It’s an hour before sunrise on an October morning when a few dozen baseball-capped birders, binoculars swinging atop their fleece sweaters, arrive on a grassy meadow on the coast of a narrow canal on the peninsula of Cape May, New Jersey.
Up to a million birds—peregrine falcons, sharp-shinned hawks, plovers called killdeer, snowy white mute swans, sea-diving northern gannets, and parasitic jaegers hatched on the Arctic tundra among them—can be spotted along the cape’s narrow peninsula as they head south on their annual migrations. Sometimes cold fronts force them to congregate, forming rivers of birds that stream across the sky for hours.
The birders have woken up at an ungodly hour to enjoy the spectacle. They are connoisseurs of wild movements. But even they reflexively defend a natural order in which movement is reserved for a select few.
The morning sky bleeds from deep blue to a thin line of orange at the horizon. The birders scan the sky with their binoculars. Suddenly someone calls out. He’s spotted something. “Flicker!” Everyone quickly turns to the patch of sky he’s pointing to, readjusting their binoculars to find the migratory woodpecker he’s identified. To my untrained eye, the flicker passing high overhead looks not unlike a black-crayon checkmark depicting “bird” in a child’s landscape drawing, but the others murmur with awe and delight.
After a few moments, someone else spots a long line of the sea ducks called scoters flying low over the water. “This is what it’s all about!” he yells triumphantly, punching his fist into the air. Later, when the group retires to a banquet hall for a buffet meal, red-cheeked and wind-tousled, someone mentions an observatory overseas, where birds can be seen migrating by at waist height, eliciting a collective gasp.
But as charmed as these bird-watchers are by the spectacle of moving birds, the movements of creatures they deem out of place do not enchant. Reeds known as phragmites grow in tall dense stands lining the edges of the canal and the bluff beside it. According to the fossil record, phragmites have been present in the United States for at least forty thousand years. A morphologically identical but more vigorously growing strain from Europe arrived around the early nineteenth century. The reeds grow deep and strong, displacing other wetland species like wild rice and cattails, but they perform useful ecological functions in the habitat, too, filtering and cleansing dirty water and providing material that can be used for thatched roofing, baskets, fishing poles, spears, and in Egypt, a little clarinet-like instrument called the sipsi. The stems can even be dried and ground into flour.
After the morning session of coastal observations concludes, the group passes by a stand of phragmites. Even the most expert among them cannot point to any specific harm the phragmites cause. But they condemn the reeds on principle, based on their foreign origins and conspicuous health.
“They are invasive,” one woman explains to me. “It’s a shame.” The others grumble their agreement. “Look at how many seed heads they have,” one says in disgust. “They’re so hard to get rid of.” If they’d been less polite, they would have spat on them.
The phragmites are just now filtering water and providing succor to the local wildlife. The sound of warblers called kinglets rummaging inside is audible. The birds, a woman next to me says, would be “better served by something native.”1
Linnaeus, whose taxonomy first conflated wild species with geographic locales, had not delved into the question of where species originated or whether and how they’d moved into their present-day habitats. For him, species belonged ipso facto wherever he found them. And he inscribed that vision, depositing each species in its place, in the way he named them in his taxonomy.
Darwin’s theory of evolution posed an early challenge to Linnaeus’s vision. His notion that all species originated from a common source required that at some point in the past, species moved across the planet, even surmounting geographical barriers to arrive at their current habitats. Monkeys, which could not swim across oceans, had spread across the Old World as well as the New. Lizards had made it to outposts across the globe. Immobile wild creatures—beetles, trees, mollusks, and the like—had flung themselves from their common origins over unscalable mountains, unlivable deserts, and insurmountable seas.
Darwin imagined a series of Kon-Tiki-like accidents dispersing species over long distances. Seeds submerged in a bit of mud could get stuck between a bird’s toes, for example, or encrusted along its feathers, before it took off for a long migration. The tiny shell of a mollusk could attach itself to the leg of a beetle, or adhere to the inside of a shell, before being swept out to sea by a storm. Rodents scavenging near coastal kelp beds could be carried away on floating rafts by ocean swells, allowing them to reach distant shores. Over time, he wrote, sufficient numbers of such accidental long-distance journeys could have dispersed species across mountains and oceans and deserts, depositing them on even the remotest shores.
Darwin had no direct evidence of these epic voyages, but he conducted experiments to prove that species could survive such journeys. He submerged seeds from eighty-seven different plant species in bottles of salt water, fishing them out after a few months to see if they still sprouted. He procured duck legs and dangled them in an aquarium to test whether freshwater snail hatchlings might cling to them. He forced seeds into the stomachs of fish, fed the fish to birds such as eagles, storks, and pelicans, then carefully extracted the seeds from the birds’ droppings, which he germinated.
His findings suggested that 14 percent of all plant species produced seeds resilient enough to travel nearly a thousand miles.
He considered the peculiar assemblages of species on islands suggestive, too. Terrestrial species could in theory distribute themselves across continental land masses by walking, but could reach remote islands only through long-distance migrations over oceans. Indeed, he noted, islands were home to those species most likely to survive long-distance journeys. New Zealand, for example, had plenty of the plants and insects that could easily weather a Kon-Tiki raft ride, and none of the mammals and reptiles that couldn’t.
Observers spotted the kind of happenstance conveyance Darwin envisioned in 1892, when a nine-thousand-square-foot floating island replete with thirty-foot-tall living trees was seen floating off the northeastern U.S. coast. They spied it again a few months later, about twelve hundred miles northeast. If it didn’t fall apart before it reached a coast, such a floating island could facilitate the kind of long-distance colonization that Darwin suggested, ferrying seeds, insects, and other creatures along to some distant shore.
Scientists rejected Darwin’s theory of long-distance dispersals regardless. Wild species moving around the planet in unpredictable and haphazard ways, irrespective of natural borders, violated the myth of a sedentary planet. While the fact of the wide distribution of wild species was difficult to square with their shared origins, that didn’t justify abandoning the sedentist paradigm and speculating about random unpredictable events that could be neither tested nor predicted.
Many were willing to entertain even more fantastic theories, so long as they were consistent with a closed-border world. One popular theory posited that wild species had traveled from their common origins to their present distributions by walking across now-disappeared land bridges that once connected continents to islands and to one another. There was no “reasonable geological evidence”2 that such land bridges had ever existed in the places where enthusiasts imagined them, the evolutionary biologist Alan de Queiroz notes. Still, nineteenth-century writers drew fanciful land bridges on maps “willy-nilly wherever closely related species were found on both sides of a sea or ocean.” One such map imagined a submerged land bridge wending over three thousand miles from southeastern Africa to the tip of India. Another connected West Africa to the eastern coast of South America; over it, a herd of elephants might stampede their way from Sierra Leone across the Atlantic directly to Brazil in a matter of days.
The conflict between Linnaean sedentism and Darwin’s theory remained essentially unresolved for the better part of the twentieth century. The biogeographical theory that finally settled the clash emerged in the 1970s. It would stifle the history and promise of migration for decades thereafter.
The idea that the continents had once all been connected into a single whole was first proposed by the early twentieth-century German meteorologist Alfred Wegener, who had noticed how the continents’ shapes could fit together like puzzle pieces. Through some mysterious process, he said, they must have split apart, the fragments drifting to their present locations.
Decades passed before anyone believed him, mostly because no known force on earth was powerful enough to pry apart large masses of solid rock and move the continents around over thousands of miles. He failed to find any convincing evidence before perishing, wrapped in a reindeer skin and buried under the Greenland snow, in 1930. But in the 1960s scientists discovered a geological force powerful enough to explain continental drift. The theory of plate tectonics is now taught in every introductory geology course.
Plate tectonics also resolved the dilemma of how species had spread3 across the planet in a sedentary world.
For hundreds of millions of years, the continents had been fused together as one, allowing the world’s species to share a single contiguous land mass. That explained wild creatures’ shared origins and biological commonalities. Then as the supercontinent fell apart—a process that continues to this day, pushing Plymouth Rock about fifteen meters farther west today than it was in 1620—the world’s species must have been carried off in different directions. That explained their scattered distribution. Biogeographers call it the theory of “vicariance.”
Vicariance obviated the need to imagine a past full of chaotic, unpredictable movements by flora and fauna across geographic borders. Any physical shift that had occurred in the past had transpired millions of years ago, without anyone moving a muscle or ruffling a pelt.
Wild creatures didn’t cross oceans, mountains, deserts, or other geographic borders on their own. Deep underfoot, below the ponds, valleys, and glens in which mollusks, frogs, and snails lived, tectonic plates had imperceptibly shifted at a rate of about 100 millimeters a year for billions of years, unknown to the denizens above. Nobody actually moved anywhere much at all—the tectonic plates had moved for them.
Biogeographers started finding clues4 in geological history that explained the mysteries in species distributions they’d long pondered. Flightless birds with clearly shared ancestry lived in the far-flung continents of Australia, South America, and Africa. How had they dispersed so widely? Their common ancestor had likely populated each continent when the three had been connected. Hoofed ruminants lived in North America, where they’d evolved into moose and caribou, as well as in Asia, where they’d evolved into elk and reindeer. Their common ancestors probably populated the two continents when they were connected, too. Marsupials were nowhere to be found in India and Africa. Why not? They likely drifted off from the supercontinent before the ancestors of marsupials could climb aboard.
Biogeographers couldn’t work out all the details of how geological forces had distributed species around. But they felt confident they would. Any number of geological changes could be called on to explain how species had been moved: the formation of a mountain range, slowly splitting a species into two; falling sea levels creating land bridges that allowed once-marooned species to colonize new territory.
Vicariance restored a “biological version of inertia,”5 as de Queiroz put it. Biogeographers allowed that the accidental, long-distance dispersals Darwin imagined might have occasionally occurred, but otherwise they dispensed with migration as a coherent explanation of where species belonged and how they’d got there. The biogeographer Gary Nelson called Darwin’s theory of long-distance dispersals the “science of the improbable, the rare, the mysterious and the miraculous.” The very notion was “negative, sterile and superficial,” the zoologist Lars Brundin added. It “offends the critical mind.”
The few biogeographers who believed6 in long-distance voyages as a viable theory of history might as well claim that “some lucky humans” would “learn to fly,” the paleontologist Paul Mazza wrote. In the story of the movement of species around the planet, biogeographers relegated random long-distance leaps to little more than “footnote acknowledgment.” Such misadventures were “almost by definition random” and “hence uninteresting,” a paper published in a 2006 issue of the Journal of Biogeography noted.
Biogeographers questioned not only the role of long-distance journeys in history but also whether wild species could even survive such voyages in the first place. Most animals could not, critics claimed. In one 2014 paper, a paleoecologist from the University of Florence described a little jackrabbit who’d been found floating on a bed of kelp set adrift by a storm, about forty miles off the coast of California. After a few days at sea, the bunny was half-dead from dehydration and heat exposure. The creature hadn’t even made it across the dozen or so miles between California and the Channel Islands. No jackrabbit ever had.7
In the history of nature envisioned by vicariance biogeographers, the movement of species had been so slow, passive, and imperceptible that active, long-distance wild movements could play no role in nature or in history. It underlined what many had known to be true since Elton warned of alien invaders after the Second World War: plants, animals, and other creatures that crossed borders and entered novel territory were trespassers, invaders, and aliens who threatened the natural order.
The U.S. government has explicitly managed the national parks as oases8 from the ravages of alien border crossers since the 1960s, when it heeded the advice of conservationists such as Aldo Leopold’s son, the zoologist A. Starker Leopold. He had recommended that the nation’s national parks “preserve, or where necessary … re-create the ecologic scene as viewed by the first European visitors,” which presumably was when Leopold suspected the long era of stillness ended.
The government extended those protections9 to the entire nation in 1999, when then-president Bill Clinton established the National Invasive Species Council, a body tasked with repelling “alien species” whose “seeds, eggs, spores or other biological material” were “not native to that ecosystem.” After the terror attacks of September 11, 2001, guarding the nation against invasive species became one of the charter functions of the newly formed Department of Homeland Security, enshrining the business of policing natural borders into the national security infrastructure.
For years, conservation-minded people around the country had cleansed their gardens of alien species and joined native plant societies to champion the cause of the endangered natives, reflexively deriding newcomers like the phragmites lining the canals of New Jersey. Scientists joined the effort in the 1980s. Three new subdisciplines emerged10—conservation biology, restoration biology, and invasion biology—all aimed at tracking the damages that border-crossing wildlife caused.
The pace of the onslaught was “unprecedented,”11 one ecologist said. Already, over the last five hundred years, newly arrived species had come to dominate some 3 percent of the earth’s ice-free surface. In many countries, these species composed 20 percent or more of the resident flora. California, England, Louisiana, and Chicago had been invaded by “German” wasps, “African” snails, “Chinese” crabs, and “European” mussels, one prominent invasion biologist warned.
In books with titles such as Immigrant Killers, Alien Invasion, and Feral Future, writers laid out the case against wildlife on the move.12
According to the “enemy-release hypothesis,” for example, intruders eluded native predators in ways that native species could not, giving them a dangerously unfair advantage; conversely, they preyed on native species in ways that native predators could not. In Hawaii, a local invasive species council noted, native species had lived “in relative isolation over … 70 million years,” evolving in the island’s “benign environment.” These native inhabitants would be ravaged by an onslaught13 “nonnative, competitive” species from elsewhere, with their thorns, sharp hooves, toxic secretions, and carnivorous appetites.
Invasion biologists pointed to the intruders’ growth as an indicator of their success in displacing native species. Argentine ants grew larger in areas they invaded than in their own native habitats, two Stanford biologists noted in a paper outlining the evolutionary impact of invasive species. Within just twenty years of arriving on the North American West Coast, fruit flies from Europe evolved altered wing sizes and extended their range from southern California to British Columbia.
The newcomers mixed with local species, which raised the specter that they’d contaminate local species with alien tissue. Mallard ducks hybridized with New Zealand gray ducks, Hawaiian ducks with Florida mottled ducks, sitka deer from Japan with reed deer from Great Britain. The interbreeding was “massive,”14 Stanford biologists wrote in a 2001 paper in the Proceedings of the National Academy of Sciences.
They took ecological jobs from the natives, as in Britain, where American gray squirrels displaced the native red squirrels, and facilitated the arrival of more of their own kind, in a sort of chain migration. Introduced species formed “synergistic relationships” with other introduced creatures, according to a new preface that appeared in a 2000 reissue of Elton’s 1958 book. As a result, if one appeared, there’d likely be many soon enough. The zebra mussel, for example, had enabled the arrival of a Eurasian water milfoil, a feathery flowering plant that lived underwater. In the arid hills of southern California, the hooves of introduced cattle destroyed the delicate crusts formed by lichens and mosses in the dry soil. That damaged the habitat of the native plants that the native checkerspot butterflies fed on. Alien plants thrived instead, pushing checkerspots to the edge of extinction.15
Intruders such as the zebra mussel, which had arrived from Russia into North America during the nineteenth century and spread into the Great Lakes, marred boat hulls, blocked water pipes, and attached themselves to native clams, preventing them from getting enough food. Invasion biologists suspected that they’d cause the collapse of native clam populations. Invasives like purple loosestrife from Europe, growing in tall vigorous stands with prominent purple flowers, would displace native cattails and harm local wildlife. Municipalities spent millions of dollars trying to suppress it.
One invasion biologist calculated that wild species moving freely across the planet would ravage large swaths of ecosystems. The number of land animals would drop by 65 percent, land birds by 47 percent, butterflies by 35 percent, and ocean life by 58 percent. Based on assessments such as this, experts described newly introduced species as the second-largest threat to biodiversity16 in the United States. Invasion biologists tallied the net cost of biological invasions at $1.4 trillion, or 5 percent of the value of the global economy. The newcomers were the “mindless horsemen of the environmental apocalypse,” the Harvard ecologist E. O. Wilson warned.
Given the stake, ecologists considered facilitating animal movement, whether on purpose or by mistake, to be so perverse and dangerous that they rejected the idea of moving species even to save them. Camille Parmesan, worried about the fate of checkerspot butterfly colonies, stood up at a scientific conference and suggested moving some threatened checkerspot colonies17 elsewhere. Her fellow ecologists erupted. The very idea overwhelmed them with horror and emotion, she remembers. “They accused her of playing God; of tampering with nature,” a Guardian article on the ensuing hubbub reported. “Her approach would set off a whole new chain of problems.”
If invasion biologists’ and other scientists’ concerns about the threat posed by species on the move sounded similar to those articulated about human migrants, they were. The corrective action required was similar, too. There’d be no relaxing of the borders, no welcome, no easing of assimilation for the newcomers. The intruders had to be eradicated.18 It was a “nasty necessity,” Stanley Temple, an ecologist and science adviser to the Aldo Leopold Foundation, wrote in 1990.
The team of scientists, wearing shorts and carrying axes and shovels, threaded their way through a tangle of jungle in the shadow of Mauna Loa, the world’s largest volcano, which sprawls across the largest island of the Hawaiian archipelago, Hawaii. Inside this dense, humid forest, beneath the flaming blossoms of ‘ōhi‘a trees, ancient biogeographical borders had been transgressed.19 The ragtag group of botanists, led by curly-haired Rebecca Ostertag and tall, wiry Susan Cordell, intended to do something about it.
The island’s original inhabitants, some twelve hundred species of plants and animals that had made their way to Hawaii on their own steam, were a special bunch, with unique qualities that allowed them to survive the searing lava that periodically poured over the island. The changeling ‘ōhi‘a could tolerate almost any kind of soil, including fresh lava flows. For millennia, it and other native Hawaiian species had lived in isolation from the chaos of the continents.
But then people started to come over, bringing cosmopolitan outsiders like pigs and dogs and rats and new diseases. They brought tree frogs from Puerto Rico, mongoose to control the rats, and ornamental plants to grow in their gardens. Birds ate their fruits and spread the seeds around in their droppings, which quickly bloomed under Hawaii’s tropical sun. The island was soon overrun by sharp-elbowed newcomers, sucking up the island’s nutrients and stealing its sun.
About half the flora clinging to the side of Mauna Loa, Ostertag and Cordell knew, were nonnative foreigners. For now, the old ‘ōhi‘a trees still dominated the overstory, but that wouldn’t be true for long. In 2010 farmers had noticed a strange fungus decimating the island’s iconic‘ōhi‘a trees. Nobody knew what it was, exactly, but most presumed it was an alien newcomer, too. Soon it would take the ‘ōhi‘a trees out. When it did, they’d be fully replaced by the outsiders. The young trees and saplings below bristled with foreigners. The forest ecosystem that the native Hawaiian species had forged thousands of years ago would vanish.
The botanists marked out four one-hundred-square-meter plots in the jungle. During a few brutal months of back-breaking labor, they destroyed every immigrant species they could find within its borders. They cut down trees with saws, then doused the stumps with deadly herbicide. They yanked shrubs and ferns, prying their roots out of the rocky ground. They set up giant funnels to capture any seeds that might rain down from above. Meticulously, they cleansed their plots of any detectable hint of nonnative tissue.
The battle against invasive species presumed their movements into new habitats via global trade and travel to be historically and ecologically aberrant. But scientists didn’t really know how far butterflies could fly, or whether wolves could surmount mountain ranges, or if crocodiles swam in ocean currents. For centuries, tracking animal movement had been a haphazard affair, relegated to the “margins of ecological research,”20 as a 2015 paper in Science put it. Experimental methods, whether by design or by necessity, could rarely capture the scale of animals’ wanderings across the landscape.
Like the British military’s inadvertent discovery of bird migrations through radar, many dramatic and long-distance movements21 had been discovered by accident. European observers first understood that storks wintered in Africa, for example, after they’d happened upon the stork with a spear of clearly African origin pierced into its side. Nineteenth-century scientists considered the owls called saw-whets, which migrate thousands of miles every year, to be a “general and constant inhabitant of the Middle and Northern states,” as one put it, until ornithologists happened to find thousands of birds in the sea after a freak snowstorm forced them out of the sky.
Even the now-famous migration of monarch butterflies22 from eastern North America to Mexico had been discovered serendipitously. In the 1930s Fred and Norah Urquhart, zoologists at the University of Toronto, had noticed the monarchs’ disappearance every winter and their reappearance in the spring with tattered wings, as if they’d been on some long journey. They started gluing little tags on the butterflies’ wings, reading “Please send to Zoology University Toronto Canada.” Over the decades, only a few handfuls had been returned. Many arrived from points south of Toronto, but whether that indicated a butterfly flight or simply one swept up in the breeze remained unclear. The mystery would have continued except that in 1975 the pair of zoologists traveled to Mexico. During a hike into the mountains of Michoacán, they saw millions of monarchs coating the trees, their fluttering wings sounding like a waterfall. A pine branch laden with butterflies happened to crash down in front of them, a tiny paper tag affixed to one of the butterflies’ wings revealing its northerly origins.
The popular “mark-and-recapture” method had scientists tying threads to birds’ feet, as the nineteenth-century ornithologist John James Audubon did, or using Magic Markers to draw dots on butterflies’ wings, as the twentieth-century butterfly biologist Paul Ehrlich did. Others used plastic ID tags, dyes, paints, and the like, or set up cameras that shot photos of animals that happened to pass by. By marking individual animals and then recapturing them later, their movements could be at least crudely inferred. But however animal trackers marked their wiggling subjects, mark-and-recapture methods allowed scientists to confirm only that subjects had moved where they thought they probably might. If a dotted butterfly or a bird with a thread on its foot evaded recapture, scientists were free to use their imaginations to determine what might have happened.
In one study, for example, Ehrlich had marked 185 butterflies and then set them free, returning to search for them a few days later. He found 97 of the marked butterflies flitting around just where he’d marked them in the first place. The other 88 eluded recapture. They could have moved beyond where he’d looked for them, but he assumed they had all perished. He concluded that checkerspots had a “remarkable lack of wanderlust.”23
Marking animals with tags that emitted signals that could be detected as they moved, like a bell on a cat’s collar, circumvented the quandary of confirmation bias but introduced other problems. The tags could be heavy, which could disrupt the animal’s behavior. They were expensive. “It cost 3,500 dollars,”24 to tag a single animal, remembers one animal-tracking scientist. “You put it on the strongest animal” and hope for the best. They had limited battery life, so after a while they’d just stop sending signals, leaving scientists in the dark about where their slippery wearers had got to.
Some tried to conserve the tags’ energy by programming them to ping only once a day or so, which gave only the most rudimentary outline of an animal’s movements. But however scientists chose to triangulate between the weight, expense, and energy needs of their tags, to get any data they still had to essentially follow the animals around, capturing the signals on their receivers. Early efforts involved chasing behind the tagged birds by car, or helming light aircraft to fly slowly behind them so the little beeps and pings could be logged. “We’d have to physically go near the elephant,”25 one animal tracker remembers, “fly over it, and locate it with the antennae on either side of our airplane until we caught sight of it. Then, by sight, we’d estimate where we were on a map and mark a little cross there. That’s just the way it was.”
The U.S. military had a much better system.26 MIT scientists had noticed that radio signals transmitted by the Russian satellite Sputnik increased and decreased as the satellite’s orbit approached and then retreated. So the military started sending signal-emitting satellites into space. By the 1990s, its Global Positioning System of satellites emitted signals continuously, and there were so many that at least four could be detected at any place on earth at any time of day. In theory, animals outfitted with GPS tags could be tracked wherever they went on the planet, with no need to follow them around with a receiver. But fearful of aiding adversaries with navigational prowess, the Defense Department introduced an unpredictable and erratic jitter into the signals, purposely degrading their accuracy. The GPS signals could be accurately interpreted only by receivers owned by the military. Everyone else got a uselessly faulty result.
And so for scientists as for the rest of us, the movements of animals remained mostly hidden. Even the ones who lived among us crept and crawled and darted around out of sight. Sometimes we’d notice, with a jolt, the faint trails they left behind—a few paw prints in the snow, an abandoned nest in the shrubbery—suggesting their passage. But usually, crossing paths with a wild animal, even common ones that lived around human habitations like deer or fox, was an occasion of surprise and delight.
A few weeks ago I saw a red fox in my driveway. It shouldn’t have been surprising, since I had heard that a pair of foxes had moved into the neighborhood. But even though we’d been sharing our patch of suburbia for a few months, I had little awareness of their whereabouts. I froze in shock at the sight.
The long, curving trunk of the highland tamarind tree27 arcs in a balletic stretch across the misty forest, thousands of feet above the sandy shores of Réunion, a volcanic island of less than one thousand square miles in the Indian Ocean. To find the tree, whose wood could be used to build fishing canoes and to roof houses, locals climbed three thousand feet up the pitched side of the volcano, until they glimpsed the trees’ strangely twisting branches looming out of the fog as if part of some enchanted forest.
The otherworldly highland tamarind tree shares a striking similarity to another tree, which similarly lives on a volcanic island ringed by coral reefs: the koa tree, which can be found growing in the deep ash deposited along the slopes of Hawaii’s volcanoes, smoky blue butterflies drinking nectar from its flowers. People in Hawaii use wood from the koa tree for their ukuleles and surfboards.
The likeness between the two species puzzled botanists28 for centuries. It seemed impossible that the Hawaiian koa and Réunion Island’s highland tamarind shared any ancestry, as scientists could conceive of no movement that could have allowed one to seed the other. The two islands, separated by eighteen thousand kilometers of ocean, had no geographic or geological connection. They were as far away as any two specks of land on earth could be. No current or wind flow or migratory bird route connected them. Even if a seed had been somehow ferried across the ocean it would have been unlikely to survive the journey. The seeds are thin-walled and can’t even float. Nor do they grow on seashores.
Botanists settled on two equally unsatisfying explanations29 for the similarities between the koa and the highland tamarind. Maybe the two trees shared no relation at all, in which case they’d somehow evolved to look exactly as if they did. Or maybe human migrants had picked them up and moved them around, although just who had done that, when and why, no one could really say.
Historical biogeography was full of such unsettled matters. Biogeographers stitched together the story of passive imperceptible movements by associating geological events with species distributions, based on fossil evidence. When it wasn’t possible, they came up with likely stories that made sense within their framework of a sedentary world.
Then, using the same molecular clock methods that had upended ideas about the timing and scale of human migrations, molecular biologists started testing those stories.
Scientists reported their findings on the genetic relationship between the koa and the highland tamarind in a 2014 Nature paper. The highland tamarind, they’d found, had directly descended from the koa. Some Réunion Island tamarinds were, in fact, more related to Hawaiian koas than to each other. And the seed that connected the two had accomplished the epic journey between Hawaii and Réunion Island 1.4 million years ago, before Homo sapiens even evolved.
The genetic evidence meant that somehow the koa tree had traveled across eighteen thousand kilometers of ocean and colonized Réunion island. The koa’s voyage was the longest single dispersal event ever recorded. And it wasn’t the only one that molecular biology findings suggested.
Vicariance theory attributed the separation of monkey species30 into New World and Old World species to the opening of the Atlantic Ocean, which had slowly and passively separated the two lineages. But according to the findings of molecular biologists, the species hadn’t diverged until 30 million years after that ocean emerged. The monkeys couldn’t have been passively separated. Their ancestors must have crossed the ocean.
Rodents in South America, which according to vicariance theory had traveled over the Panamanian isthmus, arrived years before geological forces formed the land bridge connecting the two American continents. The rodents must have surmounted the sea.
Vicariance theorists presumed that the geological breakup of Gondwanaland, which split southern South America from Australia, and Madagascar from India, had gradually rent once-contiguous plant species. But that didn’t match up with when the plant species diverged from one another. According to an influential 2004 study that one botanist dubbed “the last great gasp of the vicariance paradigm,” the plants hadn’t been carried on tectonic plates, either. They’d actively moved.
The molecular findings suggested a host of long-distance journeys in the deep past. Monkeys had made their way from the Old World to the New, at a time when that journey required crossing the Atlantic Ocean. Polynesian sweet potatoes, which had diverged from American sweet potatoes tens of thousands of years before humans could have carried them to Polynesia, colonized the Pacific on their own. Rodents catapulted themselves from North to South America before any land route existed. This kind of non-geological movement, irrespective of geographic obstacles, was exactly the kind of improbable, rare, and mysterious movement that Darwin had been talking about.
The koa tree’s journey may have been a “giant fluke,” de Queiroz commented, “but that’s part of the message of a lot of recent biogeographic studies,” he said. “Giant flukes happen.”31
As new molecular techniques recovered the dramatic story of animals’ past migrations, other new technologies transformed scientists’ ideas about how they moved in the present. The revolution in animal-tracking technology that made it possible began a few minutes after midnight on May 1, 2000. That was when the Defense Department stopped adding a jitter to its GPS32 satellites’ signals, allowing them to flow uninterrupted to anyone in the world with a receiver. (They had figured out a way to selectively block the signals as necessary to deter adversaries.)
An $8 billion GPS technology industry sprang up, unleashing a blizzard of new products, including solar-powered GPS tags so small and light, they could be attached to the furry ear of a baby bear or the slippery shell of a sea turtle. New solar-powered GPS tags allowed people to track the once-undetectable movements33 of animals continuously, in real time, over their entire ranges and lifetimes. Animal-tracking scientists such as the ornithologist Martin Wikelski, who had grown up on a farm in Bavaria amazed by the local barn swallows that flew all the way to South Africa, quickly outfitted their roving subjects—cranes, dragonflies, oilbirds, and more—with the new tags. The new GPS data were augmented with observations from people around the world who were newly connected by social media. Whale-watchers shared observations with others in Iceland; bird-watchers uploaded millions of bird sightings via apps on their phones. By 2016, more than three hundred thousand birders had logged 11.8 million bird sightings around the world on one such app, eBird.
The results were stunning. “Every time we look,” Wikelski says, “we find totally amazing new information …34 that turns around our knowledge.”
Arctic terns logged 70,900-kilometer migrations, nearly twice as long as previous estimates. A few years later, another tagging experiment found that terns traveled nearly a third farther than even that. Jaguars in the Peruvian Amazon, whose ranges scientists had estimated to be about the size of Manhattan based on studies with camera traps, ranged across areas ten times bigger. Tracked zebras on annual migrations traveled five hundred kilometers round trip, one of the longest land migrations ever logged. Estuarine crocodiles in Australia, which were presumed to avoid ocean travel, swam over two hundred miles into the sea, traveling along ocean currents. Dragonflies migrated from the eastern United States to South America, flying hundreds of kilometers every day. Tiger sharks, assumed to be permanent residents of the coastal waters around Hawaii, turned out to travel thousands of kilometers out into the sea. Scientists’ assumptions about their provincialism, a shark researcher from the Hawaii Institute of Marine Biology said, “were completely wrong.”
In one particularly epic journey tracked by satellite, a wolf collared in Trieste, Italy, trotted over one thousand kilometers across frozen rivers, six-meter-deep snows, and 2,600-meter-high mountain passes, into Austria, traveling continuously for four months.
Wild species regularly roam beyond the borders35 that scientists have defined for them. Giraffes in Ethiopia spend most of their time outside the borders of the park that was specifically designed to protect them. Green turtles in the Chagos Archipelago swim beyond the boundaries of the marine protected area meant to contain them. Forty percent of the birds in the limestone caverns of Venezuela’s El Guácharo National Park roost and forage outside its borders. Elephants that were thought to restrict their movements within Kenya wander across the border into Tanzania.
Their movements are not simple. The more extensively scientists track animals, the more complexity they discover. A three-year study in the Himalayas of tragopans, bright red pheasants that scientists understood to move uphill in summers and downhill in winter, revealed that they move both uphill and downhill in winter; some even migrate elsewhere altogether. One biologist, who mapped out badgers’ underground burrows by checking on the animals’ locations once every twenty-four hours, found more burrows in direct proportion to the frequency with which he checked. Even checking badgers’ movements every three seconds, he discovered, wasn’t enough to reveal the labyrinth of passages the badgers created to facilitate their movements. To accurately capture it, he’d have to sample ten times every second.
The physiological ease of animals’ movements has been underestimated. Pythons from Southeast Asia deposited in Florida navigated back to the precise location of their release, a journey of over twenty kilometers of Florida swamp, straightforwardly and with speed, months later. A tracked leopard made it across three countries in southern Africa, successfully circumventing the towns, cities, and roadways that scientists had presumed would make the journey impossible. Bar-headed geese that flew over the Himalayas ascended from sea level to over six thousand meters not during the day, when tailwinds would help boost their flight, but at night, against headwinds. Researchers dubbed it the “most extreme migration on Earth.”
The myth of a sedentary world had cast wild species as so limited in their capacity to move that their most far-reaching movements could only be mediated by humans. In fact, their ability to transport themselves in complex, sophisticated ways eclipses ours.
Movements that were once dismissed as robotically controlled by genes appear to be the result of dynamic interactions between individuals, each responding to subtle cues in the environment and from one another. Songbirds whose movements were thought to be controlled by genetic instructions to head south at a certain time coordinate the timing and direction of their movements according to subtle factors in the environment and cues from one another. Black warblers follow complex routes through the sky, taking advantage of a network of wind highways that swoop over the seas and continents. Baboons thought to robotically follow their leaders decide dynamically between different pathways. When two baboons move in different directions, their followers split the difference between their trajectories, plotting a course in between the two. Even the spiders36 thought to be passively carried around on winds actively climb to the tops of plants, where they attach their silken threads and stand on tiptoe, waiting for the breeze to gather them up.
“Humans trying to achieve this37 in the absence of modern technologies,” noted the ecologist Iain Couzin, referring to the mass movements of insects, birds, and other animals, “would be unthinkable.”
The study of animal movement, once relegated to the margins of biological research, has shifted toward its center. In 2006 a group of scientists gathered at the Israel Institute for Advanced Studies in Jerusalem to sketch the outlines of a new approach that would situate movement as one of the central features in the behavior of animals and the functioning of ecosystems. They called the new field “movement ecology.”38 The following year Wikelski and his colleagues started Movebank, a public database where scientists can share their animal tracking data. Animal trackers add about a million data points every day.
On a February morning in 2018, a small knot of scientists stood on the edge of a snow-covered field in the steppes of Kazakhstan, clad in heavy black parkas and fluffy hats with ear flaps. The subzero wind chapped and reddened their exposed faces. They watched the horizon, where a Russian Soyuz rocket is about to blast into the dreary gray February sky. When the slim white cylinder lifts off, the fiery blaze behind it like a gash in the firmament, they stamped their feet, flung their arms around each other, and hollered with delight.
The rocket, speeding at seventeen thousand miles per hour toward the International Space Station, carried on board two hundred kilograms of antenna. A few days later two Russian cosmonauts on the station would embark on a five-hour space walk, for which they’d been training for months, during which they’d mount the antenna on the exterior of the station. And with that, they’d commence a new phase in human understanding39 of the scale and tempo of wild species’ movements across the planet.
During every orbit, the antenna would scan the surface of the earth sixteen times, picking up data from thumbnail-size solar-powered tags ecologists across the globe had fitted on the backs of fish, the legs of birds, and behind the ears of mammals. The scientists would be able to control and reconfigure the tags whenever they liked. To begin with, they’d stream a continuous log of the animals’ locations along with clues to their behavior embedded in data about their orientation, as well as factors such as the temperature, humidity, and pressure around them. The new satellite-based system is called the International Cooperation for Animal Research Using Space, or ICARUS. It’s been described as a kind of “internet of animals,” illuminating in real time the intricate web of animal tracks across a dynamic earth.
Given movement ecologists’ insights into biological processes gleaned from tracking individual species, Wikelski predicted even deeper insights from tracking many species simultaneously. Seeing disconnected patches of sky hadn’t allowed astronomers to understand the universe. That became possible only after they set up a network of telescopes to survey all of space at once. Movement ecologists hope to effect a similar revolution in understanding with ICARUS. “We see the whole network of animals40 around the world as one big information system,” he said, “that is so far untapped.”
Wikelski stood in the Kazakhstan snow wearing a black knitted hat with large white polka dots, his neck snug in a thick knitted scarf. After a round of bear hugs, he and the others went off to enjoy a celebratory shot of vodka.
As new data about animal movements past and present pile up, ecologists have started to reevaluate their theories41 about the damage caused by border-crossing species on the move.
Invasion biologists who predicted ecological Armageddon caused by species on the move had underestimated the scale and tempo of wild movements, most of which were not disruptive. One analysis showed that only 10 percent of newly introduced species establish themselves in their new homes, and only 10 percent of those flourish in ways that can threaten already resident species. Condemning all newcomers as inevitably damaging blames them all for transgressions committed by 1 percent or less of their members.
When the Suez Canal artificially joined the Mediterranean with the Red Sea, which had been separated for millions of years, over 250 species moved from one side to the other. Their movement had led to the single extinction, according to scientific assessments a century later, of a sea star called Asterina gibbosa. The introduction of eighty marine species into the North Sea, and seventy species into the Baltic Sea, led to zero extinctions among the locals.42
Because displacements don’t happen at the scale that invasion biologists predicted, the arrival of newcomers increases biodiversity.43 In a paper that was rejected by Nature because of the way the public might misinterpret it, the Canadian ecologist Mark Vellend found that wild newcomers generally increase species richness on a local and regional level. In the continental United States, four hundred years of open borders to wild migrants has increased biodiversity by 18 percent.
To calculate the burden caused by wildlife on the move, invasion biologists had included not just the damage that newcomers cause but the cost of preemptively getting rid of them, too. They excluded the economic benefits of wild migrants. Including the benefits only of the introduced plants that contribute to the global food supply added $800 billion to the plus side of the equation.
When the botanist Ken Thompson compared the impact44 of successful introduced species to that of successful resident species, he found “in almost all respects they were the same.” Even some of the most prominent newcomers failed to fulfill invasion biology’s predictions. Zebra mussels cannot be blamed for the collapse of native clams, which face a number of challenges besides the newcomer’s appetites. And besides disrupting local ecosystems, the mussel also contributes to them, by filtering water and providing food for fish and waterfowl. “If zebra mussels were native,” Thompson noted, “there’s every reason to expect they would be hailed as environmental heroes, rather than vilified as public enemy number one.”
When Canadian researchers compared plots with and without purple loosestrife, they found that the plant neither reduced diversity nor displaced native species.45 There is “certainly no evidence that purple loosestrife ‘kills wetlands’ or ‘creates biological deserts’ as it is repeatedly reported,” a 2010 review paper concluded. Their biggest crime, Thompson writes, is being conspicuously successful. Even that hasn’t lasted: in places where they’ve been present a while, they tend to decline.
“The classifications of species as either ‘native’ or ‘alien’ is one of the organizing principles of conservation,” a 2007 review paper noted, but “the validity of this dualism has increasingly been questioned.” Species move around in ways that belie any simplistic categorization as “native” and “alien,” Thompson says. In his book Where Do Camels Belong?, Thompson noted the case of the camel, depicted on animal maps as being “native” to the Middle East. But the camel family evolved and attained its greatest diversity in North America, is presently most diverse in South America, and occurs in the wild only in Australia.
Thompson and other critics of invasion biology don’t dismiss displacement of local species as an urgent problem. On remote islands, for example, introduced species could effect dramatic displacement of already resident species. But even in such places, it isn’t only newcomers that disrupt and encrouch on locals. Natives do, too.46
In the jungles along Mauna Loa, efforts to rid a patch of forest of intrusive newcomers failed miserably. Rebecca Ostertag and Susan Cordell had tried for years, but to no avail. Even if the botanists removed every foreign weed and captured every alien seed that rained down from above, the newcomers would keep coming back. The invisible seeds and spores of the newcomers infest the ground all around. Keeping even one of the tiny plots free of foreign contamination required forty hours of hard labor a week. “It was absolutely way too much,” Ostertag said.
The invasion of species on the move appeared unstoppable. Finally, Ostertag just gave up. “Getting back to an all-native system,” she said, “is completely unrealistic.”47
But it wasn’t just futile. It also seemed unnecessary. Ostertag and Cordell came to realize that the native species in Hawaii are not necessarily any more ecologically functional than other species. When Ostertag graphed the functional traits of Hawaii’s native species on a chart, they all clumped together in a corner. The native ecosystem had been “dysharmonic,” Ostertag says. It was like a picnic consisting entirely of potato salad. Whole functional groups were missing. There’d been no amphibians, no mammals, no reptiles, no ants, and no gingers among the plants. Because they all had to be able to survive under Hawaii’s harsh conditions, the native species of Hawaii were a peculiar group. That’s why the newcomers who followed thrived the way they did. It wasn’t that they were rapacious aliens with sharp elbows. They filled ecological openings that the locals had left open.
A few days before I arrived in Hawaii, the fungus that had been decimating Hawaii’s ‘ōhi’a trees was identified. Based on its behavior, most scientists had presumed the killer would be an interloper. But it turned out that the fungus could be found nowhere other than Hawaii. No one came out and admitted it, but the killer could only be called a “native.”
Ostertag and Cordell devised a new experiment,48 taking into account how natives and aliens might live together. Instead of ridding the forest of newcomers, or letting them take over entirely, they aimed to rebuild a stretch of the forest into a mixed, diverse community comprising both newcomers and old-timers, natives and aliens. They chose which plants to grow and nurture based not on where they had come from or when, but on their traits and what they could contribute to the ecosystem. They spent three years setting up the experimental hybrid ecosystem. By the time I visited, the trees there had grown more than twenty feet tall, and the canopy was beginning to close. The rate of new arrivals had stalled as the forest floor shaded over, depriving new seedlings of light, a sign that the hybrid forest was maturing into a self-sustaining one.
As we made our way to the field site, I asked them what the native forest might have looked like. Neither Ostertag nor Cordell claims anymore that natives are any better ecologically than newcomers. But their affection for the old-timers is still palpable.
Cordell, wearing gold hoop earrings and her hair pulled back by sunglasses perched on her head, describes how she imagines these jungles before the newcomers arrived. The ‘ōhi‘a and other native trees would have dominated the overstory of the primeval Hawaiian forest, she says, lianas and vines dripping from their canopies to the forest floor. The ground itself would have been blanketed by lush stands of tree ferns, she tells me.
I look down at the ferns at our feet. They are scattered across the forest floor, sprouting alone and in little clumps. They’re still here, I say to Cordell.
“Yeah,” she says, drawing out the word. “But these aren’t native unfortunately.”
Is there something wrong with them, I ask, besides that?
“I don’t know,” she says. “That’s a hard question. I mean, when I started in conservation, I would have said everything nonnative is bad. But I don’t think that way anymore. This project has really turned my world.” She pauses. “I mean, this is our life! This is where we are in the world! And we’re scientists. And isn’t this interesting to study?”
It’s possible, I think, that she’s trying to convince herself.
She looks down at the fern. “And a lot of them are pretty, you know.”
Funza, Colombia, is a small town in the Andes, about eight thousand feet above sea level. What’s now a high plain lay, during the Pleistocene era, at the bottom of a lake. In 1989 the geologist Lucas Lourens and his team positioned a Portadrill truck just outside the village, drilling a narrow hole nearly six hundred meters down, until they hit the bedrock.
The sedimentary core they brought up represented a record of the species that had lived in the area over millions of years. The remains of animals had long disappeared, but the pollen shed from the plants, trees, herbs, and shrubs that had grown in the area had settled in those sedimentary layers, one atop the other, sinking deeper over time.
I learned about what Lourens and his team found thanks to a casual mention of their 2013 paper in Ken Thompson’s book about the folly of splitting wild creatures into natives and aliens.49 As far as I know, its findings had never reached a broad audience, the way alarmist stories about foreign plants and animals often did. There’d been no magazine stories or radio episodes about Lourens’s study. But the glimpse into biological history that it offered struck me as deeply moving.
The pollen revealed a continuous process of climatic change and migration. As the landscape transformed, new species migrated in and out. There was pollen from swamp forest trees, buttonweed, cypress-like shrubs, heathers, and medicinal herbs. When the Panamanian isthmus rose up, unleashing migrant flows between the North and South American continents, pollen from oak trees arrived.
The types of species that Lourens and his colleagues found, and the combinations in which they found them, never repeated. At each moment in time, the species that inhabited this slice of land in Colombia were completely new, living in mixed communities that would have been unrecognizable to denizens that had been there before and that would come afterward. Each layer of the sedimentary core represented a single “frozen moment,” they wrote, in a “long and dynamic process of almost continuous reorganization.”50
Climatic regimes came and went. Geological eruptions made their slow-motion advances and recessions. Sea levels rose and fell. Monkeys crossed oceans. Ferns colonized Hawaii. Koa trees sired their progeny on Réunion Island. Homo migratio left Asia and canoed into the Pacific, guided by the stars.
With each change, new opportunities for species on the move opened up. As those opportunities arrived, the migrants came.
That’s because nature transgresses borders all the time. And with good reason.