Alien Species: Common, Costly and Destructive

GLAND, Switzerland, May 14, 2001 (IUCN) - Invading alien species are responsible for a worldwide biodiversity crisis, driving large numbers of native plant and animal species to extinction on every continent. The damage is documented by IUCN - the World Conservation Union in a new survey of the 100 worst alien species issued in time for Biodiversity Day, May 22.


European starling (Photo by J.S. Spendalow courtesy U.S. Geological Survey (USGS))
The problems are common species such as the domestic house cat or the starling, grown familiar with daily contact, but they and hundreds of other invasive alien species are moving outside their natural range and threatening the existence of native plants and animals.

"After habitat loss, this biological invasion constitutes the greatest threat to biodiversity, and it has already had devastating consequences for the planet," says Jeffrey McNeely, the Union's chief scientist.

"The economic bill runs into tens of billions of dollars every year. Pests, weeds and pathogens, introduced deliberately or accidentally, reduce crop and stock yields, and degrade marine and freshwater ecosystems," McNeely says.

The United Nations Environment Programme and the Secretariat of the international agreement to conserve the diversity of species, known as the Convention on Biological Diversity, are using this year's Biodiversity Day to promote awareness of the perils facing all the world's species - focusing this year on the management of invasive alien species.

To pinpoint those species where management and control is most needed, the Invasive Species Specialist Group of the IUCN Species Survival Commission has gathered a list of the world's worst 100 alien invaders.


Asian longhorned beetles arrived in the United States in wooden shipping crates from China. (Photo courtesy U.S. Dept. of Agriculture)
They include the grey squirrel, the domestic cat, the Indian myna bird, the Asian longhorned beetle, the sweet potato whitefly, the Asian tiger mosquito, the yellow Himalayan raspberry, Koster's curse, the starling, mimosa, the shoebutton ardisia, the red-vented bulbul, the erect pricklypear, and the mile-a-minute weed.

"The species in the booklet were selected for their serious impact on biological diversity and/or human activities, and for how they highlight the important issues involved in the alien invasion," says Dr. Mick Clout, a New Zealand professor who heads IUCN's Invasive Species Specialist Group.

"Some particularly notorious cases are listed, but that does not mean that a species absent from the list is any less dangerous. Our purpose in publishing the booklet is to draw attention to the scale and complexity of the rapidly growing invasive species problem. But it is really only the tip of the iceberg."

The crazy ant, the brown tree snake, the small Indian mongoose, the Nile perch, strawberry guava, the water hyacinth, the zebra mussel and the brushtail possum are all wonderful species in their own habitats, says Dr. Clout. But like unwanted house guests they can take over ecosystems to which they are alien species.

Crazy ants, so-called because of the way they move, have invaded native ecosystems and caused environmental damage from Hawaii to the Seychelles and Zanzibar. On Christmas Island in the Indian Ocean they killed three million crabs in 18 months. These red land crabs played an important role in the island's forest ecosystem by eating leaves and seedlings of rainforest trees. Crazy ants also prey on, or interfere with, the reproduction of a variety of reptiles, birds and mammals on the forest floor and canopy.


Brown tree snake (Photo by G.H. Rodda courtesy USGS)
Brown tree snakes lived in Australia, Indonesia, Papua New Guinea and the Solomon Islands until one day in the late 1940s or early 1950s one hitchhiked on a military aircraft to Guam, a previously snake-free island. The lack of natural predators and the presence of ample prey allowed the brown tree snake population to explode. By the 1970s this poisonous reptile was found island wide and had done extensive economic and ecological damage. It has nearly exterminated Guam's native forest birds. The brown tree snake is a serious threat to the biological diversity of other tropical islands because it can conceal itself in the cargo of ships and planes, or in aircraft wheel-wells. It has reached Micronesia, Hawaii, the mainland U.S. and Spain.

The Nile perch was introduced to Africa's Lake Victoria in 1954 to counteract the drastic drop in native fish stocks caused by over-fishing. It promptly eliminated more than 200 native fish species by preying on them and competing for food.

Processing of the Nile perch for food set off a cascade of environmentally destructive events. This fish's flesh is oilier than that of the local fish, so more trees had to be cut down to fuel fires to dry the catch, the IUCN found. The subsequent erosion and run-off contributed to increased nutrient levels, opening the lake up to invasions by algae and water hyacinth. In turn these invasions led to oxygen depletion in the lake, which resulted in the death of more fish. Commercial exploitation of the Nile perch has displaced men and women from their traditional fishing and processing work.

"It is true that great economic benefits of millions of dollars per year in export income are flowing to a few people from the introduction of the perch, but none of the money is being spent on managing the considerable economic and ecological cost imposed on the poor or on the Lake Victoria ecosystem," McNeely says.


Water hyacinth blocks the Kisumu ferry terminal in Winam Gulf on the Kenyan part of Lake Victoria, 1997. (Photo courtesy USGS)
The large purple and violet flowers of the South American water hyacinth make it a very popular ornamental plant for ponds. But it is one of the worst aquatic weeds in the world. Now found in 50 countries on five continents, water hyacinth is a very fast growing plant, with populations known to double in only 12 days. Infestations of this weed block waterways, interfering with boat traffic, swimming and fishing. Water hyacinth also prevents sunlight and oxygen from reaching the water column and submerged plants. Its shading and crowding of native aquatic plants dramatically reduces biological diversity in aquatic ecosystems.

The voracious and opportunistic small Indian mongoose is native to the area from Iran through India to the Malay Peninsula. In the late 1800s it was introduced to Fiji, Mauritius, Hawaii and the West Indies to control rats, which themselves had been accidentally introduced and became pests of sugar cane and other crops. This early attempt at biological control had disastrous effects, among them the extinction of a number of native birds, reptiles and amphibians, and the rare Japanese Amami rabbit.

Dr. Wendy Strahm, the World Conservation Union's plants officer, warns, "The effects on biodiversity are immense and often irreversible, and yet awareness of the problem is alarmingly low."

The IUCN has also published "The Great Reshuffling - Human Dimensions of Invasive Alien Species," a global study of the subject edited by McNeely, to give the issue a more prominent place on the agenda of conservationists, economists and planners, and in the thinking of the general public.

IUCN was founded in 1948 and now includes 79 states, 112 government agencies, 760 nongovernmental organizations, 37 affiliates, and some 10,000 scientists and experts from 181 countries in a worldwide partnership to influence, encourage and assist societies throughout the world to conserve the integrity and diversity of nature.

IUCN has a website on the alien invasive species issue at:

The Convention on Biological Diversity is online at: