US-based science writer Sonia Shahs new book discusses why migration is a biological imperative as necessary as breathing, and the physical and invisible barriers that come in the way of it
Sonia Shah
On about 3.6 per cent of the planet's surface, geographic barriers prevent wild species from migrating as effectively as the desert borderlands barred the polo-shirted young man. Take, for example, the mosaic-tailed rat, which lived on tiny Bramble Cay, an uninhabited island on the northern edge of the Great Barrier Reef off the coast of Australia. Increasingly violent storm surges steadily wiped out the island's plant life. But like other terrestrial creatures living on remote islands, or at the top of mountains, the mosaic-tailed rat had nowhere to go. The rodents' numbers diminished. By 2002 there were only 10 mosaic-tailed rats left on the island. A fisherman spotted one in 2009, but when scientists returned in 2016 to survey the island, they couldn't find even one. In 2019, with 97 per cent of the vegetation on the island destroyed, officials in Australia declared Melomys rubicola, the Bramble Cay mosaic-tailed rat, officially extinct. It was the first mammal we know of to be wiped out by climate change. Experts agreed it would not be the last.
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The more potent barrier to wild species' movement is us. So far, our cities, towns, farms, and sprawling industrial infrastructure have swallowed up over half the planet's land surface. We transformed another 22 per cent of the earth's habitable land in just the last decades, mostly by cutting down forests and turning them into farms, as a recent analysis of satellite images from 1992 to 2015 showed. Our massive footprint makes life impossible for so many wild species that an estimated 150 go extinct every day, speeding up the background rate of extinction by a factor of one thousand. Species that have not lost their habitats entirely must move through a landscape disfigured by human developments. Black bears in the hardwood swamps of Louisiana must cross a highway to reach others in their population. Instead of striking out across the highway to find new mates, they've started to mate with those in their own cut-off group, becoming increasingly inbred. Cougars living in the mountains around Los Angeles must cross two freeways, including one with eight lanes of speeding traffic, to meet others of their kind. None of the cougars that scientists fitted with GPS collars could do it. Four died attempting the crossing, five turned back, and one was shot by police. Birds on the wing smash into industrial structures, each building regularly racking up corpses, like the half-dozen or so birds felled every week by the Thurgood Marshall Federal Judiciary building in Washington, D.C.
Migrating butterflies, lured off course by electric lighting, perish and flutter to the ground. A 2018 paper in Science analyzed the movements of fifty-seven different mammal species outfitted with GPS devices over landscapes rated according to a "human footprint index," which incorporated data on human population density, the extent of built land, roads, nighttime lighting, and the like. New York City garners a score of 50, while the vast and wild tropical wetlands of the Brazilian Pantanal scores 0. The bigger the human footprint, the researchers found, the more constrained animal movements became. In places with the largest footprints, animals managed to cover as little as a third of the distance of those in places with little or no human impact.
A member of Border Angels hangs a banner reading “Trump we will not pay for your wall” during a demonstration against him at the US-Mexico border in Playas de Tijuana, Baja California state, Mexico, on February 2, 2020. Newly built walls, fences, and gates had risen over 60 international boundaries, blockading the movement of over four billion people around the world, says Sonia Shah. PIC/AFP
Besides the inadvertent obstacles of geography and industrial development, the next great migration must overcome purposeful barriers. Before 2001 fewer than 20 of the invisible boundaries that define nearly two hundred nation-states were physically marked by fences or walls. Animals, winds, currents, and waves could freely travel across their imaginary lines. In 2015 an unprecedented surge in construction of new border walls began. By 2019 newly built walls, fences, and gates had risen over 60 international boundaries, blockading the movements of over 4 billion people around the world. More borders are fortified by walls and fences today than at any time in history. Tunisia has built a wall of sandbanks and water-filled trenches along its border with Libya. India and Myanmar have fenced their borders with Bangladesh. Israel has enclosed itself with razor wire, touch sensors, infrared cameras, and motion detectors. Hungary's fence along its border with Croatia, built by prisoners, delivers electric shocks to any migrant foolhardy enough to touch it. Security officials patrol the barrier, tear gas canisters in hand. Austria has built a fence along its border with Slovenia. Britain plans another one along the channel separating it from France. Norway has fortified its border with Russia. In the United States, the hundreds of miles of sixteen-foot-high concrete and steel walls marking the southern border would be extended with even taller, longer, more impregnable walls, U.S. president Trump insisted, perhaps even the entire length of the two-thousand-mile border. Walls don't necessarily function as the impregnable barricades they're meant to.
In one study, for example, researchers set up camera traps along the U.S.-Mexico border, tracking the movement of people and wild species across open stretches and comparing their movement across stretches blocked by border walls. The walls effectively deterred the pumas and coatis. According to conservation biologists, the extended walls proposed for the U.S.-Mexico border, for which the government has waived scores of environmental regulations, will endanger the life-saving movements of most of the 93 species that live on either side of it. But in the study comparing open and walled stretches of the border, the walls had no effect on people's movements. Whether crossing the border means scaling a wall or not, people keep moving, regardless. If border obstructions fail to stifle movements, they do effectively deflect them. People on the move take more circuitous routes than they otherwise would in order to circumvent barriers, moving like water around a boulder in a stream. Attempting to bar migration, a European border official said, "is very much like squeezing a balloon. When one route closes, the flows increase on another."
Excerpted with permission from The Next Great Migration: The Story of Movement on a Changing Planet by Sonia Shah, published by Bloomsbury India
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