June 23, 2018
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Scientists are miscalculating how fast species are disappearing, study says

By Amina Khan, Los Angeles Times

LOS ANGELES — Hit the snooze on that ecological doomsday clock for a minute: The world’s species may not be going extinct quite as fast as we thought they were, according to research published online Wednesday in the journal Nature. Scientists may be overestimating it by as much as 160 percent.

While stressing that the global extinction crisis is still indeed a crisis, the study’s two authors called for a better mathematical model to predict how fast the world’s diversity is disappearing.

The massive loss of species occurring today may constitute the sixth mass extinction that life on Earth has seen. The cause is in large part habitat destruction due to human encroachment. The rate of biodiversity loss, however, is very difficult to estimate, said study co-author Stephen Hubbell, an ecologist at the University of California, Los Angeles.

One of the models that scientists use flips around what’s called the species-area relationship: This starts with the number of species in a certain area and then measures how many new species pop up as you increase the area surveyed, by counting the first member of each new species encountered.

Extinction rates, at first glance, should be the opposite of that relationship — how many species you lose as land is lost. So researchers run the equation backward to calculate how fast species are disappearing.

But that’s not an entirely accurate way to measure extinction, Hubbell said. The species-area relationship counts the first member of each species to appear — so running it backward would count the first member of each species to disappear. Just because one individual dies doesn’t mean all of them have, he said.

He and his co-author Fangliang He, an ecologist at Sun Yat-sen University in Guangzhou, China, used a different model in their study. Rather than count the disappearance of the first member of a given species, their method relied on finding the absolute last member of a species — a better fit with the definition of extinction.

Using range maps of North American bird species assembled by Natureserve, a nonprofit conservation organization, and data from the Center for Tropical Forest Science’s network of rainforest plots in Asia, Africa and the Americas in which every tree is tagged, the scientists then charted both models to see how well each fit the real-life data.

They found that the model they used closely matched the known data, whereas the other model overestimated the extinction rate by as much as 160 percent.

The authors and other scientists hastened to add that extinctions were still occurring at an alarming rate.

“This does not mean that there’s not an extinction crisis — just because it’s slightly less disastrous than we think doesn’t mean it’s not disastrous,” said Robert Colwell, an ecologist at the University of Connecticut who co-authored a commentary on the study. “We’re narrowing in on a more accurate way of estimating something that’s difficult to predict … it’s a refinement, not a revolution.”

But some scientists questioned the study’s findings. “There are dozens of studies that have appeared over the past 15 years that show a surprisingly good match between the theoretical predictions and what you actually observe,” said Stuart Pimm, a conservation ecologist at Duke University who has used the standard way of predicting extinctions in his own research.

These new findings, Pimm argued, might be fine for assessing those extinctions that occur right after habitat is destroyed, but not for those species that fade from existence over a longer period of time. For example, he said, even if a particular species survives destruction of most of its habitat, it still may not be able to reproduce, and thus could die out when the current generation  does.

“Dozens of people have been using the right method for quite a long time, and it works very well, thank you very much,” Pimm said.

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