What killed the Ice Age beasts? Disappearing grasslands when the glaciers thawed

The huge herbivores that roamed much of the world in the last Ice Age were wiped out at the end of it, between about 11,500 and 15,000 years ago. These megafauna died because their landscapes were suddenly swamped with moisture as the glaciers covering much of the northern hemisphere melted, a new study finds.

Exactly what pushed the megafauna of many continents to extinction at the end of the last Ice Age has been hotly debated for decades. Was it mostly down to the changing climate that disrupted the animals' ecosystem? Or was it innovations in human hunting that pushed their populations over the brink?

Analysis of megafaunal bones from Europe, Siberia and the Americas has shown that a spike in the moisture availability in the ecosystem happened directly before waves of extinctions began. The nitrogen isotope signature of the megafaunal bones acted as a proxy for the amount of moisture present in their diet. The results are published in a paper in the journal Nature Ecology & Evolution.

"We didn't expect to find such clear signals of moisture increases occurring so widely across all of Europe, Siberia and the Americas," said study author Alan Cooper of the University of Adelaide.

"The timing varied between regions, but matches the collapse of glaciers and permafrost and occurs just before most species go extinct."

The abundance of moisture and the lush vegetation it created seems to be something of a paradox on first sight. The large grazers of the end of the last Ice Age would indeed have had plenty to eat, for a short while.

But the persistence of a wetter climate eventually meant a shift in vegetation away from the grasses to which the megafauna were adapted. Their grasslands eventually became wetlands, and eventually peatlands. Herbivore-resistant, perennial foliage began to dominate, restricting the herbivores' ranges.

The link between moisture and the extinctions explains one puzzle of the end of the Ice Age: why megafauna on some continents died before others, and why many survived relatively unscathed in Africa.

"The idea of moisture-driven extinctions is really exciting because it can also explain why Africa is so different, with a much lower rate of megafaunal extinctions and many species surviving to this day," said Cooper.

"Africa's position across the equator means that grassland zones have always surrounded the central monsoon region. The stable grasslands are what has allowed large herbivores to persist - rather than any special wariness of hunters learned from humans evolving there."