on native species. Above all, invasive species threaten biodiversity in many habitats. For instance, the climbing plant species kudzu (which is native to East Asia) can easily replace a habitat that had a wide range of plants with a monoculture consisting solely of kudzu, as has started to happen in the south-east of the USA.

The phenomenon is not an exclusively modern one. Humans have always transferred a variety of species from one region to another, but the development of rapid means of transportation has increased the frequency of such introductions. Aquatic organisms can be shipped across the oceans in the water that fills the wooden crates that are used to transport goods, while insects can easily get into the cargo itself. In addition, climate change has enabled some invasive plant species to colonise new areas. Other invasive species include pets such as snakes or turtles which are intentionally set free into the wild.

Invasive species do not all present the same level of threat to native ecosystems and can be classified into three types. The first of these may be introduced species which can maintain themselves in a limited range of habitat without upsetting the ecological equilibrium of the area. Some scientists have even argued that in these cases the introduction enhances the diversity of animal and plant life in that specific area. A second group of invaders present a greater threat because they spread at the expense of native species. The North American grey squirrel, for example, was introduced to the UK in 1876 largely because wealthy landowners thought it would be a fashionable and attractive addition to local wildlife on their estates. It spread widely, leading directly to the dramatic decline in the population of the native red squirrel. However, it would appear that this is the only definite negative impact of the grey squirrel.

There is a third level of threat in which the dominance of the introduced species has an extremely destructive effect on the entire ecosystem. One of the most damaging examples involved a species of comb jellyfish. Native to estuaries along the western Atlantic coast from the northern United States to the Valdes Peninsula in Argentina, this species was released from a Black Sea ship into the Black Sea in Eastern Europe in 1982, almost certainly by accident. The Black Sea has levels of industrial waste which are by international standards exceptionally high. Despite this, fishing boats were still able to catch healthy numbers of fish. But when the invading jellyfish underwent a population explosion in the space of just six years, the entire marine ecosystem was transformed, and fish numbers declined dramatically because they were in competition with the jellyfish, preying on exactly the same microscopic creatures. The jellyfish had a more serious impact on the ecosystem than the heavily polluted water.

More than a century after its introduction outside its native range in the Amazon River in South America, a plant known as water hyacinth can now be found on tropical lakes, streams, and rivers around the world. It was imported to a horticultural exhibition in New Orleans in 1884. Visitors were so impressed by its beauty that they planted it in many locations during the 1880s and 1890s, resulting in the aquatic ecosystems of the south-eastern United States being progressively colonised by vast, floating carpets of water hyacinth. Today it is present around the globe, damaging boat engines and even blocking cooling pipes for power plants, occasionally leading to massive blackouts. The impact of the water hyacinth on native water plants is largely unstudied, as is the case for most invasive species introduced water.

In the United States, more than 7,000 introduced species have established themselves, of which at least 15 percent cause ecological damage. As the number of invasive species expands, legislation to deal with this problem is rare or non-existent in the majority of countries. Unfortunately, ordinary people outside the scientific community have a very limited understanding of the threat posed by invasive species, which means that other environmental threats receive considerably more media attention. The introduction of new species can initially seem highly desirable, but the full extent of their impact is consistently underestimated.

Although ultimately measures need to be taken at an international level, limited action is possible by individuals. One way is for people to plant native plants in their gardens rather than species from abroad. It was also useful to learn to identify invasive species and report any sightings to wildlife organisations. Regularly cleaning clothing, boots, boats, tyres, and any other equipment regularly used outdoors can remove insects and plant parts that may introduce invasive species into new locations.