Until recently, the cause of the woolly mammoth extinction 4,000-10,000 years ago has been unclear. But according to new research, the species fell to a lethal combination of climate warming, encroaching humans, and habitat loss. These are many of the same threats facing most species today.
“We were interested to know what happened to this species during the climate warming at the end of the last ice age because we were looking for insights into what might happen today due to human-induced climate change,” said Glen MacDonald, director of UCLA’s Institute of the Environment and Sustainability (IoES). “The answer to why woolly mammoths died off sounds a lot like what we expect with future climate warming.”
The new research shows that although hunting may have contributed to the extinction, it didn’t cause it on its own.
The researchers created the most complete maps to date of all the changes happening during that time period, and found that the extinction didn’t line up well with any one of the factors, but seemingly from a combination of the new, simultaneous pressures facing the woolly mammoth.
When the last ice-age ended about 15,000 years ago, woolly mammoths were on the rise. The melting glaciers, but still chilly temperatures, allowed a lot of plant growth, which helped them to thrive. It was just the right balance for them, allowing good plant growth but without allowing the growth of thick forests or marshy peatlands.
Humans had been hunting mammoths in Siberia for millennia, but it’s currently thought that it wasn’t until the end of the last ice-age that people began hunting them in North America.
By about 13,000 years ago, the climate had gotten pretty warm, and the mammoths preferred food sources — grasses and willows — began to be replaced by low-nutrient conifers and birches. And marshy peatlands developed, and forests expanded, greatly limiting the mammoths’ range and available nutrition.
“It’s not just the climate change that killed them off,” MacDonald said. “It’s the habitat change and human pressure. Hunting expanded at the same time that the habitat became less amenable.”
The majority of the mammoths died out 10,000 years ago, with fragments remaining until around 4,000 years ago.
Most previous extinction theories focused solely on only one causative agent at a time; either hunting, climate changes, meteors, disease brought by humans and dogs, or melting glaciers. This new research is the first time that all of these potential causative agents have been mapped onto a single timeline. The researchers used radiocarbon dating to follow the changing locations of peatlands, forests, plant species, mammoth populations, and human settlements. They then cross-referenced all this data with climate change data.
“The research used 1,323 mammoth radiocarbon dates, 658 peatland dates, 447 tree dates, and 576 dates from Paleolithic archaeological sites. Scientists from IoES and other UCLA departments obtained samples and worked on radiocarbon dating of the peatlands and the forests, and they created a database uniting information on hundreds of previously dated mammoth samples, developing the final map from thousands of dates and latitude and longitude records.”
“Glen’s project combined paleobotanical, paleontological, genetic, archaeological and paleoclimate data and did it in a bigger way, with many more data points, than has been done before,” said Blaire Van Valkenburgh, UCLA scientist who interpreted the archaeological record. “I was excited to be able to contribute to such an ambitious and exciting study.”
“It’s a dramatic advance in the amount of data,” said Robert Wayne, a UCLA scientist who reconstructed mitochondrial DNA from radiocarbon-dated woolly mammoth remains. “Essentially, larger populations should have greater genetic diversity. However, in this case, the extent of fossil remains provided a more high-resolution picture of how the population size changed through time than genetic diversity.”
“Mapping the size and location of both mammoth and human populations alongside temperature changes and plant locations through time gave the researches a uniquely complete view of what happened.”
“We are, in a sense, time-traveling with our maps to look at the mammoths,” Macdonald said.
It’s something that Macdonald said he had been dreaming about since he held a woolly mammoth tooth in Siberia a few years back.
“We looked at it and held it, and just the thought that those immense creatures had been there not that long ago in geologic time and yet completely disappeared was really amazing,” MacDonald said. “How warming in the past has been involved in extinction might help us prevent extinctions in the future.”
Image Credits: Dave Bellman, Glen Macdonald