The University of Nebraska State Museum's Elephant Hall highlights the differences in current elephants (left) and mammoths (middle and right). Pictured (from left) is an African elephant; an Asian elephant with a juvenile; dwarf mammoth; Archie, a Columbian mammoth; and a Jefferson mammoth. Photo: Troy Fedderson / University of Nebraska

By Scott Schrage
19 April 2018

(University of Nebraska–Lincoln) – Homo sapiens, Neanderthals and other recent human relatives may have begun hunting large mammal species down to size — by way of extinction — at least 90,000 years earlier than previously thought, says a new study published in the journal Science.

Elephant-dwarfing wooly mammoths, elephant-sized ground sloths and various saber-toothed cats highlighted the array of massive mammals roaming Earth between 2.6 million and 12,000 years ago. Prior research suggested that such large mammals began disappearing faster than their smaller counterparts — a phenomenon known as size-biased extinction — in Australia around 35,000 years ago.

With the help of emerging data from older fossil and geologic records, the new study estimated that this size-biased extinction started at least 125,000 years ago in Africa. By that point, the average African mammal was already 50 percent smaller than those on other continents, the study reported, despite the fact that larger landmasses can typically support larger mammals.

But as humans migrated out of Africa, other size-biased extinctions began occurring in regions and on timelines that coincide with known human migration patterns, the researchers found. Over time, the average body size of mammals on those other continents approached and then fell well below Africa’s. Mammals that survived during the span were generally far smaller than those that went extinct.

The magnitude and scale of the recent size-biased extinction surpassed any other recorded during the last 66 million years, according to the study, which was led by the University of New Mexico’s Felisa Smith.

“It wasn’t until human impacts started becoming a factor that large body sizes made mammals more vulnerable to extinction,” said the University of Nebraska-Lincoln’s Kate Lyons, who authored the study with Smith and colleagues from Stanford University and the University of California, San Diego. “The anthropological record indicates that Homo sapiens are identified as a species around 200,000 years ago, so this occurred not very long after the birth of us as a species. It just seems to be something that we do.

“From a life-history standpoint, it makes some sense. If you kill a rabbit, you’re going to feed your family for a night. If you can kill a large mammal, you’re going to feed your village.”

By contrast, the research team found little support for the idea that climate change drove size-biased extinctions during the last 66 million years. Large and small mammals seemed equally vulnerable to temperature shifts throughout that span, the authors reported.

Off the face of the Earth

The team also looked ahead to examine how potential mammal extinctions could affect the world’s biodiversity. To do so, it posed a question: What would happen if the mammals currently listed as vulnerable or endangered were to go extinct within the next 200 years?

In that scenario, Lyons said, the largest remaining mammal would be the domestic cow. The average body mass would plummet to less than six pounds — roughly the size of a Yorkshire terrier.

“If this trend continues, and all the currently threatened (mammals) are lost, then energy flow and taxonomic composition will be entirely restructured,” said Smith, professor of biology at New Mexico. “In fact, mammalian body size around the globe will revert to what the world looked like 40 million years ago.”

Lyons said that restructuring could have “profound implications” for the world’s ecosystems. Large mammals tend to be herbivores, devouring large quantities of vegetation and effectively transporting the associated nutrients around an ecosystem. If they continue to disappear, she said, the remaining mammals would prove poor stand-ins for important ecological roles.

“The kinds of ecosystem services that are provided by large mammals are very different than what you get from small mammals,” Lyons said. “Ecosystems are going to be very, very different in the future. The last time mammal communities looked like that and had a mean body size that small was after the extinction of the dinosaurs.

“What we’re doing is potentially erasing 40 to 45 million years of mammal body-size evolution in a very short period of time.”

Smith and Lyons authored the study with Jon Payne of Stanford University and Rosemary Elliott Smith from the University of California, San Diego. The team received support from the National Science Foundation.

Contact

Kate Lyons, Assistant Professor of Biological Sciences
773-841-6521

Unprecedented wave of large-mammal extinctions linked to ancient humans


By Steve Carr
19 April 2018

(UNM) – Researchers have demonstrated that mammal biodiversity loss, a major conservation concern today, is part of a long-term trend lasting at least 125,000 years. As archaic humans, Neanderthals and other hominin species migrated out of Africa, what followed was a wave of size-biased extinction in mammals on all continents that intensified over time.

A new study, titled Body size downgrading of mammals over the late Quaternary, released Friday in the prestigious journal Science, is the first to quantitatively show that human effects on mammal body size predates their migration out of Africa and that size selective extinction is a hallmark of human activities and not the norm in mammal evolution.

The research, funded by a National Science Foundation (NSF) grant, was led by Dr. Felisa Smith at The University of New Mexico, along with colleagues from University of California San Diego, University of Nebraska-Lincoln and Stanford University. The researchers showed that body-size downgrading – the loss of the largest species on each continent over time – is a hallmark of human activity, both in the past and present. If this trend continues into the future researchers warn, the largest terrestrial mammal in 200 years will be the domestic cow.

“One of the most surprising finds was that 125,000 years ago, the average body size of mammals on Africa was already 50 percent smaller than on other continents,” said Smith, a professor in the UNM Department of Biology who has studied megafauna extinction for more than 15 years. “We suspect this means that archaic humans and other hominins had already influenced mammal diversity and body size in the late-Pleistocene.”

This find was particularly surprising because Africa is a larger continent and typically, larger land masses house and support larger mammals. But, it appears that by the late Pleistocene, hominins had already reduced the diversity of mammals there. Over time, as humans migrated around the globe, extinctions of the largest mammals followed. These giant mammals included the woolly rhinoceros, mammoths, llamas, camels and giant ground sloths as well as ferocious predators such as the short-faced bear, and the scimitar and saber-toothed cats.

In their research, Smith and her colleagues found that this decline follows the global expansion of hominins over the late-Quaternary, including the Pleistocene and Holocene Periods. A number of theories have been developed over the years to explain more recent extinctions such as those at the end of the last ice age, including human hunting, climate change, disease, and even a cosmic impact such as an asteroid or comet. However, earlier workers had not focused on extinctions this far back in time.

“Our study suggests that all of these mammal extinctions are part of a long-term trend. This was fascinating because it only occurred after the arrival of early humans,” said co-author Rosemary Elliott Smith.

By quantifying mammalian extinction selectivity, the researchers documented what happened to mammals as early humans left Africa through the compilation of extensive data including mammal body size, climate, extinction status and geographic location over the last 125,000 years. They also used the conservation status of modern mammals to model diversity and body size distributions for 200 years in the future. The researchers investigated and demonstrated the role of body size and diet on the likelihood of extinction. These data were evaluated in light of climate change and human migration patterns over the same time frame.

They demonstrated size-selective extinction was already underway in the oldest interval, occurred on all continents, within all trophic modes, and across all time intervals. Moreover, the degree of selectivity was unprecedented in 65 million years of mammalian evolution. The distinctive selectivity signature implicates hominin activity as a primary driver of taxonomic losses and ecosystem homogenization.

The researchers also examined the potential influence of climate on extinction risk and selectivity over time. They found that for 65 million years, changes in climate did not result in more extinctions, nor was there a greater tendency for large-bodied mammals to go extinct. “You just don’t see extreme size selectivity for mammals until the late-Pleistocene,” said Kate Lyons, a co-author on the study. “Past climate changes don’t result in size-selective extinction.”

‘We suspect that in the past, shifts in climate led to adaptation and movement of animals, not extinction” said co-author Payne, “Of course, today ongoing climate change may result in extinction since most megafauna are limited in how far they can move.” If the loss of large-bodied mammals continues into the future and all the currently threatened animals are lost, the largest mammal on earth in 200 years may be a domestic cow.

Because megafauna have a disproportionate influence on ecosystem structure and function, past and present body size downgrading is reshaping Earth’s biosphere. By comparing extinction events with the entire record of mammal turnover over the past 65 million years, the researchers demonstrated that body size and diet did not influence extinction risk for mammals for most of their evolutionary history. These results highlight a startling point. The role of ancient and modern humans on large mammals has been vastly underappreciated researchers say.

“Megafauna play a really important role in ecosystems,” said Smith. “which we are just beginning to appreciate. For example, as they walk their massive size compacts the soil, which can lead to changes in gas exchange or water tables. They change the structure of vegetation through their browsing and help maintain open grasslands. They burp methane, a greenhouse gas and even influence the distribution of nitrogen and phosphorous on the landscape. We are not entirely sure what the potential loss of these ‘ecosystem engineers’ could lead to. I hope we never find out.”

Contact

Steve Carr, Supervisor, Communications
University of New Mexico
505-277-1821
scarr@unm.edu

UNM researcher explores influence of ancient humans on mammal body size


ABSTRACT: Since the late Pleistocene, large-bodied mammals have been extirpated from much of Earth. Although all habitable continents once harbored giant mammals, the few remaining species are largely confined to Africa. This decline is coincident with the global expansion of hominins over the late Quaternary. Here, we quantify mammalian extinction selectivity, continental body size distributions, and taxonomic diversity over five time periods spanning the past 125,000 years and stretching approximately 200 years into the future. We demonstrate that size-selective extinction was already under way in the oldest interval and occurred on all continents, within all trophic modes, and across all time intervals. Moreover, the degree of selectivity was unprecedented in 65 million years of mammalian evolution. The distinctive selectivity signature implicates hominin activity as a primary driver of taxonomic losses and ecosystem homogenization. Because megafauna have a disproportionate influence on ecosystem structure and function, past and present body size downgrading is reshaping Earth’s biosphere.

Megafaunal loss

Today, it is well known that human activities put larger animals at greater risk of extinction. Such targeting of the largest species is not new, however. Smith, et al., show that biased loss of large-bodied mammal species from ecosystems is a signature of human impacts that has been following hominin migrations since the Pleistocene. If the current trend continues, terrestrial mammal body sizes will become smaller than they have been over the past 45 million years. Megafaunal mammals have a major impact on the structure of ecosystems, so their loss could be particularly damaging.

Body size downgrading of mammals over the late Quaternary

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