Huge mammals such as mammoths, woolly rhinos and saber-toothed tigers once roamed over every habitable continent, but died out. New research shows that such a loss of mammal biodiversity – a major conservation issue today – is part of a long-term trend that lasts at least 125,000 years.
When archaic humans, Neanderthals and other hominin species emigrated from Africa, they were followed by a wave of the extinction of large mammals. This happened on all continents and intensified over time. The study, published in the journal Science on April 20, is the first to show quantitatively that human effects on mammalian body size prior to emigration are from Africa, and that size-related extinction reflects human activities
funded by a Foundation of the National Science Foundation, was designed by Professor Felisa Smith, a paleo-ecologist at the University of New Mexico, along with colleagues from the University of California, San Diego, the University of Nebraska-Lincoln, and Stanford University School of Earth, Energy & Environmental Sciences.
Deep Impact on Ecosystems
To document what happened to mammals when the first humans left Africa, over the past 1
The study showed that the downgrading of height – the loss of the largest species on each continent over time – is a hallmark of human activity, both in the past and in the present. If this trend continues into the future, the largest national mammal in 200 years will be the domestic cow.
"We – both modern and old – have had a profound impact on ecosystems," Smith said. "Our results clearly show that the extinction of size is a hallmark of human activity and not a general feature of mammalian evolution," she said. "The big picture is that as things go on, not only does biodiversity decrease, but so does the way that energy changes through ecosystems."
Why large mammals play a role
Large-bodied animals have different aspects of the landscape. For example, mammoths weighed as much as a Caterpillar tractor. They compacted the soil – and that changed the groundwater level. As herbivores, they also kept grasslands by keeping certain invasive shrubs outside.
What would happen if we lost all large mammals? "If we lose elephants, we also lose their environmental interactions – how their foraging affects plant structure and vegetation, how their massive weight compacts soils and affects gas exchange," Smith says. "We refer ecosystem engineers of large mammals because of their strong influence on almost all aspects of the habitat, and because large animals change landscapes and transport nutrients to an extent that can not be replicated by smaller relatives."
The proportion of species endangered today exceeds by far the percentage of species that became extinct in the geological periods of the researchers. "That's disturbing," said co-author Jonathan Payne, a paleobiologist at Stanford Earth, Energy, and Environmental Sciences (Stanford Earth). "From an evolutionary perspective at random species removal leaves much of the evolutionary information unaffected and gives you many different starting points for recovery," he said. "If you selectively remove some of the diversity, it's very hard to get back to where you were." Some estimates suggest that in 50 years up to 50 percent of mammals could be extinct.
Fossil Database Review
To find out what role hominin activity might play in eradicating extinction patterns and mammalian body size distribution, the authors were Rosemary Elliott Smith of the University of California San Diego and Kathleen Lyons of the University of Nebraska-Lincoln, sorting extinction in the late Quaternary in five time intervals from 125,000 years ago to 200 years into the future – a period that includes the migration of hominins from Africa and the eventual expansion around the world. In the five intervals, their analysis found that the average body mass of victims and survivors was two to three orders of magnitude different.
As part of the study entitled "Reducing the size of mammals in the late Quaternary" researchers collected data sets of nearly 6,000 species and often extrapolated body size from measurements of teeth or a single bone.
"I was surprised at how consistent the pattern of selectivity is," Payne said. "It shows up on every continent and in every time interval where enough extinction times are available for analysis."
The analysis clearly showed that the entire fossil record does not resemble the periods after the appearance of hominin. None of the five previous mass extinction events or climate changes so greatly increased the extinction risk for animals based on size, large or small.
"Global Warming, Climate Cooling, Long-Term Heat, Long-Term Cold, Highly Variable Climate – none of this is associated with highly selective extermination of fossils," added Payne. "This indicates that you get a signal that is very much dominated by the hunt."
In earlier geological periods, landscape changes and hominin populations remained relatively small. Today, less targeted extinction processes such as climate change, urbanization, habitat changes, and introduced predators combine with the large-scale effects of hunting, poaching, and urban intervention.