Книга: The Human Age

The Slow-Motion Invaders

The Slow-Motion Invaders

Named P-52, as if she were a bomber or a precious fragment of papyrus, the Burmese python recently found in the Everglades weighed 165 pounds and stretched 17 feet in length, setting a local record (not a world record—that’s held by a 403-pound, 27-foot-long python residing in Illinois). A tan beauty, with black splotches that resemble jigsaw puzzle pieces, dry satiny skin, and a body like a firm eraser, P-52 had a pyramidal head, a brain surging with raw instinct, tiny black Sen-Sen eyes, and a mind like a dial tone. In her heyday, she could squeeze the life out of an alligator or a panther. And she was pregnant.

Standing shoulder to shoulder at the dissecting table, amazed University of Florida scientists uncovered eighty-seven eggs in her womb. Not all the hatchlings would have survived. But with such fecundity it’s easy to understand the flourishing of pythons throughout the southern region of the Everglades—slipping through the sawgrass, sibilant as sassafras, slanting up to their prey, and then—slam!—seizing hold with back-curving teeth, crushing and slowly swallowing every morsel.

No one knows precisely how many pythons inhabit South Florida, but reliable estimates run to thirty thousand or more. Over the last ten years, snake wranglers removed 1,825 pythons from as far north as Lake Okeechobee and as far south as the Florida Keys. In the picturesque, if amusingly named, Shark Valley (no sharks, a valley only a foot deep) in the heart of the Everglades, visitors may glimpse a python plying the river of grass, or even wrinkling across the road. Pythons will also be busy hunting, sun-swilling on the canal levees, mating (in spring), coiling around their eggs and trembling their muscles to incubate them, occasionally wrestling with alligators, and absorbing warmth from still-toasty asphalt roads at night.

Alas, they’ve vanquished nearly all the foxes, raccoons, rabbits, opossums, bobcats, and white-tailed deer in the park; also the three-foot-tall statuesque white wood storks. A survey conducted between 2003 and 2011, and published in the Proceedings of the National Academy of Sciences, reported that raccoons had declined 99.3 percent, opossums 98.9 percent, and bobcats 87.5 percent. Marsh rabbits, cottontails, and foxes completely disappeared. Last year, one python was found digesting a whole 76-pound deer.

Where did all the pythons—native to India, Sri Lanka, and Indonesia—come from? Some were wayward pets or hitchhikers in delivery trucks. Others escaped from ponds overflowing in heavy rains, from pet stores during hurricanes, or from international food markets. They hid in foreign packing materials for plants, fruits, and vegetables, or clung to boat hulls or propeller blades. Some may have freeloaded in the ballast of large ships, which take on water and who-knows-what aquatic species in a foreign port, and release alien life forms when they reach their destination. Others sneak a ride on board globe-trotting pleasure or military planes.

Many invasive life forms arrive legally, as desirable crops or companion animals that help to define us or just strike our fancy. Burmese pythons have become popular pets in the United States, credited with a pleasant personality, as snakes go. They’re sometimes bred as stunning yellow-and-white mosaics—like the one Britney Spears slipped around her shoulders and slither-danced with at the MTV Video Music Awards in 2001. But many python owners, chastened by the twenty-year commitment, or alarmed by how quickly the reins of power can shift as the snake grows, turn them loose in the Everglades, assuming it will offer an Edenic home. It does. Undaunted by anything smaller than a mature alligator, they eat everything with a pulse, ravaging the whole ecosystem. Native species haven’t yet evolved to resist or compete with them, the strongest toughs around.[10]

OF COURSE, MOST of us humans are transplants, too, perpetually bustling between cities and taking familiar plants and animals with us—by accident or by design—without worrying much over the mischief we may be unloosing. We are like witches, leaning over the cauldron of the planet, stirring its creatures round and round, unsure about our new familiars—not wildcats, but pythons?—and waiting to see what on Earth may bubble up next.

Accidental hobos, exotic species travel with us everywhere. A list of known invasive species would fill pages, and their handiwork volumes. Because, like the Burmese python, they can wreak havoc with an ecosystem, we scorn them as marauders, as if it were their fault. But most often we’re the ones relocating the planet’s life forms.

Invasive species may carry hobos of their own, contagious ones we’re not immune to. When a San Francisco woman’s pet boa, Larry, fell ill recently, scientists studied the genome of boas and to their shock discovered a genetic mishmash of arenaviruses, which spawn such human nightmares as Ebola, aseptic meningitis, and hemorrhagic fever. It’s entirely possible, they surmise, that Ebola began in snakes and spread to humans. Or that, somewhere along the evolutionary road, snakes became vulnerable to Ebola, just as we did. Now we know that reptiles can harbor some of the world’s deadliest human viruses, yet we still ferry them from one locale to another.

Pythons aren’t the only brawny Floridian invaders. In Cape Coral, monitor lizards—which can reach six feet long—threaten the protected, and altogether winsome, burrowing owl. Gambian pouched rats are overrunning Grassy Key. Cuban tree frogs devour smaller native frogs. Giant African snails dine on five hundred different plants. Jumbo green iguanas are driving the Miami blue butterfly toward extinction. And monk parakeets flock across the Florida skies, flat-nosed as aging prizefighters, making otherworldly shrieks that sound like people prying the lids off cans of motor oil. Unfortunately, their large colonial nests can damage residential trees and electrical power lines, and not everyone is a devotee of squawks, so they’re regarded as a nuisance. Florida boasts more invasive species than anywhere else on Earth, from wild boars and Jamaican fruit bats to squirrel and vervet monkeys, nine-banded armadillos, and prairie dogs.

The same thing can happen in freshwater, and the Finger Lakes now teem with zebra mussels (Russian natives) that clog boat engines and water intake pipes and weigh down buoys. In Tampa Bay, green mussels (New Zealand natives) are smothering the local oyster reefs. Asian carp are turning the Great Lakes into their private dining room. With great relish, the rainbow-sheened Japanese beetles are snipping rose leaves into doilies. Although the Nile perch was introduced into Lake Victoria to appear on the tables of locals, no one counted on its predatory gusto, and it’s decadently feasting on a hundred species of native fish.

We’ve been wantonly shuffling life forms for tens of thousands of years. Migrating bands of Homo sapiens carried plants, animals, and parasites with them on their travels, and ancient texts often speak of importing exotic delicacies and species from foreign lands. Vagabond species travel in our luggage, cuffs, and cars—shadowing us around the block and around the world. During the seventeenth- and eighteenth-century voyages of exploration, along with ideas and goods we spread vermin and disease. We colonized every continent and all but the most extreme ecosystems, reconfiguring them at speed. Just breezing through our lives—hiking across a meadow, commuting to work, flying or sailing overseas—we keep rearranging nature like a suite of living room furniture.

So invasive species have been running riot for ages, some a plague and a nuisance, others a delight. We’ve transplanted a great many plants and animals on purpose, for their beauty, novelty, taste, or usefulness—from starlings and poison ivy (a nonallergic European found it pretty and took it home with him) to exotic reptiles and azaleas. Charmed by the climate and organisms at their new locale, they’ve taken hold, sometimes fiercely (as is the case with eucalyptus, bamboo, and Indian mongooses), to the distress of local species and human residents. People love their English ivy, Norway maple, bullfrogs, Japanese honeysuckle, oxeye daisies, St. John’s wort, dog roses, Scots pine, etc. In contrast, such alien invaders as African bees, tiger mosquitoes, fire ants, water lettuce, burdock, lampreys, loosestrife, bamboo, kudzu vine, and dandelions (which apparently accompanied pilgrims on the Mayflower) are scorned, cursed, and uprooted.

We insist that invasive species don’t belong in wilderness, but native ones do—even if they’ve died out. Inspired by that notion, we’ve reintroduced wolves into Yellowstone, moose into Michigan, European lynxes into Switzerland, musk oxen into Alaska, Przewalski’s horses into Mongolia and the Netherlands, red kites and golden eagles into Ireland, cheetahs into India, black-footed ferrets into Canada, brown bears into the Alps, reindeer into Scotland, northern goshawks into England, Bornean orangutans into Indonesia, condors into California, giant anteaters into Argentina, Arabian oryx into Oman, peregrine falcons into Norway, Germany, Sweden, and Poland—and a great many more. At the same time we’re destroying some ecosystems, we’re busy recreating many others.

People may talk about rebalancing an ecosystem, but there is no perfect “balance of nature,” no strategy that will guarantee perpetual harmony and freedom from change. Nature is a never-ending conga line of bold moves and corrections. Hence the continuing debate about whether or not the Everglades should be python-free or allowed to evolve into whatever comes next. Ever since the 1920s, we’ve been transmogrifying Florida swamps into houses. So the real question is what sort of pocket wilderness we prefer.

I’m of two minds about this case. On the one hand, I don’t want to disturb the dynamic well of nature. Habitats keep evolving new pageants of species, and we shouldn’t interfere. Yet I also sympathize with those who argue that we should capture the pythons in the Everglades and allow the ecosystem to return to its admittedly idealized state, where foxes, rabbits, deer, and a host of other vanishing life forms may flourish. We’re losing biodiversity globally at an alarming rate, and we need a cornucopia of different plants and animals, for the planet’s health and our own. By introducing just one predator into a beloved habitat, we’ve doomed a shockingly large segment of species and all those that depend on them.

The tug-of-war we secretly feel between our animal and human natures is part of what makes us endearingly compassionate, and mighty strange primates. Unlike other animals, we care deeply about scores of life forms with whom we share the planet, even though they’re not family members, not even species members, for that matter, not possessions, and not personal friends. We care abstractly about whole populations we may not have seen firsthand, determined to help fellow creatures survive. We feel a powerfully mingled kinship.

Whatever interventions or restorations we might plan, our unplanned intervention, in the form of climate change, is rearranging habitats in ways we can’t begin to control, spawning migrants everywhere. We may notice more pine or spruce beetles this year, or fewer familiar butterflies poised like pocket squares atop the flowers, or thinner fire-crisped forests with dusty winds and jaw-dropping heat. We may wonder where all the slender-necked corncrakes have gone. We may obey the rules and not water lawns or wash cars or let the faucet run while brushing teeth. But we may not connect the dots and link less water and missing butterflies and corncrakes to early spring and snowpacks melting too soon, leaving little water for parched forests during the long torrid summer, when already weakened trees face an armada of beetles and incendiary drought.

This is not so much a vicious cycle as a carelessly torn fabric. You notice a ripped seam, and though you may procrastinate about fixing it, it annoys your senses, it picks at your awareness, something isn’t as whole. The foxes have moved north, there are new snakes in the yard, field mice have either waned or multiplied to Pied Piper of Hamelin status, West Nile virus is slaying the local crows, and you spotted something long with eyes and scales swimming in the canal. Lured by the warmer, north-spreading swamps, alligators have begun slithering up from Florida into North Carolina. In time, they may well become native to Virginia, maybe venturing as far as Virginia Beach, with some trailblazers swimming up the Potomac to D.C.

One keystone species, plankton,[11] at the heart of the ocean food chain,[12] tells a tale of the changing times. Tiny shrimplike flagellates in the trillions, without a thought among them, they’re barely visible to us and seem far too weak to act as a keystone, without which hazel-waved ocean life would collapse. But they are one of the largest biomasses on Earth, drifting everywhere on the currents like pointillist clouds.

In Arctic waters, where polar bears travel the corridors of sea ice with their young, resting and hunting, and seabirds nest on icy cliffs, flying to fish through cracks in the ice, seals give birth and raise their young atop the floes. Walruses ride on magic carpets of ice to fish farther afield. With warmer water, there are fewer icebergs where algae cling, and fewer algae-eating plankton as a result. According to a recent study published in Nature, worldwide levels of plankton are down 40 percent since the 1950s, which means less food for the plankton-feeding fish, birds, and whales.

Less plankton leads to fewer krill, tiny crustaceans whose numbers have also plummeted, and fewer petite Ad?lie penguins, which feed on krill and squid in Antarctic waters at the other end of the globe. Untidy masons of the penguin world, Ad?lies build nests of stones along gently sloping beaches and raise fluffy, brown, yeti-shaped chicks in those miniature craters. When I visited one large, squawksome colony twenty years ago, stones were a precious commodity. But, according to the ornithologist Bill Fraser, that Ad?lie population has dropped by 90 percent in the past twenty-five years. With so few couples courting, there are stones abounding, but less food for the orcas (killer whales) and leopard seals that prey on the penguins.

Yet the Anthropocene (like nature itself) rarely tells simple stories. In Alaska, our bestirring of the weather is good for the nearly extinct trumpeter swans, who are using the longer summers to feed and raise their young. Orcas will also profit from the warmer waters. As Arctic seam ice shrinks to a record low, undulating orca shipping lanes open up across the pole via the once-fabled Northwest Passage, changing the ecology of the northern ocean. The melt allows the orcas to widen their range and catch more of the white “singing” beluga whales, the canaries of the ocean, and the unicorn-tusked narwhals, two of the orca’s favorite meals. But both the belugas and the narwhals are endangered.

How astonishing it is that just one warm-blooded species is causing all this commotion. Creating hives of great megacities and concrete nests that tower into the sky is impressive enough. But removing, relocating, redesigning, and generally vexing and bothering an entire planet full of plants and animals is another magnitude of mischief beyond anything the planet has ever known. The first is just brilliant niche building, something other animals do on a much more modest scale. For instance, beavers fell trees and dam up streams to create ideal ponds for their underwater huts, and in the process some flora and fauna are dislodged. But no other animal widens its niche to disturb every life form on every continent and in every ocean.

The addled climate is boosting some species and harming or extinguishing others, and not in faraway places, but close to home, in signposts as plain as the jamboree of Canada geese on your lawn. This news of climate change isn’t accusatory, jargon-ridden, arguable, or even verbal. It’s local and personal when eagerly awaited butterflies—the ones that captivated your parents, you, and your children every Good-Humor-jingling summer you can remember—have fled. Some things are more visible in their absence.

In England, the once-rare Argus butterfly has been extending its range northward over the past thirty years, and altering its diet in habitats free of its natural enemies (parasitoids). It’s a marvel with brown wing tops fringed in white, and bright-orange eye dots; the underside is paler brown with black and white eye dots plus the orange. The North Country nurse, leaving her local pub, won’t see her favorite winged pub-crawlers flitting across the meadows. Until she visits her sister thirty miles farther north, where the rare beauty is now plentiful. How come you now get the butterflies and I don’t? she may be thinking with a touch of sibling eco-rivalry. Parasitoids used to finding Argus caterpillars on certain plants haven’t kept up with Argus’s northward migration.

Imagine if you woke up one morning and discovered that the restaurants across the street, the nearby deli and groceries—in fact, all your usual food pantries—had moved several hours north during the night. Would you make long tiring shopping trips, change your diet, or follow the food and resettle in the north? Like our hunter-gatherer ancestors, you’d probably pack up and follow the herds. (Our herds may lie motionless on shelves these days, but we’re still out there hunting and gathering.)

The Spanish ornithologist Miguel Ferrer estimates that around twenty billion birds of many species have altered their migration pattern because of climate change. “Long-distance migrators are traveling shorter distances; shorter-distance migrators are becoming sedentary,” he reported at a conference of two hundred migration specialists. “The normal summer temperature in your city twelve months ago is now normal four kilometers further north. It doesn’t sound like a lot, but that’s twenty times quicker than temperatures changed in the last ice age.”

An Audubon Society study found that roughly half of 305 species of North American birds are wintering thirty-five miles farther north than they did forty years ago. The purple finch is wintering four hundred miles farther north. Birds are fun to watch and beautiful, of course, but they’re also essential pollinators, seed dispersers, and insect eaters whom we need to help crops and ecosystems flourish.

It’s not always easy for birds to migrate in our citified, fragmented landscapes. Some species, fooled by warm weather into traveling too early, arrive in new environs before the food is sprouting in the fields. Once upon a time corncrakes filled the skies from northern Europe to South Africa with long-necked speckled charm and hoarse calls, nesting in field edges, or in dense vegetation and grasslands. As we’ve mown those fields to plant crops, corncrakes have lost their foothold and become scarce. Fortunately, through the synchronized efforts of fifty countries, from Belarus to Tasmania—such tactics as asking farmers not to mow their grasslands until after the chicks have fledged—corncrakes and their tottery black young are making a comeback.

There’s often little harm done by nudging plants and animals to new locales. That’s how we (the most successful invasives of all time) came to settle the continents, planting apples, peaches, horses, and roses. Only crab apples are native to North America. The sweet crisp honey-scented apples we wait for all summer, the fleshy Red Delicious we shine on our jeans, and the 7,500 other cultivars, each with its own taste, fragrance, crispness, and uses—all are invasives, and we’ve bred them like show dogs to taste, feel, look, cook, and smell the way we prefer. Since I’m an apple maven, I’m grateful for two old tart apple trees growing in my backyard.

I also find horses a wonderful addition to the North American landscape, even though I know they’re an invasive species that arrived on Spanish ships and adapted to the vast grassy habitats. It’s a mystery how this happened, but if we look at one instance—the wild ponies on the East Coast barrier islands of Chincoteague and Assateague—there are intriguing theories. Did pirates turn horses loose to graze while they were off looting, and return to find they’d vanished into the dense thickets and woods? Did seventeenth-century growers, who imported European horses, decide to pasture them on the island to avoid taxes, and discover that some wandered off to begin a herd? Did a Spanish galleon, bound for the English colonies, encounter a hurricane and break up on the island, where local Indians came to their rescue, but the ponies ran free? Did a boatload of Spanish horses, blinded for work in the mines, sink in a hurricane, and the terrified horses, despite the raging storm and their blindness, somehow manage to swim to shore? No one knows.

The ponies’ ancestors faced poor food, scorching summers and cold damp winters, sandpaper winds, relentless mosquitoes, and cyclical storms. In response they grew thick furry coats and learned a host of survival skills, such as sensing a drop in barometric pressure, seeking shelter in hilly areas, and huddling together with their rumps facing the high winds. As a result, only the fittest and smartest ponies survived, and their genes live on in the current herd, which is vigorous, canny, and well adapted to the rigors of a maritime landscape.

I may be able to trace my beloved rosebushes, which I raise organically, tend minimally, and almost never count, back thirty-five million years to fossils, or five thousand years to the Chinese gardeners who first dreamt of breeding them. They’re not native to North America either, but also highly successful invasives, which ran wild to the surprise and merriment of settlers.

Chinese mitten crabs may be destroying the San Francisco Bay habitat, but European green crabs are helping to revive the ecosystem in the salt marshes of Cape Cod. We’re restoring prairie ecosystems by reintroducing the grasses that used to fan slowly in the summer haze, and reclaiming wetlands by diverting streams and planting native flora. When we do, of course, it changes the climate and the migration patterns of birds and insects; some animals find a new home, while others decamp. In most cases, the ecological restoration is successful, but we sometimes get it wrong.

The southernmost of the Mariana Islands, about 1,500 miles south of Tokyo, Guam has been a U.S. dependency since the Spanish–American War of 1898. People think of it mainly as a military base, but it is also a lush tropical paradise. Yet at dawn all a visitor hears is an eerie silence, no birdsong, because most of the native birdlife has gone extinct. What’s been causing the extinction? Pesticide poisoning? Habitat destruction? Exotic disease? It took scientists years to figure it out. The answer is one pregnant Indonesian brown snake that slithered off a cargo ship in 1949. The nocturnal brown tree snake is an arboreal predator, up to eleven feet long, and poisonous. Equally happy in forests and on rocky shores, today the snake has multiplied into the hundreds of thousands. Birds evolved on Guam with no predatory snakes, and so most of the forest birds have been devoured, along with native reptiles, amphibians, and bats. The flightless Guam rail survives only in captivity, rescued in a last-ditch effort to save the species by starting a captive breeding program on Guam and in some mainland zoos, so that it doesn’t go the way of the Guam flycatcher, a species once unique to the island. Because birds were the chief seed-spreaders of native fruit trees, those are vanishing, too, and the island is aswarm with forty times more spiders, which the missing birds used to hunt.

The same fiddling with evolution can happen on much larger islands. In Australia, cane toads—native to the Americas, from south Texas to the Central Amazon basin—were imported as assassins to eat the plague of cane beetles, which they did with rousing success. In their native habitat, cane toads can grow as big as catchers’ mitts and weigh five pounds. In their new home, they also evolved longer legs to cross the vast outback. Only big-mouthed snakes could swallow the poisonous toads, and those snakes died in the process, leaving their smaller-mouthed, toad-shy cousins to pass on their genes. As a result, Australian snakes began evolving smaller mouths.

Sometimes we breed and move animals around the landscape to save their species from extinction. In Wild Ones, Jon Mooallem writes about his experience with Operation Migration, where endangered baby whooping cranes are hatched in incubators and taught wild crane behaviors, including how to migrate, and then are led thousands of miles by crane-suited humans flying ultralights. “This work,” Mooallem writes, “this wholesale manufacture of wild birds by human beings—turns out to be so ambitious, tedious, and packed with perplexing arcana that, after ten years, it’s hard for those who have given their lives to the project even to agree on how well it’s working and what they should do next.”[13]

Usually, however, we breed and transport animals not to save them but to suit ourselves: domesticating dogs, cats, and even pythons as pets, enslaving horses and oxen for work, or drafting a cavalcade of creatures—from camels and horses to pigeons and bats—to fight beside us in our bloodiest wars.