Countless biology students have dutifully learned to associate the Galapagos Islands with finches. Here Darwin noticed that birds on different islands had different beak shapes, and ta-da, theory of evolution. But galápago is Spanish for “tortoise,” and young Darwin also learned from watching these huge reptiles lumber across the archipelago. Today, the galápagos are only a fraction of their former population. And as they’ve disappeared, the landscape of the islands has transformed—because although Darwin didn’t know it, the tortoises were driving the evolution of an entire ecosystem.
The story starts before Darwin ever reached the Pacific island chain. So to get details from a time before naturalists were taking notes, Swansea University ecologist Cynthia Froyd and her colleagues searched a different set of records: fossilized tortoise poop.
There used to be 100,000 to 250,000 tortoises living and relieving themselves in the Galapagos. Those numbers dropped after European settlers arrived in the 16th century—the slow-moving giants were eaten, hunted for oil, and tormented by invasive egg-eating rats. By the 1970s their numbers had dropped to 14,000 or fewer.
Now Galapagos tortoises are being reintroduced to the islands. But has the ecosystem changed in their absence? Froyd wondered specifically about the islands’ highest points. These areas are mostly empty of tortoises today, even though the animals are known to travel to higher ground for water during the dry season.
Froyd took sediment samples at lofty bogs on the island of Santa Cruz. (This island is also called Indefatigable, like a tortoise climbing an 800-meter volcano.) These bogs are packed with moss, surrounded by lush vegetation, and frequently covered in a cold, thick mist called garúa.
The researchers scoured the ancient mud samples for fossilized fungus spores, pollen, and plant remains. At all three of their sample sites, they found “dung-affiliated” fungi—species that grow on the droppings of herbivores. This was a clue that a large plant-eater used to live and poop at those spots. Judging by radiocarbon dating, the animal had lived in the bogs for thousands of years, but disappeared around 500 years ago. Dung-rich areas were also full of plant pollen, as from the gut of a grazer. All signs pointed to the Galapagos tortoise, the only large herbivore around. (There’s also an “extinct giant rice rat” that could have left enough dung, the authors note, but it wasn’t known to hang out in swamps.)
When the researchers collected fresh tortoise dung and examined it in the lab, they saw similar patterns of fungus to those in their ancient samples. The same was true of sediment samples taken from a pond where tortoises still live today.
At the same time the dung fungi disappeared, about 500 years ago, certain plant species disappeared from the dirt samples too. The plants that vanished were those that prefer a muddy, churned-up environment—like the home tortoises would have provided as they trampled and sloshed through a wetland. Some of these plant species are now rare or extinct in the Galapagos.
All this evidence added up to tell a story: Tortoises used to cover Santa Cruz Island, from the coasts to the highlands. At the top of the island they wallowed in wetlands with open ponds or lakes. Here they drank, grazed on plants, and kept their bodies cool. Then, around the time humans settled on the island, the turtles left the highlands. It’s still not clear why—their reduced numbers from hunting may have meant less competition from other tortoises, and thus less need to travel for water. There might also have been a shift in the island’s climate that discouraged tortoises from hiking the volcano.
As tortoises left the wetlands, they filled in and became peat bogs dense enough to walk on. Other plant species that had lived there were choked out. Open, freshwater wetlands became rare all across the Galapagos. Charcoal found in the soil samples suggests that as tortoises munched away less of Santa Cruz’s plant material, fires may have become more common too.
Today humans are bringing tortoises back to the islands—though with 5 of the original 14 subspecies now extinct, those tortoises aren’t always the same ones that lived there in the past. The results at Santa Cruz show that just replacing the missing animals won’t turn back the clock. Globally, Froyd says, “we may be missing some of the impacts that past loss of large herbivores has had on ecosystems.” Conservation scientists around the world can learn from the tortoises that when even one animal species leaves, it may carry an entire ecosystem on its back.
Image: by Pandanus (via Wikimedia Commons)
Froyd CA, Coffey EE, van der Knaap WO, van Leeuwen JF, Tye A, & Willis KJ (2014). The ecological consequences of megafaunal loss: giant tortoises and wetland biodiversity. Ecology letters, 17 (2), 144-54 PMID: 24382356