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Humans are causing the sixth mass extinction in the history of the Earth, say biologists from Stanford University, the University of California, Berkeley, Universidad Nacional Autónoma de México, the University of Florida, and Princeton University.
"Although biologists cannot say precisely how many species there are, or exactly how many have gone extinct in any time interval, we can confidently conclude that modern extinction rates are exceptionally high, that they are increasing, and that they suggest a mass extinction under way — the sixth of its kind in Earth's 4.5 billion years of history," the researchers write in the journal Science Advances. "If it is allowed to continue, life would take many millions of years to recover, and our species itself would likely disappear early on."
The total number of vertebrate species that went extinct in the last century would have taken about 800 to10,000 years to disappear had natural extinction rates prevailed — making the current rate of mass extinction the most rapid since the one 65 million years ago that wiped out the dinosaurs. Known as the Cretaceous-Tertiary extinction, scientists believe it was prompted by an asteroid impact near Mexico's Yucatan Peninsula. The four mass extinctions that occurred prior to it were tied to drastic and diverse geological events, such as melting glaciers, global cooling and warming, and volcanic eruption associated with shifts in the tectonic plates.
The researchers say human activities are driving the latest mass extinction event. A ballooning human population, habitat destruction to make way for settlements or agricultural production, climate change, ocean acidification, and soil, water, and air pollution are all wiping species off the face of the Earth.
Paul Ehrlich, a professor of population studies at Stanford University and author of The Population Bomb, co-wrote the report. He told VICE News that "overpopulation and overconsumption, particularly by the rich" were the biggest factors driving the sixth mass extinction.
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Species go extinct all the time, even without asteroid impacts or volcanoes. But estimates of the average rate of extinction throughout the history of the Earth vary widely. And that makes it difficult to determine how much human activities have ramped up the rate at which animals are vanishing.
Ehrlich and his co-authors used a conservative figure for the natural rate of extinction in order to avoid an overestimation of human impacts. They assume in their study that two mammal extinctions occur per 10,000 species every century, which is twice as high as previous estimates. They compared this with a conservatively low estimate for the rate of extinctions today.
The current rate of extinction, they found, was as little as eight to as much as 114 times higher than the natural rate.
"We emphasize that our calculations very likely underestimate the severity of the extinction crisis because our aim was to place a realistic 'lower bound" on humanity's impact on biodiversity," the researchers say. They add that avoiding a truly catastrophic mass extinction would require intense efforts to conserve already threatened species and alleviate the pressures on their populations. "However, the window of opportunity is rapidly closing," they say.
Since 1900, the researchers say, over 450 more vertebrates went extinct than what would have occurred normally, included 69 mammals species, 80 bird species, 24 reptiles, 146 amphibians and 158 fish.
The International Union for Conservation of Nature (IUCN) produces the world's most comprehensive measure for extinction risk among species. In the latest update to it's so-called Red List, the group said conservation efforts had helped to pull several species from the brink, but the gains were overshadowed by devastating losses in many other populations.
The IUCN Red List now includes 77,340 species that were assessed for extinction risk, of which 22,784 are threatened. Loss and degradation of habitat was identified as the main threat to 85 percent of all species on the list, with illegal trade and invasive species as other key drivers of population decline.
Paul Olsen, a paleontologist at Columbia University's Lamont-Doherty Earth Observatory, focuses on mass extinction and climate change. "What we don't understand about extinctions is at what point do ecosystems get so badly damaged that life as we know it can't continue," he told VICE News.
Olsen, who was not involved in the study, pointed out some of the uncertainties in working with fossil records, which are used to determine the rate of past extinctions. For instance, scientists are not sure about pace of insect extinctions because they do not leave much of a fossil imprint. But insects are one of the biggest groups impacted by human activities.
Yet, such limitations aside, Olsen concurs that a sixth mass extinction is underway. He hesitates to say what impact human activities might ultimately have on us as a species. "Humans are phenomenally adaptable," he told VICE News. "You can wipe out 99 percent of all human beings and 400 years from that time there might be essentially no real, visible effect."
Follow Esha Dey on Twitter: @deyesha