by Lauren Frisch
A new study published in Nature Ecology and Evolution reveals that increased moisture levels may have been a primary cause of death for giant herbivores approximately 10,000 years ago.
“The mass extinctions of mega-herbivores across the globe have been an ongoing puzzle for scientists,” said Professor Matthew Wooller of the University of Alaska Fairbanks College of Fisheries and Ocean Sciences and UAF’s Alaska Stable Isotope Facility. “We looked at carbon and nitrogen isotopes in ancient animal bones to learn about what the herbivores were eating, which can also tell us about what climate was like around the time that the megafauna died.”
Wooller was part of an international research team led by University of Adelaide researchers Alan Cooper and Tim Rabanus-Wallace that synthesized data from hundreds of bone samples from around the world to show that moisture changes played a role in the extinctions.
Mega-herbivores—large vegetarian animals including some species of horses, bison and mammoths that used to tromp around Alaska—rapidly disappeared or declined at the end of the Pleistocene era about 10,000 years ago. The mass extinction coincided with a period of significant environmental change, when the earth transitioned from the last glacial period to the current interglacial period. This was also the time that modern humans began to spread into the Americas.
A lot of environmental changes were happening around the world, and it is likely that many of them played a role in the mega-herbivore extinctions. That’s why it has been exceptionally difficult to piece together the story of why these giant vegetarians disappeared. Understanding how species respond during periods of rapid environmental changes may shed light onto what would happen to modern animals affected by the rapid environmental changes we are currently facing.
‘‘This extinction event had been going on for awhile, but we wanted to study a particularly sharp spike,” said Tim Rabanus-Wallace, a PhD student at the University of Adelaide who spearheaded this project. “That’s when we lost all the really cool things, like mammoths, Steppe bison and giant short-faced bears.”
Digging into this story relies on understanding a number of changing variables. The researchers looked at bone samples from a number of species that lived in a variety of environments on different continents. In order to figure out what killed the mega-herbivores, the researchers had to be able to account for differences in species, environments and continents. This can be accomplished by having a huge number of samples.
“We were fortunate to have access to a large data set,” said Wooller. “When you get a new fossil, one of the first things you want to do is date it. This can be done by radiocarbon dating a fossil’s collagen. And this process often generates bonus data in the form of stable nitrogen isotopes.” Scientists all over the world have been collecting valuable isotopic information, but until now nobody has looked at it all together to study the mega-herbivores.
The research team used the radiocarbon data to place fossils from megafauna all over the world on a timeline. The nitrogen isotope data helped reveal what the environment was like when the animals were alive. In essence, the fossils tell us about the conditions that these animals lived in, and the timeline helps researchers study how these conditions changed.
Isotopes refer to the number of neutrons that an element like nitrogen carries. Different isotopes of the same element have different numbers of neutrons, which affects the element’s weight. Distinct environments involve varying amounts of heavy and light isotopes of an element. Wet and dry environments can have very different isotopic signatures in their soils, plants and subsequently the collagen of the herbivores that are eating those plants. Collagen is a protein, some of which remains in bones long after an animal has died. As a result, nitrogen isotopes found in the collagen of herbivores’ bones illustrate the nature of the environment they lived in.
“You are what you eat,” said Rabanus-Wallace. “When you consume food with a certain ratio of heavy and light isotopes, your bones develop a related ratio. So we can learn what and where different animals ate based on the isotopes in their bones.”
The researchers measured isotopes from the bones of animals all over the world with funding from the National Science Foundation, Norges Forskningsråd, Australian Research Council and Australian Centre for Ancient DNA.
Across the board, these isotopes show a spike in moisture just prior to the extinction of megafauna.
“This makes sense for a lot of reasons,” Wooller said. “This change in moisture could have affected the dominant environment that the mega-herbivores were living in.” Large herbivores likely preferred living in cool, dry grasslands. Increasing moisture could have radically affected the ecosystem that they thrived in. Areas that were primarily grassy likely became swampy and eventually transitioned into forests.
The mega-herbivores may have lost their primary food source as landscapes changed. “If you’re adapted to grass, you can’t live in a forest,” Rabanus-Wallace said. “The plants are full of plant toxins specifically designed to fend off herbivores.”
This trend was observed across continents, even though the timing of the extinctions varied.
“We find that on different continents the climate changes happened at different times, but they all showed a similar kind of feature, that moisture levels changed just prior to extinction,” Wooller said. “That’s recorded in the fossils themselves.”
Africa appears to be the outlier, possibly due to the structure of the environment at the time. “Northern Africa has desert on the top, grassland in the middle, and forest at the bottom,” Rabanus-Wallace explains. “It is possible that given the change in moisture levels, the grasslands in Africa just shifted up and down in latitude, rather than disappearing altogether, which would allow species to move in order to keep up with their food source.” The researchers did not have enough isotope data on African species to confirm whether this might be the case.
These results are a good indication that increasing moisture had a significant role in the extinctions, and support the findings from previous, regionally-focused research on the mega-herbivore extinctions. However, this does not rule out that other environmental changes, including the spread of humans, also played significant parts. As the researchers continue to collect a broader range of global samples, it will be easier to piece together the whole story on what caused the extinctions at the end of the Pleistocene.
A press release for this story was published in the UAF Cornerstone.
ADDITIONAL CONTACT: Matthew Wooller, firstname.lastname@example.org, 907-474-6738.