Organisms living outside their usual range are called nonnative species. Introducing them is generally considered undesirable — not surprising considering the harm that some have caused. The brown tree snake in Guam is one example. This predator arrived on the island of Guam, probably as a stowaway in ship cargo, in the mid-twentieth century. It has since caused the extinction of most of the island’s native birds. The cane toad (see Box 1) is another example. This amphibian was introduced deliberately to Australia as a form of biological control for the cane beetle pest. It has since wreaked ecological havoc by killing many native species. The global cost of damage by non-native species to agriculture, forestry, fisheries and waterways is estimated at well over £63.3 billion annually.
So why are some biologists now suggesting introducing non-native species as substitutes for extinct species, all in the name of ecological restoration? Well, it is important to recognise that only a small percentage of introduced species become invasive and harmful — most are relatively benign, some can even have beneficial effects. Many of our food crops and livestock were introduced, and many of the species we think of as ours are not native. For example, the pheasant was introduced into the UK from eastern Europe in the tenth century…and the rabbit arrived with the Normans.
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