An interview with Alan Burdick
There's irony in even the grandest designs. NASA recently announced that Spirit and Opportunity, the robotic rovers that have been exploring Mars for signs of life, may inadvertently have introduced it there themselves. Microbiologist Kasthuri Venkateswaran has discovered a new breed of microbe capable of surviving the space agency's cleanrooms, space travel and even the bitterly cold climate of the red planet itself, where it may be living even now inside the landers' circuitry.
For science journalist Alan Burdick, this is one more proof of life's endless talent for discovering new habitats. For his new book, Out of Eden, Burdick traveled the world collecting instances of biological invasion here on Earth, such as the brown tree snake that leapt from Australia to Guam and has now devoured most of the island's native birds. The phenomenon, he finds, is accelerating as other species piggyback on an increasingly mobile human culture.
American Scientist Online managing editor Greg Ross interviewed Burdick by e-mail in July 2005.
The invasive species you describe tend to stow away on ships or planes; in some cases people deliberately import them to new areas. Are humans ultimately responsible for most modern invasions?
Yes. Of course, plants and animals have been moving around the world and invading organisms since time immemorial—that's the root of biogeographical diversity in the first place. Think of Darwin's finches in the Galapagos, or the 40-odd species of honeycreeper in Hawaii, each group the evolutionary descendents of a bird (or pair of them) that somehow colonized those island groups eons ago. Roughly 95 percent of the animal species living today in South America evolved from species that traveled from North America—by wing, or by foot across a land bridge—millions of years ago.
And the process continues today. Frogs and insects are transported on the winds of hurricanes; snails and seeds travel on the feet of birds. A few years ago, in the journal Nature, scientists described an incident where a small group of iguanas had traveled from one Caribbean island to another on a natural raft of trees and flotsam, and had successfully colonized a new island.
So there is this background rate (I don't like calling it the "natural" rate) of invasions. The problem today is that this background rate pales in comparison to the rate of invasions mediated by humans. In Hawaii, for example, it's estimated that prior to the arrival of humans (about 1,500 years ago), a new plant or animal successfully colonized the islands once every 20,000 years or so. Today, something like 20 exotic plants and insects are newly established in Hawaii each year. So while the phenomenon of invasion may be old, perhaps even "natural," the rate at which we're making it happen is unprecedented. And while some of these invasions, given sufficient time, conceivably could happen on their own, many more—particularly the small marine organisms being moved around the world in ships' ballast water—could never have made their voyages without aid from us.
Aren't humans also isolating ecosystems that were formerly in communication? Could this interplay of isolation and mixing foster speciation over the long term? Would this be a good thing or a bad thing?
Certainly we are creating isolated ecosystems—and in some cases, fostering speciation within them. A few years ago, for Natural History magazine, I interviewed a biologist in London who had discovered at least three subspecies of mosquito unique to different sections of the London Underground subway system—one each for the Baker, Waterloo and Victoria lines. They evidently are related to a species that is found above ground but became isolated below when the subway system was first excavated a century or so ago and were subsequently pushed on cushions of air to isolated corners of the network.
In the course of researching Out of Eden, one of the most remarkable places I visited was NASA's spacecraft-assembly facility in Pasadena, California. NASA has a stringent "planetary protection policy" that involves decontaminating the spacecraft we send out into the solar system, to make sure we don't infect other planets with our germs. Their effort is only so successful: Microbiologists have identified numerous microbes thriving in the SAF, including at least one that appears to be unique to it—that is, it evolved there and adapted to the harsh conditions that NASA created expressly to eliminate it. It eats titanium and aluminum and was found thriving on the surface of the Mars rovers, then under construction. The microbiologist I spoke with believes it is quite likely that this hardy organism is now on Mars, and is still (in spore form) alive.
So yes, even as we humans move around and beyond Earth, we do leave isolated islands of habitat in our wake. But it's important to note that we're talking about small organisms—from insects on down in size—and very small pockets of real estate. Nobody is accidentally isolating tracts of habitat vast enough to hold large mammals like tigers or rhinoceroses, then forgetting about them long enough for speciation to occur. (However, one could argue that an aim of wilderness conservation is to do precisely that.) On the whole, whatever isolated habitats we create are far outnumbered by the previously isolated islands that we are continuously reaching toward and intruding upon. As a species, our sights are set on globalization—making everywhere accessible to everyone—not localization.
As for whether human-mediated speciation is a good thing or a bad thing, that's tough. Where we get into trouble is in talking about what's best for "nature," as if nature were some third-person entity. What's good for "nature" isn't necessarily good for us (penicillin-resistant microbes are doubtless happy that we're fostering their emergence, but we aren't), nor is it necessarily what we want. One of the things I tried to do in Out of Eden was point out the extreme difficulty—perhaps impossibility—of talking about nature in anything but the human first person.
Has this project changed your conception of nature? If a snake hitches a ride in a freight container, is it "playing outside the rules"? Or adapting to new ones?
It's complicated my view of nature, that's for sure—but I expected that going in, and it's the main reason I took the project on in the first place.
Most other books about ecological invasion paint the subject in black-and-white terms, as a stark choice between bad species (invaders) and good ones (natives). Of course, many invading species are detrimental, and that deserves emphasis. But to focus solely on that subset seemed to me to present an incomplete, and frankly far less interesting, portrait of the larger phenomenon. In a way, alien species are biogeographical winners. The snake that crawls into an airplane wheel well and so spreads from Guam to Hawaii, or the purple loosestrife that is introduced from Eurasia for its ornamental properties and then grows wildly out of control: They represent nature at its fittest, spreading and thriving—even if we don't always appreciate the outcome. Rats have fleas; bees have mites; birds' nests have bedbugs. Evolutionarily, it pays to hitchhike. In spreading alien species, we are favoring the rise of organisms that can seize the opportunities to disseminate that we provide through our own global travels. In a way, they're using us—perhaps not willfully, but they profit nonetheless. That struck me as admirable, if not always desirable.
So instead of demonizing these globe-trotting organisms, I felt it was important to observe them in a more neutral light. I wanted to give myself, and by extension the reader, permission to be impressed by them—to sympathize with them, even. I've taken flak for this. Some critics have accused me of promoting a "What, me worry?" attitude, arguing that by engaging alien species on more equal terms I'm thereby dismissing them as a problem. That's simply not the case. Some invaders do indeed do bad things, and deserve our attention accordingly. But only by appreciating alien species—acknowledging their appeal, which is what draws many of them into our company in the first place—can we begin to understand the true nature of the threat they pose.
Likewise, to ecologists, the study of alien species has offered a terrific window on the inner workings of ecosystems; some of their conclusions certainly came as a surprise to me. Since the publication of Charles Elton's pioneering book The Ecology of Invasions in 1958, the chief question among invasion biologists is why invasions occur where they do, when they do. Do certain organisms make better invaders than others? Are certain ecosystems—for example, species-poor ecosystems, or "weedy," heavily invaded ecosystems—more susceptible to invasions? The answer in both cases is basically no. The main force behind ecological invasion is propagule pressure: Invaders invade because human traffic provides them with sufficient opportunity to do so. It's much less about ecology per se—the superior competitive traits of the invader, or open niches in the receiving ecosystem—and much more about statistics.
In effect, Elton proposed that biological diversity offered a kind of resistance against invasion. Ecosystems that were rich in species, and especially ecosystems full of native species that have forged tight interrelationships over evolutionary time, had a kind of advantage, or so he believed. But that doesn't really seem to be the case—at least, the evidence is far less clear that it once seemed. That probably isn't news to most ecologists nowadays—but I don't think it's quite trickled out to the mainstream audience that I write for; certainly it was news to me. And it's challenging news to process. We've come to think of biological diversity as something to be valued for the benefits—stability, "biotic resistance"—that allegedly accrue to nature. But what if those don't apply? What if the main reason to value biodiversity is because, well, we like it? How does a person begin to explore and assess that value? And how, as a writer, can I express that to the reader without lecturing and wagging my finger? Those were some of the big challenges I faced in following invasion biologists around the world and then turning my experiences into a book.
Of course, I'm not a scientist, and I don't mean to suggest that the scientific debate over the intrinsic value of biodiversity is fully settled. But I did end up thinking that when we humans talk about our relationship to nature, what we mean to discuss is our relationship to time. By and large that seems to be what natural scientists have in mind. Do ecological communities that formed over a geological time span differ in some fashion—in their productivity, in their potential stability—from those that were tossed together last month, last year, last century? Do recombinant communities differ from "normal" ones? Does time matter? That's the big question. The paradox is that the study of this crucial subject is itself compromised by time. Smart as we are, humans have yet to devise a way to study and measure ecological communities over time periods longer than the average human life span.
Therein lies the value of unadulterated nature, according to some of the biologists I spent time with. Human scientists have not yet had time to determine if time matters. And until such a time comes when time is deemed indisputably irrelevant to the structure of nature, it pays to be prudent and keep some unspoiled nature around.
You describe Out of Eden as a travel book about the natural consequences of travel. Can humans themselves be considered an invasive species?
Absolutely. Historically, perhaps evolutionarily, that's been one of our winning traits—for better and worse. Of course, insofar as we define evolutionary success as the ability to disperse and disseminate oneself, the same could be said of any of Earth's existing species; the species that (prior to human involvement) were most widespread and abundant are, de facto, ones that have done the best job of dispersing and colonizing new ground. Of all species, we've probably done a better job of invading farthest in the least amount of time—except maybe rats and cockroaches, but they travel in our wake.
How well do invasion biologists understand these patterns? The trends seem difficult to measure or even detect.
The chief questions among invasion biologists are big ones: How many exotic organisms are in motion at any given time, and where are they moving to? What fraction of those organisms successfully invade? How and to what extent do those invaders affect their new environments? What fraction of the successful invaders will have detrimental impacts?
By way of an answer, the longstanding axiom is something called the "tens rule": roughly one in 10 invasion attempts results in successful establishment, and roughly one in 10 established invaders will become a serious pest. But invasion biologists would be first to admit that that's a back-of-the-napkin estimate. The fact is, reliable numbers are hard to pin down, for several reasons. One is epistemological. Historically, biologists have tended to study only the successful invasions, particularly the flamboyantly detrimental ones—partly because they are the most apparent and easiest to study and partly because that's often where the funding is. As a result, far less is known about that larger group of failed invaders, beginning with their numbers.
Still, I think one can safely say that more invasions fail than succeed, and that more invaders have negligible impacts than detrimental ones ("detrimental," of course, being a highly subjective judgment). At the same time—in case it sounds like I'm dismissing the problem—it only takes one detrimental invader to turn an ecosystem on its ear: the Australian brown tree snake in Guam, or the Eurasian zebra mussel in the Great Lakes. So numbers alone don't capture the threat.
Can we even stop this process?
Can we stop it utterly? No. Remember, even without humans, there always was—and still is—a background rate of invasions. Life disseminates; it aims to spread, expand its range, move around; that's what any organism or species must do to perpetuate itself over generations. People are no different—and the alien-species problem is one price we pay for being like every other species in this regard. Background rate aside, alien species move because we do: Some organisms we move intentionally, because we like their company; others we move unintentionally, with our own migrations and commerce. Human-aided invasions won't stop until we—with all of our planes and ships, cargo and mail—stop moving, and we as a species clearly have no intention of doing that.
However, even if we can't (or don't want to) stop the process cold, we can greatly reduce the rate at which we move alien species around. Legislation like the National Aquatic Invasive Species Act of 2005, which would regulate the movement of ships' ballast and provide funds for various invasive species monitoring programs, is an overdue step in that direction. Invasions may be an inevitable cost of our human ambitions on Earth, but that doesn't mean we can't slow their pace and minimize that cost.