(Online Desk)

Who killed the giant ground sloth? Or the mammoth and sabre-toothed cat, come to that. Was it humans or a natural event, like the end of the last ice age? The question is endlessly debated. But the answer, at least in the Caribbean, now seems certain: it was humans.

It is hard to distinguish the effects of humans and natural climatic shifts on wildlife. The trouble is that changes in climate impact on where and how humans live. So if many species die off at the same time, we are none the wiser about the culprit.

In most of North America, humans arrived around the end of the last ice age, and a menagerie of large mammals disappeared in short order. The casualties included dire wolves, short-faced bears, the American lion and ground sloths. Attributing blame is near-impossible.

But the Caribbean islands are different, say Siobhán Cooke at Johns Hopkins University in Baltimore, Maryland. The ice age ended 12,000 years ago, but archaeological evidence shows humans only made it to most of the islands around 5000 years ago.

Cooke and her colleagues have performed a detailed analysis of the chronology of extinction and human occupation. Their data shows it was the arrival of humans that saw off the giant ground sloths, plus the region’s enigmatic monkeys, giant rodents and much else.

Two waves of death: “Human arrival and climate changes occur at similar times on many continents,” says co-author Alexis Mychajliw of the Los Angeles Natural History Museum. “However, islands around the world were generally colonised later.” The research shows “the pervasive ecological impacts of humans, whenever they finally arrive.” Large mammals almost all disappeared in the Caribbean after humans arrived and began hunting and farming.

Of 130 Caribbean mammals in the fossil record, only 73 survive today. But the slaughter of large mammals was not the only catastrophe for wildlife of these islands. There was a second mass extinction just 500 years ago – when Europeans showed up.

This time, it was the small mammals that succumbed. The endemic rodents, shrew-like insectivores and many species of bat didn’t stand a chance, says Cooke. The newcomers let lose cargoes of rapacious Old World mammals like cats, rats, dogs, cows and mongooses.

“This is a most elegant study,” says sloth expert Ross MacPhee of the American Museum of Natural History in New York. “There is rarely a smoking gun where prehistoric extinctions are concerned, but the authors show that most West Indian losses can be correlated with the arrival of people.”

MacPhee has long suspected that the Caribbean was the last refuge of giant ground sloths, once one of the most successful creatures in the Americas, and that it was the arrival of humans that ended their long reign.