The Flight of the Iguana (16 page)

Insular Biogeography and the Scourge of Guam

Ever since young Charles Darwin first made sense of his Galápagos data, small islands have played a disproportionately large role in the thinking of evolutionary biologists.

Alfred Russel Wallace spent eight years in the Malay Archipelago, finding patterns of evidence that led him (independently but simultaneously) to the same great idea that Darwin had been incubating. Later Wallace produced an opus called
Island Life
and became known as the father of biogeography, that branch of science concerned with the geographical distribution of plants and animals. K. W. Dammermann published a classic study of the recolonization of Krakatau after the big blast, while Hawaii, Easter Island, and the Channel Islands off southern California have also come in for especially careful investigation. More recently, Robert MacArthur and E. O. Wilson have influenced a generation of ecologists with a work called
The Theory of Island Biogeography.
And it can be no mere coincidence that Stephen Jay Gould, whose mind soars like a condor over the whole vista of biological sciences, has focused his own research efforts on land snails of the West Indies. Why all this attention to remote tropical isles? Is it simply because academic biologists like to get
away from the telephone and come home from their fieldwork with a tan?

No, there's a still better reason. Speciation and extinction tend to happen more rapidly on islands. At the same time, the level of species diversity (the number of different species present in a given area of similar habitat) is almost always lower than on the continental mainlands. Therefore the complex relationships balancing life against death, stasis against change, the success of one species against the decline of another, show themselves more clearly in such places. The history of life on islands reflects—in a heightened and simplified way—the entire evolutionary process. You could think of it as the pantomime version of a Shakespearean drama. And now lately, on a remote Pacific island, one scene of that stark pantomime is being acted out again.

Something is killing the birds of Guam.

•   •   •

Until just a few years ago Guam had six endemic species of bird—
endemic
meaning they were found nowhere else in the world. Today the bridled white-eye is extinct. The Guam broadbill is extinct. The rufous-fronted fantail is almost certainly extinct. And the other three species are not much better off. The Guam rail seems to have disappeared from the island in early 1984, though it is still represented in zoos on the U.S. mainland. The Micronesian kingfisher is also being protected and bred over here, in captivity, with only a single forlorn and unmated male left, at last report, in the wild. The Mariana crow, most numerous of the survivors, is down to about fifty wild birds. Six other species of land bird on the island, native but not endemic, are also threatened. More birds are vanishing daily. This whole trend of decline was first noticed in 1978 and grew worse very quickly, but for a long while no one could see what was causing it.

Pesticides were suspected. Disease was suspected. Lab analyses were done on some bird specimens, though, and no Guam avian epidemic could be found. Then in 1982 a graduate student from
the University of Illinois named Julie Savidge went out there to study the die-off, and she came upon one other possible answer:
Boiga irregularis.

Boiga irregularis
is a snake. It is an exotic species—not native to Guam—that seems to have arrived just after World War II. It may have stowed away on a shipment of military supplies, or it may have been introduced on purpose by some misguided individual or agency with a notion about rodent control. No one seems to know. At least no one is stepping up to claim credit. For close to forty years it was casually misidentified as the “Philippine rat snake,” a mistake that allowed people to think of it tolerantly. But it turns out that
B. irregularis
is not a rat snake and is not from the Philippines. It is a bird-eating tree snake, native to New Guinea and coastal Australia. Arboreal and nocturnal and stealthy, reproductively prolific, it is also flexible enough in its habits to eat the occasional rodent or lizard if fowl isn't available. In other words,
B. irregularis
is just the sort of species that is supremely qualified for invading a strange island.

The snake is mildly venomous but not dangerous to humans, and for decades after its first appearance on Guam no one gave it much heed. It has long since grown familiar to the native Guamanians. Finally Julie Savidge began paying some bounties for these “Philippine rat snakes” and, when she cut them open, in virtually every one she found birds and eggs.

Back before the arrival of
B. irregularis,
Guam had been almost as snakeless as Ireland. The only native species of serpent was a poor little blind thing that burrowed in the soil, fed on termites, and looked more like a worm than like a snake. So the birds of Guam had evolved in the absence of snake predators, a halcyon situation that eventually cost them mortally. Better train for ill and not for good, says A. E. Housman, since “Luck's a chance, but trouble's sure.” These birds hadn't trained. They were behaviorally naive. They had never been forced to learn certain hard lessons. For instance, they did not place their nests
on the far ends of tiny branches or suspend them as elaborate hanging baskets; they did not carefully limit their trips to and from the nest site, so as to avoid giving away its position; they did not gather in colonies with a system of mutual warning calls. They were therefore defenseless against
B. irregularis.

By the mid-1980s there were maybe a few hundred birds left on the island of Guam, maybe a few thousand, and an astonishing proliferation of
B. irregularis.
Rough estimates placed the snake population between one and three million—and Guam is not a big area, only ten miles by twenty. That figures out at about 10,000
B. irregularis
to every square mile, which would be bad enough if they were mosquitoes. No one can know the real number, but the snakes have grown so common that they now cause frequent electrical outages. “I've caught snakes climbing guy wires to power poles,” one biologist told me by phone from the island. “I've seen snakes hanging on power lines. I can go out here and in two hours collect more arboreal snakes than I could anywhere else in the world.”
B. irregularis
has attained this astonishing abundance, we can safely guess, for three reasons: 1) the pickings have been so easy among those naive Guam birds that a bountiful food supply has enhanced reproduction; 2) the snake has left its troubles behind, escaping from all its own natural predators and competitors back in New Guinea and Australia; and 3) even if the
B. irregularis
population now suffers from overcrowding, there is nowhere it can expand to—because it too is stuck, after all, on an island.

The birds, on the other hand, are being offered a last-ditch getaway. Under a program sponsored cooperatively by the American Association of Zoological Parks and Aquariums and the Guam Division of Aquatic and Wildlife Resources, survivors of those last endemic species have been taken from the wild, before the snakes could finish them off, and airlifted to zoos on the U.S. mainland. The goal is to breed them back up to viable populations for eventual release back on Guam—that eventuality
arriving only when (and if) a way has been found to control or eliminate
B. irregularis.
The airlift began in 1984, and within a year the captive breeding program yielded some young birds. The Micronesian kingfisher at that point numbered twenty-three in captivity. The Guam rail was up to about forty. Since then the breeding program has been fairly successful, producing small but steady increases in the numbers of both species. On Guam itself, though, the snake has continued its population explosion and the birds have continued to suffer. Only the Mariana crow seems to have held on in safe numbers, possibly because it is larger and less vulnerable. Meanwhile, in the Philadelphia Zoo, the Bronx Zoo, and a few other mainland zoos now reside almost the entire avian heritage of Guam. For an indefinite future those birds will endure their exile, patient and helpless, like an émigré community of Czarist Russians. Maybe someday their descendants will go home. Maybe.

What if they do? What if the snakes are successfully controlled and the birds are repatriated? From the standpoint of world ecological vigor and the conservation of the gene pool at large, will anything useful have been accomplished?

Arguably, no. One of the curious things about this emergency rescue program is that—by a coldly pragmatic standard of measure, according to prevailing ideas of island biogeography—it is evolutionarily futile.

•   •   •

If the snakes were allowed to eat every bird on Guam, then those six endemic species would have come to an evolutionary dead end, disappearing without trace from the main lines of bird evolution. And if the rescue succeeds, if the snake is controlled, if the rail and the kingfisher are finally returned home, then their likely eventual fate is . . . exactly the same. Resettled on a safer and more pristine Guam, what they face is the likelihood of disappearing without trace from the main lines of bird evolution.

Their native habitat—by its very nature, being a small and
remote island—is what promises them this evolutionary dead end. As natural processes take their course, the flow of species between mainland and island is almost purely a one-way movement. Small islands especially are the black holes to oblivion.

Speciation proceeds more rapidly under island conditions, it's true. Mature species arrive by all forms of dispersal, evolution progresses, and the luckiest of the pioneers adapt to their new habitats. They colonize. They specialize. They succeed. They become more sedentary than they were when they arrived—because the most restless individuals among them are constantly taking their genes elsewhere, flying off to escape or else to die trying. The others stay and stay. If they are birds or insects, very possibly they may lose the power of flight, like the dodos of Mauritius, the moas of New Zealand, the elephant birds of Madagascar. It is no coincidence that those extinct, flightless species all came to their end on islands. Hard to reach but still harder to escape, an island is generally the last stop. Once a species has landed, and settled in, and transformed itself in response to the local requirements, it has nowhere to go but extinct.

So by a coldly pragmatic standard of measure, the Guam airlift might be called futile. Nothing useful accomplished on behalf of the world's natural genetic vigor. That particular rail, that particular kingfisher—they had each, in a sense, already destined themselves to oblivion. If it wasn't the snake, it would have been a disease. If not a disease, then a fire or a climate change or an eruption. If it wasn't that, it would have been man, worst of all invaders, turning their habitat into parking lots and time-share condominiums. Face it, those two little species are doomed. They are islanders, therefore doomed. To rescue them now, temporarily, is only to say no to the inevitable.

Which is precisely why it's worth doing, of course.

A coldly pragmatic standard of measure is exactly the wrong sort, in this case as in so many others, to apply. Practicalities are not the issue. Two flickering candles of life are the issue. Dylan
Thomas would understand: Rage, rage against the dying of the light. So would Karl Wallenda and Harry Houdini and Ed Abbey. Saying no to the inevitable is one of the few precious ways our own species redeems itself from oblivion—or at least tries to. For mortal creatures, on a slow-dying planet, in the ocean of space, there's really no other option.