Where the Wild Things Were

by William Stolzenburg

  In general, the missing birds were either the larger species of their particular guild or those that typically foraged or nested on the ground. Of the eighteen, sixteen were still to be found on the mainland, which was in some places only a little more than a quarter mile away. He could imagine that the larger species of birds, with their greater demands for food and space, and their typically low numbers, would naturally be more prone to extinction. But the missing ground dwellers stumped him. “Why did that particular set of birds go extinct?” he asked. “They were just some garden variety songbirds. It seemed there was no particular thread that made them vulnerable.” In concluding, Terborgh all but threw up his hands. “These are nontraumatic population declines compelled by unrelenting forces that are yet to be identified.” That was as far as Terborgh could go in 1974. But for a long time afterward, that particular cast of missing birds on Barro Colorado haunted his mind.

  Six years later, Terborgh was back with another stab at the answer. In the book Conservation Biology, a seminal volume announcing the formal emergence of a discipline spurred by the biodiversity crisis, he and Blair Winter contributed a chapter titled “Some Causes of Extinction.” They noted two essential kinds of extinction: primary and secondary. Primary extinctions arose from such forces as fragmentation, the chopping of big populations into small and dangerously isolated populations, with their inherent susceptibility to freakish and fatal accidents. Primary extinction described the more obvious assault on the diversity of life, the fallout from the ubiquitous human saw as it cut through the last broad bastions of wilderness. But it was the more subtle and unheralded phenomenon of secondary extinctions that most intrigued Terborgh and Winter.

  These, they noted, were of the kind that Robert T. Paine had famously demonstrated when he tossed the predatory starfish Pisaster from the splash zone of Mukkaw Bay, and watched half the species of its community disappear shortly thereafter. “Herein lies a potentially important area of research that has scarcely been breached,” wrote Terborgh and Winter. “We know next to nothing about what consequences follow the loss of top predators in terrestrial ecosystems.”

  This was their cue to introduce Barro Colorado Island, in all its splendidly disguised decay. “Completely protected for more than 50 years, Barro Colorado has become a veritable zoo without cages,” wrote Terborgh and Winter. “Medium and large mammals are unwary and remarkably abundant. These include the collared peccary, agouti, coati mundi, three-toed and two-toed sloths, tamandua, armadillo and howler monkey.” The prolific, carefree denizens of Barro Colorado were of the same genetic stock as their scarce and skittish counterparts on the mainland. Why this menagerie remained so remarkably abundant and unwary, Terborgh proposed, was at least partly due to an unnatural freedom from being chased and eaten. Barro Colorado, at just seventeen square kilometers, was too small to support jaguars or pumas or the massive harpy eagle, the topmost predators of the tropical forest. (All three of them good examples of primary extinction via fragmentation.)

  It was only conjecture, but it came well educated. Terborgh’s tropical canvassings had calibrated his eye. No other place, not even those reserves banning hunting, approached Barro Colorado for the tameness and profusion of its mammals. What Terborgh saw in Barro Colorado was something other than a peaceable kingdom of tropical splendor.

  Cases in point were those missing birds from the forests of Barro Colorado Island. Of the fifteen to eighteen birds that had vanished, many shared a certain facet of life history. Many nested on or near the ground. And of the suite of mammals whose populations had conversely soared since the island was born, two of them, the coati and peccary (southern cousins to the raccoon and pig, respectively), were notorious gourmands of eggs and nestlings. Along with the ground-nesting bird’s typical and sundry difficulties of raising young in such a competitive environment, the added weight of an abnormal load of predators could easily drive the yearly production into the red zone. “All this is merely speculation,” noted the authors, “but it has a persuasive ring to it.”

  One year later, two graduate students from the University of Illinois set out a line of wicker bird nests on the ground and shrubs of both Barro Colorado and the adjacent Panamanian mainland. They lined their artificial nests with leaves and placed a pair of domestic quail eggs in each. When they checked a day or two later, 6 percent of the ground nests on the mainland were missing eggs. On Barro Colorado, the losses tallied 88 percent.

  Manu

  Shortly after fleeing the Apurímac Valley in 1973, as his paradise came crashing down under the flood of colonos and drug lords, Terborgh came upon a far grander prize in Peru’s Manu National Park. Nearly six thousand square miles of Amazonian lowland jungle extending unbroken to the thirteen-thousand-foot heights of the Andes, Manu circumscribed one of the wildest places left on earth. Manu was an embarrassment of biological riches, harboring nearly half as many species of mammals as all of North America and nearly twice as many species of birds. In a single acre of Manu, one could count a hundred species of trees. It was likely that Manu housed more species of life than any other park in the world.

  The nearest semblance of civilization at Manu was a rustic biological research station called Cocha Cashu, which Terborgh promptly adopted as his tropical home and laboratory, becoming its chief scientist and administrator when not teaching in the States. The accommodations of Cocha Cashu included a desk, a kitchen with two kerosene stoves, a lake to bathe in, and a choice of several million acres in which to pitch your tent. It took three days’ travel in a motorized dugout canoe to reach the inner sanctum. For the first seven years, there was no phone, no two-way radio, no outside communications whatsoever. Nobody ever just happened upon Cocha Cashu; the only primate visitors were the monkeys that gamboled before the open windows of the research station. It was John Terborgh’s private villa on the Seine.

  What mattered most for Terborgh’s purposes was the fact that Manu, in its relative purity and wholeness, was a baseline against which to measure all other tropical forests of its kind. Manu was fully stocked with jaguars, the third largest cat in the world; and the puma, slightly smaller but more versatile, its species spanning latitudes from Patagonia to the Yukon, making game of everything from bite-size rodents to quarter-ton elk. Lording over all life in the trees was the harpy eagle, the world’s most powerful and impressive raptor, an eleven-pound, monkey-seeking missile with warrior plumes and the talons of a tiger. Manu still harbored these superpredators of the New World tropical forests. But Barro Colorado, save for their sporadic visits from the mainland, no longer did. And that got Terborgh again pondering the maddening disappearance of Barro Colorado’s birds.

  In the early 1980s, mammalogist Louise Emmons of the Smithsonian Institution conducted a study of Manu’s pumas and jaguars, as well as a smaller spotted cat called the ocelot. Emmons endlessly walked the trails, tracking the big cats by radio signals, collecting their scat, analyzing their diets. She tracked them day and night, and they, at times, tracked her, the steady beeping of the jaguar’s collar following Emmons’s footsteps through the dark.

  Emmons also censused the jungle for the cats’ potential prey. She took note of agoutis and pacas, two stout, seed-eating rodents of the forest; and capybaras, semiaquatic, nearly sheep-size rodents of the riverbanks. She tallied the piglike peccaries and the raccoonish coatimundis. And then she noted how often each got eaten.

  Emmons discovered the predators sampling prey in proportions almost identical to their densities in Manu—lots of agoutis crossing the path meant lots of agoutis getting eaten. This surprisingly random mode of hunting meant to Terborgh the cats might be capable of regulating their prey. And by extension, it suggested that in places where the cats no longer hunted, those prey were likely to boom.

  And indeed, the differences between Manu and Barro Colorado were astounding. The Barro Colorado agoutis outnumbered those on Manu by twenty times, the coatis by twenty-five times. As avid seekers of eggs and nestlings, the hyper
abundant coatis became prime suspects in Terborgh’s search for Barro Colorado’s missing birds. While not suggesting the coatis had single-handedly exterminated the island’s missing avifauna, Terborgh found it hard to imagine such a talented nest raider, in such abnormal densities, as anything less than an accomplice in the bird murders of Barro Colorado.

  Over the years, challenges to Terborgh’s reconstruction of Barro Colorado’s demise would be many. Some questioned the accused coati’s role, after finding no greater rates of nest predation on Barro Colorado than on the adjacent mainland. Others maintained that the little animals on Barro Colorado—extinctions notwithstanding—were doing just fine without big predators, thank you.

  As stubbornly as Terborgh held his ground, he was well aware of that conspicuous lot of if’s and but’s in his argument. He realized the enormous and tenuous reach between Manu and Barro Colorado, two vastly different paradigms of wilderness, hemispheres apart. In his “Big Things” essay of 1988, in which he had openly lauded the powers of the superpredators, he had also openly opined for the “major research program … not yet undertaken.”

  Terborgh dreamed of a new laboratory—a fresh system of detached fragments, along the lines of Barro Colorado but with more islands and simpler faunas to sample. And he needed to be there from the start.

  Lago Guri

  In the spring of 1990, the primatologist Warren Kinzey mentioned to his friend John Terborgh that he was heading down to Venezuela to his new research site. The monkeys of Kinzey’s interest had been stranded on some islands in the midst of a huge hydroelectric impoundment that had sprung up just a few years before.

  “I’m going down with you,” blurted Terborgh.

  Terborgh spent two weeks on the giant lake with its nascent archipelago, growing giddy with its possibilities. The water that had risen behind the Raul Leoni Dam created a lake fifty miles wide and five hundred feet deep over what had once been the valley of the Río Caroní. Only the highest peaks of the hilly countryside had been left dry, surrounded by cemeteries of treetop tombstones. The islands came in all sizes, from one quarter acre to one and a half square miles, each harboring its own community of castaways. Most critical to Terborgh, many of the islands were decidedly too small to house the superpredators that he suspected ran the world.

  Stepping ashore for the first time on the little islands of Guri, Terborgh was struck by their extreme asymmetry and exaggeration, flip-flopping between obliterating barrenness and explosive abundances. The islands presented a patchwork of incongruities, marrying the work of phantom bulldozers to some satanic gardener of hell’s thickets. It was, in Terborgh’s own words, “a god-forsaken place.”

  Terborgh returned to his teaching job at Duke University in a fervent state of anticipation. And for the next two years … he waited. And he steamed, as one grant proposal after the next was rejected: Too ambitious, too speculative, too long a wait for returns, came the replies. Terborgh bristled at their meekness—How skeptical, how reluctant, how cautious, he thought. In 1992, fortune stepped in, rewarding Terborgh with the coveted MacArthur Fellowship (popularly known as the “genius grant”), which provided him with a big sum of discretionary cash for his intellectual pursuits. Terborgh immediately poured the money into his Guri dream project. He assembled a team of scientists from across the continents. And the very next field season, one of the great experiments in tropical fragmentation was launched.

  Beginning in 1993, and for two months every year thereafter, the field crews—Terborgh’s students from Duke, local biologists, colleagues from institutions as far away as India—would descend upon the giant lake. From the mainland they would boat hours across the water to pinpricks of land, erecting makeshift camps in the Terborgh tradition, sheltering in tents, eating daily rations of rice or beans cooked on a gas stove, and bathing in the tepid waters of Lago Guri. The crews censused animals of all stripes—ants, butterflies, tortoises, monkeys, birds. They also surveyed the plants, counting trees and seedlings, following the forests’ succession of life through the maturing of Lago Guri’s newborn archipelago.

  Guri’s lack of living space had simplified the surveyors’ task. Three quarters of the animal species found on the mainland were already missing from the islands by the time the crews arrived, most of them short-lived creatures unable to maintain themselves in such limited confines. As the islands got smaller, the casts naturally got shorter. What remained were skeleton crews of creatures surviving, in some cases, to numerical extremes.

  They found trees crawling with Cebus olivaceus, the olive capuchin monkey, an artful, agile omnivore foraging from canopy to forest floor. And in time, those same trees began losing species of birds. On one island, during a period in which the capuchin population doubled, more than half of the avian species was lost. The Guri crew put out artificial bird nests stocked with quail eggs only to have every egg disappear. It was of no marginal concern that many of these disappearing birds had previously been employed as pollinators and seed dispersers of the Guri forests.

  But among the primates, it was Alouatta seniculus, the red howler monkey, that came to most personally symbolize the depravity of Guri’s bottom-heavy world. The Guri researchers would step onto an island the size of a suburban lawn, peer up, and see five howler monkeys staring down from a naked tree. Those five amounted to twenty to thirty times the typical densities on the mainland. But in order to appreciate the strangeness of that arrangement, it helps to first consider the brief textbook résumé of a mainland howler monkey.

  Alouatta seniculus is one of a suite of six species of howler monkeys found throughout New World tropical forests from southern Mexico to northern Argentina. Prehensile-tailed primates typically living in bands of three to seven related individuals, howler monkeys are led by a large male, a husky, throaty monkey whose territorial roars can travel three miles through the jungle, letting neighboring troops of howlers know to stay away, and otherwise giving neophyte tourists the unnerving impression of lions in their midst.

  The howler monkeys of Guri had been put under intensive watch by Terborgh’s researcher Gabriela Orihuela. Nothing Orihuela had ever seen or heard of monkey society on the mainland could prepare her for the sights awaiting on Lago Guri’s littlest islands. Free of fearsome predators—and hounded by hunger—the howlers of Lago Guri no longer lived in coherent groups but instead they slept in separate trees. For lack of contact, they seldom groomed. Those that did come together sometimes fought, inflicting savage wounds. Their babies never played. The monkeys of Guri grew thin. There were ominous signs of infanticide. The howler monkeys of Lago Guri no longer howled.

  The commonly held and unscientific impression among Orihuela and the rest of the Guri crew was that the howlers of Guri simply didn’t like each other anymore. In this supposed paradise free of predators, the group-hugging howler monkey had been sentenced to a solitary confinement in hell.

  Even the favorite trees of the Guri howlers began biting back. Denuded by the relentless browsing, the trees started sending forth leaves increasingly spiked with bitter, nauseating toxins. Breakfasts for the monkeys became rituals of self-administered poisonings, with bouts of hapless gorging followed by episodes of perfunctory vomiting. Ostensibly freed from the top-down control of predators, the howler monkeys had entered a far nastier bottom-up realm run by plants.

  Atta

  Among the more striking features of the distorted Guri islands were the forests themselves. Denuded trees and tangles of vines and flesh-shredding thickets of thorns—these were ecological aberrations for which the howler monkeys could not be ultimately blamed. Something other than the monkeys had been eating the forest to such severity. In the predators’ absence, the title of the most fearsome animal on the Guri islands had been passed to an ant.

  The variety of concern was a leaf-cutter ant of the genus Atta. Dedicated farmers of fungus, Atta ants stream into the trees each day and return with mandibles grasping leaves they have harvested. Descending into their subter
ranean nests, the leaf-cutters chew the leaves to a pulp, preparing the compost upon which their sustaining gardens of fungus grow. In more typical forests of the tropics, the work of the Atta ants often goes unnoticed, their harvestings meager, their forays conducted under cover of night. On Guri, Atta society had taken a turn.

  The Guri forests had left too little room for the chief predators of the leaf-cutter ants. Army ants—those fearsome Mongol hordes of the tropical forest floor that consume fauna like a ground fire—were absent in the inadequate confines of Guri. Another notorious plunderer of the leaf-cutter, the armadillo, with its tremendous foreclaws excavating like backhoes, went missing from all but the biggest of the Guri islands. A lethal family of parasitic flies, practiced at laying their eggs in the leaf-eater’s heads, like so many others tended to be scarce on Guri.

  The ants’ predator-free life on Guri thus took on a frenzied pace. The Atta colonies spread like a rash, to densities a hundred times beyond the mainland norms. On the tiniest islands they merrily streamed about in broad daylight, undeterred by any pesky head-piercing flies, unchecked by the pillagings of army ants or the excavations of the armadillos. The rash as metaphor conveyed as well to the wreckage Atta wrought upon the Guri forests. The leaf-cutters’ leafy trees gave way to thorny vines and walls of climbing lianas, the bane of the Guri researchers. The barren ground ran bright red with the soil of the ants’ excavations. Unlike the howler monkeys, who at least recycled some of their nutrients by way of dung, the ants sequestered the greenery of the forest in their underground gardens, beneath even the reaches of the roots. While otherwise devouring the trees from above, they were starving them from below.