Above: The cane toad, a small invader that has caused big problems in Australia (istockphoto.com/JohnCarnemolla)
Cane toads were introduced into Australia in 1935 to control a type of beetle that was destroying sugar cane crops. The toads never showed much interest in eating the beetles, but they are still expanding their range at a rate of 55 kilometres per year. They now cover over a million square kilometres of the continent, causing other species to decline because of their toxicity, and costing the Australian economy a half a million dollars per year.
Did you know? Invasive species in the United States cause approximately $120 billion in environmental damage per year.Invasive species are plants or animals that have been moved beyond the area they normally inhabit because of human activities. Most species that are transported to a new area do not cause much damage, but some thrive and can spread very rapidly. Are there specific characteristics that make species "successful invaders"? Many come from places with similar climates to their new home, which means they are already adapted to succeeding in that type of habitat.
Invasive species are especially harmful when they drive native species to extinction through competition, predation, or the spread of disease. They can also interfere with human activities. Invasive zebra mussels in the Great Lakes clog water intake pipes and damage electric generating plants. They can reach densities of 700,000 per square metre and cost hundreds of millions of dollars a year to control. Since the emerald ash borer, a beetle that kills ash trees, was introduced to Michigan and Ontario in 2002, it has spread and killed up to 15 million trees. This has changed eastern North American forests and caused significant losses in the forestry sector.
Species can be transported to areas beyond their natural range accidentally or on purpose. Some of the best known invasive species, like the cane toad, were deliberately introduced as pets, decorations, or a way of controlling pests. Others have been introduced accidentally, such as the brown tree snake, which invaded Guam by hitching a ride with military equipment being moved after World War II. These snakes have become so numerous that they regularly cause power outages when they crawl on electric lines, costing the island territory more than one million dollars per year.
Did you know? Invasive species have contributed to the extinction of 91 species of animals across the globe.Removing invasive species is often unsuccessful and extremely expensive. However, there are several islands where invaders like rats and disease-carrying flies have been successfully removed. For example, New Zealand has eradicated Norway rats and house mice from more than 200 islands. In order to succeed against invasive species, control efforts need to be well-funded, have clear objectives, and be based on a good understanding of the biology of the invader.
Invasive species are not a new problem. For example, the extinction of the dodo on the island of Mauritius in the 1600s was likely caused by invasive pigs brought by humans to the island. However, living in an age of global travel means that there is a constant risk of new invaders. Because goods such as food, decorative plants, and equipment are moved across international borders faster and more frequently than ever before, there is more opportunity for hitchhiking organisms to be transported to new areas. Predicting, preventing and managing invasive species is an important task for biologists to reduce damage to the environment and the economy.
Invasive Species Centre (2014)
Canadian organization supporting research, outreach and education activities related to invasive species. Website includes information on a variety of invasive species.