very fall for years, three or four times a week, I have climbed an hour-long trail up the side of a low-slung ridge in the Pennsylvania Appalachians, to run a bird-banding station on a boulder field along the summit. In October the path -- crunchy with newly fallen leaves of oak and black birch -- would turn a luminous, bronzy yellow, when the hay-scented ferns growing along its margins began to die for the winter.
The trail still turns autumnal yellow, but the reason has changed in a subtle but, to me, disturbing way. The ferns have been replaced by a low, shaggy grass known as Microstegium vimineum, or Japanese stilt grass. It's rumored to have come to the United States almost a century ago when it was used to protect porcelain en route from Asia. It has spread widely, crowding out native species from New York and Florida to Texas and Wisconsin.
I first noticed the stilt grass in the 1980s, near the base of the mountain. For five or six years, I watched it ascend the trail, climbing several hundred feet a year. Even though I yanked it out by hand whenever I could, I suspected I was partly to blame for its progress. My thrice-weekly trips represented by far the greatest portion of the foot traffic up and down that trail, the mud caked in my boot soles rich in tiny Microstegium seeds. Eventually the grass reached the top of the ridge and began to spread out along the main stem of the Appalachian Trail, where there were far more boots than mine to carry it. Well adapted to shady forest conditions, it quickly shouldered aside the ferns, first along the path and then farther and farther into the woods.
Few hikers, I suspect, realize that this innocent-looking grass at their feet is a small example of one of the dominant themes in modern global conservation: the homogenization of the world's biota, as invading species -- carried about the planet by commerce or design -- reshape ecosystems in profound ways. Until recently, no one doubted that such invasions were almost always harmful, varying only in their degree of damage; many ecologists consider alien species to be second only to habitat destruction as a threat to global biodiversity.
In Out of Eden, the science writer Alan Burdick explores this endlessly fascinating topic, but his isn't the usual story of good native versus bad alien; he finds a world of messy grays in which scientists struggle with their own conceptions of what constitutes a natural ecosystem and how invaders fit into the puzzle. Biologists long believed that some inherently aggressive quality propels alien species into dominance, but now experts say it may simply be a numbers game. The sheer volume of organisms inadvertently introduced into new habitats is such that one or two biological Genghis Khans are bound to emerge from the mix. Burdick even offers a heretical but comforting vision, one voiced by a few biologists: Invaded ecosystems are merely expanded and altered, sometimes radically so, but not destroyed, in the sense that a forest is clearcut or a marsh is drained.
Of course, a drained marsh is still technically an ecosystem, albeit a vastly simplified one. But Burdick suggests that the greatest threat posed by alien species may actually be aesthetic; at risk is our love for unique natural places. As more and more organisms hitchhike around the planet, our world becomes blander, more uniform.
This sanguine attitude is hard to swallow, and given his thesis, it seems odd that Burdick begins in Guam, the site of one of the most chilling examples of an invader run horribly amok. Islands -- small, isolated, and home to naturally limited populations of species often found nowhere else on earth -- are at particular risk from invaders, and Guam was invaded by one of the worst. The brown tree snake from Australasia, which apparently stowed away on shipments of World War II matériel, has rendered the island's forests virtually silent, having eaten nine of Guam's thirteen species of forest birds into either complete extinction or exile. One remaining species, the Mariana crow, now nests only in a few trees bristling with electric fencing. The tree snake exemplifies the challenges of reining in an out-of-control organism, for like many exotics, it has morphed in its new environment into something more lethal, eating a vastly more catholic diet than it ate in its homeland. (It will, in the absence of live prey, feed on dog food, table scraps, and even soiled tampons; the smell of human breast milk draws it to the cribs of infants, which it tries -- unsuccessfully, thank heavens -- to eat.)
Although the decline in Guam's birds began to worry locals as early as the 1960s, it took 20 years for anyone to realize the snakes were the culprit. Such is often the way of invasives, coming in under the radar and remaining unnoticed unless they reach scourge levels. Figuring out what to do about the snakes is not simple; while one zoologist is pursuing sci-fi schemes in which mosquitoes would transmit some yet unknown DNA -- corrupting virus or chemical to the reptiles, others slog along designing better snake traps or more effective snake fences.
Although the brown tree snake represents a worst-case scenario, the vast majority of aliens slip into their new homes without much fuss and, Burdick contends, without doing much permanent damage. Eurasian ring-necked pheasants have lived for more than a century on farms and grasslands in North America without having much impact on native birds. As one scientist points out, ecosystems seem to be more elastic than we thought, able to absorb many new species. This may be true, but is perhaps overly optimistic: Surely there is some intrinsic value beyond aesthetics in preserving intact the threatened forests of Hawaii, a place to which Burdick devotes much of his book. Certainly there is an argument to be made for saving the unique community of honeycreepers, flowers, trees, and insects that makes the archipelago one of the most exceptional ecosystems on earth, a model of evolution surpassing the Galápagos. Life forms are being killed off or crowded out by a flood of exotics that began with the Polynesians and is only accelerating.
The world is irrevocably smaller -- humans are "unraveling the borders," Burdick says -- and if not all introduced species are as destructive as the tree snake, enough of them wreak havoc on native communities that it makes sense to stop them before they arrive -- if we only knew how. We can bar a few small doors: Consider the trap-studded fence around Guam's airport, painstakingly designed to keep tree snakes out of planes headed off the island. But the closer you look, the more you realize that the global economy has become a great engine, as efficient at moving species around as it is at generating wealth.
Take ballast water, sloshing in the holds of innumerable ships around the globe -- picked up in one part of the world and flushed out in another -- replete with trillions of planktonic organisms, from larval crabs to jellyfish. This is how Eurasian zebra mussels reached North America's Great Lakes. Once there, they multiplied exponentially, clogging water intakes, edging out native clams, and short-circuiting the food chain by siphoning off nutrients. European green crabs were established in New England by 1817; along with another exotic, the common periwinkle, they have so completely overtaken the intertidal zone that few realize they don't belong.
Although we didn't notice these newcomers until they became a problem, coastal ecosystems have been among the most altered by alien species. They are also among the least studied, underscoring the difficulty, which Burdick acknowledges, of accurately assessing the impact of invasives. Hundreds of widespread marine organisms like shipworms and barnacles, traditionally considered native to much of the world, were hitchhikers in the days of the Phoenicians.
And yet, while organisms leapfrog the world willy-nilly, exactly why a species establishes itself in a particular place at a particular time remains largely a mystery; ships were entering the Great Lakes and dumping ballast water for decades before one of them flushed its tanks in the 1980s, bestowing upon us the zebra mussel. Why at that moment? Why not long before or many years hence? We simply don't know; our ignorance about the dynamics of invasive species is vast.
Because we cannot yet predict whether the introduction of species A into ecosystem B will be relatively benign or harshly virulent, we must guard against everything that may slip through -- yet that is both expensive and ultimately impossible. But failure could be even more costly; by one Cornell University estimate, aggressive invasives cost the U.S. economy $138 billion a year in losses to crops, timber, fisheries, and even human health (think West Nile virus), a figure likely to increase. If the preservation of biodiversity and aesthetic considerations aren't reason enough to protect natural communities from unruly colonists, the daunting financial risk ought to warrant far more attention than it has so far received. Chronicling, as it does, an emerging and often fractious science and a complex subject with profound implications for the natural world, Out of Eden may leave you with unanswered questions, but it is at every turn thought provoking and timely.