When new species are introduced to places where they have never existed naturally, they can sometimes cause a real problem for native wildlife. Not only can they affect local species by competing with them for food and resources, or even eating the natives, but they can also be very damaging to crops, people's homes and even cause diseases.

A recent estimate of the damage to the United States economy from introduced pest species was as much as 123 billion US$ per year. Before humans arrived in Hawaii, the islands received a new species by sea or wind once every 70 000 years - these days 100 new species are introduced there every year causing great damage to the native plants and animals.

Many rare birds have been threatened by introduced rats, cats, snakes and other species. Native plants can find it hard to compete with other exotic plants. But humans are also affected. Termites, aggressive bees, insects that damage our crops, and even diseases that originate in foreign lands can be very harmful. The Nile perch was introduced into Africa's Lake Victoria for food and sport fishing. It has eaten its way through 200 native fish species, and is still going. The brown tree snake from Papua New Guinea arrived on the Pacific Island of Guam and has eliminated 12 out of 14 of the local bird species there.

There must be tight regulation on bringing plants and animals across international borders, airports and shipping ports. The public mustn't bring exotic species back home with them without a licence. New Zealand is leading the world in ridding their country of pest species such as rats and cats, but it is a slow and expensive process, and preventing new arrivals is the cheapest and easiest solution of all.

| habitat loss | introduced species | pollution | | over harvesting | fragmentation |