The alien invasions

In 1958, the distinguished ecologist Charles Elton published a seminal book, The Ecology of Invasions by Animals and Plants . It covered many of the obvious invasions that had occurred up to that time, such as the choking of European waterways by the Canada water-weed Elodea , and the transformation of the Australian landscape by European rabbits. Elton was of course concerned about the impact of such invasions, but he was more interested in the ecological principles that they illustrated.

These days, Elton's book reads like a quaint and charming glimpse of a peaceful past - perhaps with a few storm clouds on the horizon. Elton could not have imagined the wrenching transformation that is now taking place throughout the world's ecosystems as a result of plant and animal invasions. This transformation is ably documented in Chris Bright's book, one of a series of Environmental Alert volumes published by the Worldwatch Institute. Bright does not cite Elton, but he has done a very workmanlike job of bringing together a wide variety of sources about what is happening to ecosystems throughout the planet. The cumulative effect is depressing - what book about the environment isn't? But his book deserves to be read by both policymakers and the environmentally concerned.

I learned a great deal from these pages. The impact of invasions can be overwhelming. Native plants now exist on the island of Mauritius in only a handful of plots that are a few hectares in extent, and even these remnant scraps must be managed by continual hand-weeding. We all know about the devastating effects of the Exxon Valdez oil spill in Alaska's Prince William Sound, but Bright points out that oil tankers are continuing to introduce new marine animals and plants into the sound in their ballast water. These organisms may in the long run be even more damaging to the sound's delicate ecosystems.

There are some gleams of hope. Monoculture in farming can lead to disastrous results, enabling introduced pests to race through millions of plants that have identical genotypes and therefore identical flaws in their defences. But in the Peruvian highlands, traditional farmers plant fields with up to 20 different kinds of potatoes, and largely circumvent this problem. People throughout the world are beginning to rebel against thoughtless introductions - a plantation of water-hogging eucalyptus in Thailand was recently burned by activists. And there are a few international treaties that address, even if inadequately, the problem of introductions.

Bright makes the central point that the greatest enemy faced by the ecosystems of the world is human ignorance. The natural world has always been considered by most people to be a savage and frightening place, even at its most benign. He uncovers a marvellous quote by William Bradford, a Puritan governor of Plymouth Colony, who observed in the 1640s that the woods in the fall were a "hideous and desolate wilderness" coloured a "wild and savage hue". These are of course the same woods that we see bursting into glorious colour every autumn, and that are visited by millions of dazzled tourists. Bradford was horrified that the woods were turning the same colour as his enemies the Red Indians.

Yet we cannot laugh at Bradford when we can be deluded by our own ignorance about the natural world. Even apparent natural glories can be deceiving. Purple loosestrife, a devastating European invader of American wetlands, turns these fragile ecosystems into a blaze of colour each year - at the same time as it is destroying the food supply of native birds. If the environmentally concerned can be fooled, what about everybody else? We are living at a time when the majority of the world's people see and know nothing about nature and what is happening to it, and this is perhaps the greatest danger of all.

At the end of the book, Bright takes us on a trip around the vestiges of nature that are still to be found near his house on the outskirts of Washington, DC. He ticks off the innumerable invaders from Europe and Asia that have transformed that world in recent decades. It is a depressing story, particularly since so few of the people who live there would be likely to have the least idea of what he is talking about.

Christopher Wills is professor of biology, University of California, San Diego, United States.