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.”