Modern animals graze on the shores of Lake Pacucha in Peru

Modern animals graze on the shores of Lake Pacucha in Peru, where Pleistocene megaherbivores once gathered.

Aaron Collins

Climate change—not humans—killed giant camels and sloths in South America

Humans and climate change were long the prime suspects in the disappearance of South America’s large mammals, which took place after the last glacial period, about 15,000 years ago. But a new study finds that humans may be off the hook―at least in the high Andes of Peru. The study suggests the extinction there happened in a one-two punch, with climatic instability landing the blows—not humans—who didn’t enter the ring until 3000 years later.

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After the extinction event, South America’s megafauna—animals like giant camels and 3-meter-tall sloths—saw their numbers plummet from 52 genera to fewer than 10. “We just don’t have the [megafauna] biodiversity we once had,” says biologist Angela Rozas-Davila of the Florida Institute of Technology in Melbourne who led the study. Though evidence of human killings has been discovered in some areas, none has been found in the Andes of Peru. “My goal is to find out what happened to all the big animals in South America, starting with my home, Peru.”

To answer that question, Rozas-Davila and her team took a look at an unusual indicator: Sporormiella, an ancient dung fungus whose life cycle depends on it passing through the stomach of a megaherbivore like bison. In paleoecology, its presence signals that large herbivores were hanging around a given area—usually ancient lakes, where the animals gathered, drank water, and defecated, allowing the fungus to multiply.

The high Andes is dotted with lakes and ancient lakebeds, many of them thousands of years old. Rozas-Davila and her team turned to Lake Pacucha in Peru, where they pulled a 14-meter-long core of sediment from the deepest part of the lake. Based on deposits of Sporormiella in the core, the team reconstructed the ecosystem of the lake area for the past 25,000 years. Megaherbivores are usually present when dung fungus makes up more than 2% of all pollen spores in a given sample. In the oldest layers of sediment, that percentage was about 35%, a clear sign of their presence. But starting around 21,000 years ago, that number started dropping. It oscillated for about 5000 years and then fell sharply about 16,000 years ago, at last dipping below 2%.

It was the double drop that shocked the scientists. “When I starting looking at the Sporormiella, I expected to see one drop, not two,” Rozas-Davila says. “I had no idea there would be this early decline at 21,000 years ago.” The oscillating numbers suggest that the extinction of megaherbivores happened in two stages, the team reported online last week in Ecology. The final stage, which finished 15,000 years ago, took place 3000 years before the first record of humans in the area. “No one can know for sure what happened to the Pleistocene megafauna in this region, but the drop in Sporormiella is a strong indicator of extinction,” Rozas-Davila says.

The team also looked for other indicators. They noticed that as the Sporormiella diminished, the composition of the vegetation changed. Based on pollen samples in the sediment, they saw that plants evolved from common edible grassland species to woody species. The latter were not likely to survive the trampling of giant herbivores.

But the researchers concede their findings might apply only to this isolated area and, further, that there could be other explanations for the rapid drop in fungal dung. Abandonment of the lake is one such possibility. But Rozas-Davila says that further study of higher Andean sites can test this hypothesis.  

“The study is quite local, but it’s valuable,” says paleoecologist Christopher Sandom at the University of Sussex in the United Kingdom, who was not associated with the work. “It’s a new in-depth study of a different place that can help us look at broader scale patterns over larger areas.”