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. Butgalpago is Spanish for tortoise, and young Darwin also learned from watching these huge reptiles lumber across the archipelago. Today, thegalpagos are only a fraction of their former population. And as theyve disappeared, the landscape of the islands has transformedbecause although Darwin didnt 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 centurythe 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 calledgara.

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 fungispecies 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. (Theres also an extinct giant rice rat that could have left enough dung, the authors note, but it wasnt 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 environmentlike 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. Its still not clear whytheir 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 islands climate that discouraged tortoises from hiking the volcano.

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We Discovered Too Late That Tortoises Are Expert Landscapers