For about 1,000 years at the end of the last ice age, the woolly mammoths, steppe bison, ancient horses and other Pleistocene animals that populated Alaska's North Slope enjoyed a golden age, thriving on plentiful plants and roaming wide spaces, according to a newly published study.
But a shifting climate changed the habitat they relied on while simultaneously cutting off retreat to Asia across what's now the Bering Strait.
The study, published Monday in Proceedings of the National Academies of Science, relies on analysis of about 4,000 well-preserved bones dating back 40,000 years, cross-referenced with information about climate changes that occurred through those millennia.
The transition period between the Pleistocene epoch and the current Holocene epoch -- a transition that happened roughly 11,000 years ago -- was a "highly favorable time for megafauna," according to the study. But after that came habitat change, fragmentation and, ultimately, extinction for many species of large mammals.
"It's like a little bloom of weeds, if you want to think about it that way," said lead author Daniel Mann, a geologist and paleoecologist at the University of Alaska Fairbanks.
As the last ice age ended for good, several centuries of maritime moisture and weather patterns caused peat to take over the landscape, choking out the long-rooted grasses and sedges the big animals needed for food. Meanwhile, melting glaciers inundated the Bering Strait area with water, cutting off land access to other potential habitat in Asia while lingering ice prevented movement to the east, the researchers found.
"You've got to have that space to move, the ability to move around," Mann said. "As soon as you flood the Bering Strait, you're in trouble."
Populations of large ice age mammals underwent a series of boom-bust cycles, with "short-lived pulses of high-quality range" occurring as climate conditions shifted back and forth, according to the research by scientists from UAF and other institutions and based on analysis of the bones, which were collected over two decades.
In some ways, Alaska's North Slope was a far different place at the end of the Pleistocene epoch.
Its climate was dry and relatively warm in summer, though cold in winter – somewhat akin to modern-day Mongolia, Mann said. Climatic conditions changed quickly, though, frequently veering between warm and cold. And those changes happened even faster than current changes, sometimes marked over just a few years, he said.
"The ice age was totally different in its climate regime than the last 12,000 years," he said. The climate stability of the Holocene led to the development of agriculture, according to some scientists' theories, he said.
Those quick transitions had some benefits for the big animals of the time, allowing grasses and sedges to periodically thrive in different places, the study says. As long as the area now known as the Bering Strait was dry, animal populations could move around to find favorable sites, Mann said.
Evidence from the bones suggests that, along with the big plant-eating mammals, there were large predators on the North Slope at that time, Mann said. Just about 13,000 years ago, ice age horses were abundant on the North Slope – and so were now-extinct cave lions, he said.
"As the horses increased, so did the lions," he said.
Missing from the scene, apparently, were many humans. Very few signs of human habitation were found in the two decades of bone collections, Mann said. That finding may rule out overhunting by humans as a cause of extinction for ice age mammals on Alaska's North Slope, he said.
That contrasts with other parts of the world, where there are many remnants and relics, such as pits used to trap and kill animals, providing great detail about organized hunts of the big ice age mammals.
Not all of the large mammals that roamed the North Slope in Pleistocene times went extinct when the climate changed. Notable survivors include caribou, musk oxen and brown bears.
Those survivors held various advantages over the extinct species, Mann said. Caribou have wide feet that can tread effectively over peat during long migrations, for example, but the ice age horses, bison and mammoths had small feet compared to their large body sizes, he said. Musk oxen – which largely disappeared from Alaska but survived elsewhere in the North before being reintroduced to the state – have the advantage of needing much less food than did woolly mammoths, he said. And a certain amount of luck may have been a factor for some survivors, such as avoidance of diseases, he said.
The findings hold lessons for modern times, as both climate change and spreading human development break up wildlife habitat and create barriers for animals to move around and find more favorable places to find food and carry out life events, Mann said.