New evidence suggests that changes in the North American ecosystem didn't kill the mammoth - their demise may have brought the changes about.
Credit: Wikimedia
SYDNEY: The extinction of mammoths in North America at the end of the last ice age was not caused by a change in the ecosystem: it's what triggered the changes, a new study suggests.
The study also elucidates a possible cause for the demise of mammoths and mastodons 15,000 years ago, and researchers say that the expanded incidence of fire in the landscape - suspected of being caused by human arrival - only appeared after the extinction.
Unmasking the last ice age
"For the first time, we have a very close linkage between this major ecological event and some evidence about the consequences of this extinction," said John Williams, a professor at the University of Wisconsin in Madison and the senior author of the study published in the U.S. journal, Science.
The researchers used fossils, pollen, charcoal and dung fungus spores, along with radiocarbon dates, to build a more accurate picture of the sequence of events at the end of the last ice age.
They found a decline of Sporormiella dung fungal spores, began around 14,800 years ago, leading to its virtual disappearence 13,700 years ago, said Jacquelyn Gill, study leader and a graduate student in Williams' lab.
Previous hypotheses
The data discounts previous hypotheses for the cause of extinction of 50% of all mammal species above 32 kg and all species above 1,000 kg in North America.
It shows that the megafaunal decline, including mammoths and mastodons, took about 1,000 years. The decline also preceded major habitat change, increased incidence of fire in the landscape, and a presumed comet or asteroid impact estimated to have occurred around 12,900 years ago.
On the other hand, the study does "not fully resolve the human versus climate debate," said Williams.
Some researchers argue that the arrival of prehistoric humans on the American continent and their voracious hunting of game, along with their widespread use of fire, helped bring about the decline of mammals in the region. Known as the Clovis people, they are thought to have arrived 13,500 to 13,000 years ago.
Human impact
They said that it was is possible that other humans arrived before the Clovis - a Paleo-Indian culture characterised by the manufacture of distinctive bone and ivory tools - which could have been the driving force for the extinction and, in turn, for the modified hunting strategy seen in the Clovis people.
After the extinctions came a habitat transformation, with an increase in woody biomass. "Our record shows, as Sporormiella declines, we see increase in the abundance of two tree taxas that have very similar pollen grains," said Williams.
This led them to believe that these trees, which according to Gill have a higher nutrient content and are softer to the palate, may have been present before the decline and extinction of the megafauna, but were being kept at bay by the large herbivores.
The results "help to elucidate the cause of megafaunal extinction in North America and raise new questions on how people and megafauna interacted," said Christopher Johnson, professor at James Cook University in Queensland, in a response to the study also published in Science.
Evidence outside of North America
"There is evidence for similar changes in vegetation following megafaunal extinction in other parts of the world, but the data from this new study are especially well-resolved, and make the cause-effect relationship very clear," Johnson told Cosmos Online.
This study also paved the way for future studies, and may generate new responses to our own flora and fauna.
"Today, we have replaced much of this extinct megafauna (biomass) with our own domesticated megafauna (live stock)," said Scott Hocknull, senior curator for the Queensland Museum in Brisbane. "Knowing the balance in the past will help us manage our own domestic megafauna into the future."
"Describing these ecological transformations, and understanding the development of today's ecosystems in light of them, is the next big challenge for ecologists," added Johnson.
