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For many visitors to Ecuador’s Galapagos Islands, their first glimpse of a giant tortoise is from the window of a bus or taxi driving across Santa Cruz Island, from the airport to the town of Puerto Ayora. The main road, a two-lane affair, rises from the dry lowlands in the island’s northern rain shadow, passes through the midlands, and doglegs across the cloud-draped highlands, where a forest of native Scalesia trees caps the remains of an extinct volcano.

This is my second visit to the Galapagos, and though I’ve seen tortoises in remote areas that most people never reach, watching these enormous reptiles never gets old. As the bus I’m traveling on approaches Puerto Ayora through a patchwork of small farms and houses, I watch the world blur past. We pass some palms and a flowering banana tree on the right. On the other side of the road, two dogs slink into a roadside bodega. Suddenly the driver hits the brakes. “Tortuga!” he says, and points with a laugh.

Nothing brings traffic to a halt like a giant tortoise crossing the road. Minutes tick by as the tortoise plods across the asphalt—plenty of time for tourists to hang out the windows and snap their first photos of an animal that many have traveled halfway across the planet to see. More than 4,600 tortoises are estimated to live on the western slopes of Santa Cruz, the Galapagos’ most populous island, and now, on a sweltering spring day in mid-March, we are seeing one of them on the move.

Western Santa Cruz tortoises, which can weigh as much as a small grand piano, have been climbing up and down these volcanic slopes for hundreds of thousands of years. During the wet season, from roughly January to May, when rains bring the parched lowlands to life, tortoises come to the lower elevations to nest and forage on new grass, herbaceous plants, and cacti. When the rains dry up, they trek as far as 10 kilometers and climb as much as 400 meters to overwinter in the highlands, where a cloak of clouds lingers until December, keeping the vegetation green.

Nowhere else on Earth do tortoises make mass migrations like they do in the Galapagos. (The closest comparison is Aldabra, an atoll in Seychelles, where some individual giant tortoises may seasonally travel a few kilometers between the island’s coast and its interior.) In the Galapagos, 12 distinct lineages of tortoises are spread across the archipelago’s 13 major islands, which lie some 1,000 kilometers off the coast of Ecuador. Traveling tortoises sustain ecosystems with their movements—munching plants, spreading seeds through their feces, and recycling nutrients so thoroughly that many of the Galapagos’ endemic plants depend on tortoises to thrive. The reptiles leave their mark in other ways, too. On Isabela Island—the largest of the Galapagos islands, 28 kilometers west of Santa Cruz—tortoises have worn lava rocks smooth and cut deep ruts that look like toboggan runs through the undergrowth.

Charles Darwin caught a glimmer of this on his first day exploring the Galapagos, in 1835. He noticed a network of trails up and down the volcanic slopes of San Cristóbal Island, and it puzzled him. “I could not imagine what animal traveled so methodically along well-chosen tracks,” he later wrote in The Voyage of the Beagle. The mystery didn’t last long. While out on a stroll—struggling across rugged black lava and fighting dense thickets of vegetation—Darwin ran into his first pair of tortoises, one of whom greeted him with a hiss.

Today, though, the routes these ancient wanderers depend on are increasingly blocked. As the Galapagos’ human population soars—from about 2,000 in the 1960s to some 32,000 today—an expanding network of farms, roads, and tourist infrastructure disrupts the tortoises’ migratory paths, particularly on Santa Cruz. And beyond these roadblocks lurks an even more challenging barrier: nonnative vegetation that threatens to halt millennia of migration.

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The tortoise finally reaches the edge of the pavement and disappears through a thicket of elephant grass, heading toward a tall stand of Spanish cedar trees locally called Cedrela. It’s the hot rainy season, and the tortoise is moving to lower elevations. Since 2009, tracking research by the Galapagos Tortoise Movement Ecology Program (GTMEP), run by the nonprofit Charles Darwin Foundation and others, confirms what Darwin suspected: giant tortoises follow the same migration routes year after year, similar to herds of wildebeest on the Serengeti. “Like migratory animals around the globe, the tortoises are chasing seasonal food supply,” says Patrick Moldowan, a wildlife biologist with GTMEP. “It’s essential for their energy budgets … to sustain their immense bulk.”

Tortoises have demonstrated a remarkable ability to maneuver around the obstacles of modern life. They regularly traverse secondary roads and pause to wallow at farms with human-made livestock ponds that mimic the natural pools tortoises use to hydrate and cool down. Though not everyone welcomes tortoises—cattle ranching is big on the island, and some ranchers view tortoises as nuisances that can knock down a fence and let cattle out—many islanders make an effort to adapt their lives and infrastructure to accommodate the giant reptiles. At a farm not far from the bus route across Santa Cruz, I witness this firsthand.

From the main road it’s a short walk down a dirt lane to Montemar, a coffee plantation and guest lodge that owners Reyna Oleas and Roberto Plaza work to keep tortoise-friendly. Plaza shows me the hedge at the edge of their farm and points out a tunnel that he and Oleas carved through the bushes to accommodate tortoises. As we’re talking, a massive tortoise emerges beneath the fence of a neighboring farm and heads up the red dirt road toward an oncoming tourist bus, which stops to let it pass. Like many farmers on the island, the one who manages this fence has removed the lowest strand of wire, giving tortoises about 75 centimeters of clearance.

Oleas and Plaza walk me toward their tortoise pond, green with floating duckweed. A tortoise soaks motionless in the muddy shallows, and three pintail ducks paddle off, leaving swirls and streaks that fade as the water settles behind them. In the distance, beyond where the island meets the sea, clouds are gathering.

The farm didn’t have a view of the ocean when they bought this land in 2009, Oleas and Plaza tell me. It was covered with invasive elephant grass and blackberries: two of the plants, along with Cedrela, that tortoises refuse to eat and struggle to travel through. The pond was just a dry hole, and only a handful of tortoises ever visited. “When we started controlling [the invasive species], naturally the water started coming back,” Oleas says. “And naturally the giant tortoises came back.” By the hundreds. The couple has counted as many as 200 tortoises at one time on their property.

But while locals are willing to accommodate tortoises—helping some populations recover from past centuries of hunting by whalers and pirates—another threat has been quietly growing.

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Back on the bus, continuing down the road to Puerto Ayora, we whiz past a grove of 20-meter-high Cedrela trees. From here, the leaves atop their straight, tall trunks seem almost fernlike. Along the sunny edge, a thorny tangle of Himalayan blackberries has taken hold; when Cedrela moves in, native species richness declines by 42 percent.

Cedrela, a type of mahogany native to much of South and Central America, was introduced to the Galapagos by settlers in the 1940s as a source of timber. The blackberry is a more recent arrival, brought in 1968 by a farmer who saw it as a potential crop. Both have gone wild, together creating a nearly impassable barrier. “Once the Cedrela invades an area, it very soon is also invaded by blackberry,” Moldowan says. “So the Cedrela is kind of a threat multiplier.”

Although Cedrela has been around for three-quarters of a century, conservationists haven’t recognized it as a serious threat until recently. In February 2024, an international team of researchers published a paper in the journal Ecology and Evolution that showed, for the first time, that Cedrela has the power to stop tortoise migrations.

Using maps of tortoises’ movements drawn from 12 years of GPS tracking data combined with plant survey data, researchers found that Cedrela forests now largely block the routes tortoises historically relied on to travel up and down the volcanic slopes of Santa Cruz. Tortoises are now funneled through the only three gaps remaining in the forest, each around a kilometer wide. And as Cedrela and Himalayan blackberries continue to spread, the gaps may be closing up.

“It’s making it impossible for tortoises to go through,” says Heinke Jäger, a restoration ecologist working with invasive plants and animals at the Charles Darwin Foundation who was not involved with the paper.

In Jäger’s view, eradicating blackberries altogether is impossible. Seeds are spread by birds, and while manual weeding might work on small properties, it’s too labor-intensive to be done at scale. The best hope is that scientists will find a rust fungus or a leaf spot disease that targets this blackberry variety and weakens it enough that native vegetation can retain a foothold. Though such research is underway, Jäger admits it’s a long shot—and if blackberries can’t be controlled, she fears that the island’s remaining Scalesia forests will be gone in less than 20 years. “In 10 years, we did not have a single Scalesia seedling in the areas with [the] blackberry invasion,” she says. “I mean, zero.”

The GTMEP, meanwhile, is pondering possible ways to help stop or slow Cedrela’s spread. “It’s complicated,” says Stephen Blake, lead author of the 2024 paper and founder of the GTMEP. Simply chopping the trees down might open the door for more blackberries to move in. New pathways could be cut through the forest, but there’s no research yet on whether that would work. Any effort would take money, machinery, and many hours of human labor—all of which are hard to come by in the Galapagos Islands, Blake adds.

What might be lost if these tortoises are blocked from migrating? The animals themselves will face diminished lives. Trapped above or below the barrier of invasive plants, they won’t be able to follow seasonal forage. Their fat reserves will shrink. Poor health leads to poor reproduction. And because tortoises grow and reproduce so slowly, it might take a century or more before any drop in numbers becomes measurable.

“One of the issues with giant tortoises is that they operate on such a different timescale compared with migratory birds or migratory wildebeest,” says Blake. “You’re not going to see … a march towards extinction with them for many centuries. That doesn’t mean that the negative consequences are any less potentially catastrophic.”

Whether the tortoise I saw crossing the road is able to find a path through the Cedrela and blackberries in the years to come will depend on the combined efforts of scientists, conservationists, and the local community already working to help keep tortoises moving around human-made obstacles. If it does—if it reaches its lowland foraging and nesting habitat, pauses for a few months, and then continues its timeless journey back into the misty highlands—it will play a role in the enduring resilience of giant tortoises and bear witness to the power of human dedication.