Horses and other large mammals who lived on Alaska’s massive northern slope during the last ice age relied on the land bridge across what is now the Bering Strait to cope with the rapid climate changes that characterized the period, according to researchers.
It allowed them to move between different continental habitats during what the study team described as boom and bust cycles.
Short periods of warm climate during the last ice age triggered these boom-and-bust cycles in the populations of large mammals in the Arctic, the researchers reported in the Proceedings of the National Academy of Sciences.
Many large mammals became extinct when these cycles and the ice age ended and spreading peatlands and rising sea levels restricted the animals’ ability to move between continents.
Scientists from the University of Alaska Fairbanks and the University of California examined the age and abundance of the bones of megafauna – a term for mammals weighing more than 100 pounds – on Alaska’s North Slope, a tundra region between the Brooks Range and the Arctic Ocean.
By radiocarbon dating the fossils and comparing their ages and abundances to climate records spanning the past 40,000 years, the researchers reconstructed a picture of what happened to horses, woolly mammoths, steppe bison and other mammals in Alaska’s Arctic.
“We wanted to know how these large animals responded to the rapid climatic changes that characterized that period of Earth’s history,” said lead author Daniel Mann, an associate professor in the Department of Geosciences at the University of Alaska Fairbanks.
To do this, the study team tested a hypothesis suggested by retired paleontologist Dale Guthrie that megafaunal populations experienced boom-and-bust cycles during the ice age as the vegetation tracked climate change.
The last ice age, which began about 80,000 years ago and ended around 12,000 years ago, was a time of diverse climates, Mann said.
The results of the 20-year study show that animal and plant communities were much more changeable during the ice age than they have been during the last 12,000 years of interglacial climate in which we live today.
“This has been a long-term effort, and it has resulted in a much larger data set of bones from one given area than anybody else has ever compiled,” said co-author Groves, a wildlife biologist with the university’s Institute of Arctic Biology.
During the last ice age, a lowered sea level drained the Bering Strait, the narrow seaway now separating Alaska and Asia.
Being able to move back and forth across Bering Strait was probably what kept the large Arctic mammals thriving for so long, Mann said. This travel corridor allowed mammals to come to Alaska when their favored foods — grasses, sedges, and rushes — thrived during warm periods.
The corridor also allowed mammals to leave Alaska for greener pastures when prolonged periods of warmer, wetter climate allowed peat to spread, which cooled the ground and discouraged the grass, sedges and rushes from growing.
“Counterintuitively, rapid climate changes during the ice age were, at times, highly beneficial for megafauna when rapid warming allowed grasses and forbs to briefly spread before peatlands had a chance to take over the landscape and degrade forage quality,” Groves said.
When the planet’s big ice sheets collapsed at the end of the last ice age, their melting caused global sea levels to rise as much as 100 meters in roughly 10,000 years, which is fast in geological time, Mann noted.
“In the case of northern Alaska, the rising seas flooded the Bering Strait, which changed the climate of the North Slope,” Mann said.
“Summers became wetter and cooler because the sea was much closer. This encouraged the spread of organic soils and peat at the same time that it cut off access between Asia and Alaska.”
The animals could not migrate elsewhere because ice sheets were still blocking their way to the east.
Mann became interested in the study when he and other researchers went to archaeological sites on the North Slope in 1994 to see what ice-age humans were eating.
He said they did not find much evidence of humans on the North Slope, which suggested they weren’t a driving factor in the mass extinction of the large mammals.
However, the researchers did find more than 4000 bones of ancient ice-age animals during the last two decades. The bones are now archived in the University of Alaska Museum of the North.
It took two decades to collect enough bones for the study and then several more years to figure out the statistical methods needed to analyze the radiocarbon data, Mann said.
Groves continued: “One of the contributions of research like this is helping humans understand the challenges large animals face today from climate change. It’s a testimony to the value of doing long-term research.”
Evolutionary biologist Beth Shapiro, of the University of California Santa Cruz, who was also a part of the study, said: “We’ve known for some time that the population sizes of these animals fluctuated throughout the last ice age. What this study provides is a possible explanation for these changes.”
The study’s conclusions have implications for present-day extinctions.
Sharpiro said: “As human populations grow, patches of suitable habitat for many species are becoming increasingly isolated from each other. If we are to preserve these species, we will need to devise strategies that allow these populations to remain somehow connected.”
Mann says Arctic climates are particularly unstable and are changing rapidly today. “This means the Arctic is an interesting place to study how climate changes cause extinctions, and the past gives us many interesting examples of extinction there. Plus, bones are exceptionally well-preserved because of the frozen ground.”
Based on current evidence, the regional extinction of the ice-age megafauna was complete in arctic Alaska around 12,000 years ago, leaving caribou, the tundra muskox, and brown bears as the only surviving megafaunal species.
Of the Pleistocene species, horses and steppe bison survived the longest. The most recent horse lived between 12,400 and 12,700 years ago. They said there was no direct paleontological evidence in the form of dated bones in mainland Alaska supporting claims based on ancient DNA that mammoths and horses survived as recently as 7500 years ago.
Their research pinpointed a surge in horse numbers on the North Slope of Alaska from around 40,000 years ago during these boom and bust cycles.
The project was supported by the National Science Foundation and the US Bureau of Land Management.
Life and extinction of megafauna in the ice-age Arctic
Daniel H. Mann, Pamela Groves, Richard E. Reanier, Benjamin V. Gaglioti, Michael L. Kunz, and Beth Shapiro.
The study can be read here.