American Scientist

Out of Eden: An Odyssey of Ecological Invasion. Alan Burdick. viii + 324 pp. Farrar, Straus and Giroux, 2005. $25.

In Out of Eden, Alan Burdick embarks on a journey to investigate the massive biological disruptions that nonnative species often bring about, traveling first to what might be considered ground zero of ecological invasion, Guam. There he engages with scientists fighting to control the brown tree snake, Boiga irregularis, which first appeared on this Pacific island shortly after World War II, most likely having arrived in military transport from an area near New Guinea. Now as many as a hundred of these snakes inhabit each hectare on Guam, where they have had dramatic effects. For one thing, the sound of birds is principally gone—9 of the 14 species of native forest birds have been locally extirpated, and three of those species are now extinct, because Guam was the only place they were found. Every year the snakes bite more than 200 people and cause as many as 100 electrical outages as they cross power lines.

So the brown tree snake is a pretty nasty creature. But Burdick can't help marveling at its capacity to penetrate barriers designed specifically to keep it out. Thus he introduces a recurring theme: that how one views the alteration of the biotic face of the Earth is a matter of values and focus—and, I might add, a matter of the lens one uses to view the natural world. For example, some of the citizens and businesspeople of Guam have taken the attitude that there's nothing wrong with cosmopolitan fauna, saying, "Well, if we can't have native birds here anymore, why not introduce [other] tropical birds, parrots, because tourists will like it."

On leaving Guam, Burdick travels to the Hawaiian Islands, where authorities are intently guarding against being invaded by the brown tree snake because of the damage it could do to resident biota and to the popular image of the islands as a paradise. However, the main battle against invasive species in Hawaii is in many ways already over. Every year some five new types of plants and about 20 new kinds of insects become established—plus other critters, including pathogens. Nearly one-half of all of the flowering plant and mammal species now in Hawaii are relative newcomers, having arrived after humans showed up about 1,500 years ago. All of the amphibians and most of the reptiles are in this sense new to the islands, as are more than 2,000 species of insects.

What about those organisms that evolved in place from those few ancestors that arrived via currents of wind and water? Many were driven to extinction by the early Polynesian settlers, and much later the introduction of avian malaria and its vector, the mosquito, took a devastating toll. Other human-driven perturbations include habitat loss and purposeful introductions of birds and mammals.

Burdick has thoroughly acquainted himself with what biologists have learned about invasive species. In the course of his encounters with scientists and other residents of Hawaii, he refers repeatedly to Charles Elton's pioneering and eloquent 1958 book, The Ecology of Invasions. Elton, the most respected animal ecologist of his time, was highly qualified to write about species interactions. "We must make no mistake," he said. "We are seeing one of the great historical convulsions of the world's fauna and flora."

Some of the conclusions Elton came to in his early work are still, even after decades of empirical investigation, subject to considerable debate and continue to be modified and refined. Most controversial is the question of whether diverse communities are more resistant to invasions, as Elton believed. Experiments conducted on small plots support this conclusion, but at larger scales, different processes come into play.

Burdick does a nice job of telling the story of the development of the science of invasion biology, blending it with his depictions of the everyday work of researchers and of the concerns of others who benefit from the bounties of nature. These narratives illustrate the conflicts that occur when groups of people with different values and objectives share the same landscapes and resources. The early colonizers of the Hawaiian Islands brought with them useful animals and plants from their native lands, including pigs, which have thrived and are now feral. Park managers, to protect native biota, have constructed fences to keep the pigs out and to control them—objectives that bring the managers into conflict with people who hunt the pigs and see them as an important part of the cultural heritage of native Hawaiians.

Burdick tags along with ecologist David Foote, who is studying the impact of the feral pigs. Foote is interested in such questions as whether soil tilled by feral pigs contains fewer native micropods and whether nonnative weeds are more likely to grow in it. Burdick, in translating what he learns from Foote about biogeochemistry, notes rather poetically that "at the lowest trophic levels . . . the animate and inanimate come . . . close to converging: geology melts into soil chemistry; soil chemistry merges with biochemistry; biochemistry begets biology. There is a river of nutrients flowing up and down the scales of perspective."

With more than 250 nonindigenous species established and more pouring in, the San Francisco Bay is one of the most highly modified estuaries in the world.

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Foote's colleague Peter Vitousek observes that ecological processes in Hawaii are being significantly altered by such nonnatives as Myrica faya, the fire tree. This plant enriches the soil by fixing nitrogen, but in doing so completely changes the dynamics of plant colonization on the volcanic flows. Myrica faya, interacting with native as well as introduced species, has created a whole new biotic world on the islands. Similarly, an invading grass species can change the fire regime of an ecosystem and in time inhibit the recovery of native forest species after a conflagration. So scientists must consider not just one invading species at a time, but many, and how they interact.

This complexity, and uncertainty, appears to trouble Burdick. Do we know enough—about pig exclusion, for example—to take expensive action? Acting in the face of incomplete knowledge, or uncertainty, is the great challenge to policy makers everywhere. But ecological systems are complex, often with nonlinear responses to change. Inaction at an early stage of an invasion can result in irreversible damage, so applying the precautionary principle is appropriate.

As he describes the current status of the ecosystems of Hawaii, Burdick reflects on Elton's conclusions. These older tenets form a counterpoint to our growing knowledge of the dynamics of invasive species and of newer ideas, such as Daniel Simberloff's invasional meltdown theory, which posits that new introductions continue to increase rather than leveling off as in the Eltonian world.

Burdick also brings up biocontrol, the use of other species to attack target invasive organisms. Once an introduced species has built up a large population, control of it by mechanical or chemical means is costly and may harm the environment. Thus it is tempting to try biocontrol instead. Unfortunately, in some cases the creatures used have attacked not only the invader but also native, and often rare, species.

Burdick comments that some of the biologists seeking to limit invasive species are motivated by a personal wish to maintain the uniqueness of a given place. He notes the difficulty of achieving this goal in a world where Darwinian competition reigns and massive global exchanges of biotic material occur. Burdick does not, however, dwell on another motive of scientists and the public alike, that of "controlling" nature to benefit society (by providing food, beauty or water purification, for example). The battles against invasive species are generally waged to reduce massive economic losses or to mitigate possible harm to human health.

Among the last stops in Burdick's journey is the San Francisco Bay. With more than 250 nonindigenous species known to be established and more pouring in, it is one of the most highly modified estuaries in the world. Many of the invaders come on ships, which account for 80 percent of all global transport. These vessels carry ballast water laden with living animals from foreign places and dump it into distant ports. The resulting changes in marine systems are not visible to the casual observer, but they are dramatic nonetheless: The whole way these systems function is transformed, with effects all along the food chain.

Here Burdick's book changes in tenor as he becomes captivated by the inspirational passion with which leading scientists James T. Carlton and Greg Ruiz pursue their joint work on marine invasions. Burdick describes well such poster children of aquatic invasion as the green crab and the zebra mussel, which have not only had major effects on ecosystems but have also fueled interest in invasive species.

Many biologists working on invasions are concerned only about aliens that are jeopardizing valuable ecosystem processes or causing harm to the economy or public health—a list that is quite long. However, managers of natural ecosystems who are charged with protecting native biodiversity may have a mandate to consider all alien species a problem.

At the end of the book, after being tutored by a remarkable collection of scientists, Burdick is still pondering what nature is and why people might care about invasions. Of course all living things are part of nature. But biologists studying invasive organisms are striving to understand why, when certain species from distant lands enter a new habitat, their population explodes, injuring ecosystem processes and native species that humans treasure. To answer this question, one must understand the whole train of events that leads to transport, establishment and spread—a daunting task.

Playing the role of investigative reporter, Burdick does a good job of describing some major battlegrounds, the scientists involved and the state of the science. Unfortunately, the book has no notes, references or index—not even a hint of the literature he consulted, even though not all of the facts he cites could have come from personal communications. And the book does not state explicitly what society should do to respond to Elton's historic convulsion.

Burdick evidently was saving his opinions on that subject for a different venue, a cover article in the May 2005 issue of Discover magazine, where he is a senior editor. This piece, titled "The Truth about Invasive Species: How to Stop Worrying and Learn to Love Ecological Intruders," repeats material from Out of Eden and adds new interpretations. Here Burdick states flatly that "invasions don't cause ecosystems to collapse." However, the caption for one figure in the article notes that a "comb jelly, native to American waters, entered the Black Sea in the 1980s and promptly ate everything—fish eggs, fish larvae, and zooplankton—in it." And the fisheries of the Black Sea did collapse in the early 1990s, falling in tonnage harvests from 250,000 to 30,000 tons. If by saying that ecosystems don't collapse Burdick means that they don't lose all primary producers, consumers and decomposers, he is right, but such a distinction may not be very meaningful for people who can no longer depend economically on what was there before.

The Convention on Biological Diversity specifically targets not nonnative species per se, but primarily ones with robust, growing populations—"those alien species which threaten ecosystems, habitats or species" and those that disrupt ecological systems that society values and may even depend on. There are certainly plenty of invaders of this ilk, as Burdick describes so nicely in Out of Eden. The fact that in this article he seems to have forgotten many of the lessons his generous mentors taught him and instead emphasizes that most intruders can be safely embraced is quite puzzling.