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The influence of primitive humans on the biodiversity of mammals occurred earlier than previously thought



Giant mammals, such as mammoths, woolly rhinos and saber-toothed tigers, once roamed all habitable continents but became extinct. New research shows that the loss of mammalian biodiversity, one of the main conservation concerns today, is part of a long-term trend that lasts at least 125,000 years.

As archaic humans, Neandertals and other hominid species migrated from Africa, a wave of extinction in large-bodied mammals followed. This happened on all continents and intensified over time. The study, published April 20 in the journal Science is the first to quantitatively demonstrate that the human effects on the body size of mammals predate their migration out of Africa and that the extinction related to the size reflects human activities.

funded by a grant from the National Science Foundation, was led by Professor Felisa Smith, paleoecologist at the University of New Mexico along with colleagues from the University of California San Diego, the University of Nebraska-Lincoln and the School of Earth, Energy and Environment of the Stanford University Sciences.

Deep Influence on Ecosystems

To document what happened to mammals when the first humans left Africa, the researchers compiled extensive data on climate and on mammalian body size, extinction status and location geographic in the last 125,000 years. They also used the state of conservation of modern mammals to model diversity and body size distributions for 200 years in the future. His study investigated the role of body size and diet in the probability of extinction. These data were evaluated in light of climate change and human migration patterns in the same time frame.

The study showed that the degradation of body size – the loss of the largest species on each continent over time – is a hallmark of human activity, both past and present. If this trend continues in the future, the largest land mammal in 200 years will be the domestic cow.

"We, modern and ancient humans, have had a profound influence on ecosystems," Smith said. "Our results clearly indicate that size-biased extinction is a hallmark of human activities and not a general characteristic of the evolution of mammals," he said. "The big picture is that if things continue as they are, the degradation of biodiversity will not only change, but the way that energy flows through ecosystems will fundamentally change."

Why large mammals matter

Large-bodied animals maintain various aspects of the landscape. For example, mammoths weighed as much as a Caterpillar tractor. They compacted the soil, and that changed the water table. As herbivores, they also maintained pastures by keeping certain invasive shrubs out.

What would happen if we lost all the large mammals? "If we lose elephants, we also lose their ecological interactions, how their feeding influences the structure of the plant and vegetation, how their massive weight compacts soils and influences the exchange of gases and so on," Smith said. "We call ecosystems engineers large mammals because of the great influence they have on virtually every aspect of the habitat, and because large animals modify landscapes and move nutrients to a scale that can not be reproduced by relatives of smaller bodies. "

The proportion of threatened species today far exceeds the proportion of species that became extinct during geological periods. to. "That's worrisome," explained co-author Jonathan Payne, a paleobiologist at Stanford Earth's School of Earth, Energy and Environmental Sciences. "From an evolutionary point of view, at random the removal of species leaves a lot of evolutionary information intact and gives you many different starting points for a recovery," he said. "If selectively eliminates part of the diversity, it's very difficult to go back to where it was." Some estimates suggest that between 25 and 50 percent of mammals could go extinct in 200 years.

Review of the fossil database

Seeking to determine the role that hominid activity may have played in the configuration of extinction patterns and body size distribution of mammals, the authors, which included to Rosemary Elliott Smith of the University of California San Diego and Kathleen Lyons of the University of Nebraska-Lincoln, classified extinctions in the last Quaternary period in five time intervals ranging from 125,000 years to 200 years in the future, a period that it covers the migration of hominids outside of Africa and their eventual expansion around the world. In the five intervals, his analysis found that the average body mass of victims and survivors differed by up to two or three orders of magnitude.

As part of the study, entitled "Reduction of the body size of mammals at the end of the Quaternary period". The researchers compiled data sets that encompass nearly 6,000 species, often extrapolating the body size from measurements of the teeth or a single bone.

Consistent pattern

"I was surprised at how consistent the selectivity pattern is," Payne said. "It is shown on each continent and in each time interval where there are enough extinctions to run an analysis."

The analysis clearly showed that the complete fossil record does not resemble at all the periods after the appearance of the hominids. None of the five previous events of mass extinction or changes in climate increased the risk of extinction for animals so strongly based on size, large or small.
"Climate warming, climate cooling, long-term heat, long-term cold, very variable weather, none of them are associated with highly selective extinction in the fossil record," Payne added. "That suggests you're getting a signal that's very much dominated by hunting."

In earlier geological periods, changes in landscape and hominid populations remained relatively small. At present, less specific extinction processes such as climate change, urbanization, habitat alteration and introduced predators are combined with the strongly biased effects of hunting, poaching and urban invasion.

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