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Plant, Animal Extinction Rates Exaggerated: Study

May 19, 2011

A projected wave of extinctions of plant and animal species this century may have been overestimated because the most widely used scientific method can exaggerate losses by more than 160 percent, according to a study published Wednesday in the journal Nature.

The scientists said that habitat destruction by humans is driving animal and plant species toward extinction at just half the pace as previously believed.

They attribute the overestimation to flaws in the most popular scientific method, which exaggerated the projected losses by more than 160 percent.

“Extinctions caused by habitat loss require greater loss of habitat than previously thought,” wrote the study’s authors.

However, while the problem of species extinction due to habitat loss is not as calamitous as scientists had originally believed, the global extinction crisis is real, said Stephen Hubbell, distinguished professor of ecology and evolutionary biology at UCLA and co-author of the study.

“The methods currently in use to estimate extinction rates are erroneous, but we are losing habitat faster than at any time over the last 65 million years,” Hubbell said.

“The good news is that we are not in quite as serious trouble right now as people had thought, but that is no reason for complacency. I don’t want this research to be misconstrued as saying we don’t have anything to worry about when nothing is further from the truth.”

Because there are very few ways of directly determining extinction rates, scientists and conservationists have used an indirect method known as “species-area relationship.”

The technique begins by determining the number of species found in a given area, and then estimating how this number grows as the area expands. Using this data, the calculations are then reversed in an attempt to estimate how many fewer species will remain when the amount of land decreases due to habitat loss.

“There is a forward version when we add species and a backward version when we lose species,” Hubbell said.

“In the Nature paper, we show that this surrogate measure is fundamentally flawed. The species-area curve has been around for more than a century, but you can’t just turn it around to calculate how many species should be left when the area is reduced; the area you need to sample to first locate a species is always less than the area you have to sample to eliminate the last member of the species.”

“The overestimates can be very substantial. The way people have defined ‘extinction debt’ (species that face certain extinction) by running the species-area curve backwards is incorrect, but we are not saying an extinction debt does not exist.”

Hubbell, along with ecologist and lead author Fangliang He, a professor at China’s Sun Yat-sen University in Guangzhou, said they were confident of their findings.

“100 percent,” Hubbell said.

“The mathematical proof is in our paper.”

In the early 1980s, some scientific predictions estimated that as much as one-half of the species on Earth would be lost by 2000.

“Nothing like that has happened,” Hubbell said.

“However, the next mass extinction may be upon us or just around the corner. There have been five mass extinctions in the history of the Earth, and we could be entering the sixth mass extinction.”

Hubbell and He’s mathematical proof addresses very large numbers of species, but does not answer whether any particular species is at risk of extinction.

“We have bought a little more time with this discovery, but not a lot,” Hubbell said.

“As we continue to destroy habitat, there comes a point at which we do lose a lot of species “” there is no doubt about that,” he said.

“However, we have to destroy more habitat before we get to that point.”

When a meteor struck the Earth some 65 million years ago, killing the dinosaurs, a fireball incinerated the Earth’s forests.  It took about 10 million years before any semblance of continuous forest cover returned, Hubbell said.  The extinctions that humans cause may be just as catastrophic, but in different ways, he added.

Humans are already using 40 percent of all the “plant biomass” produced by photosynthesis on the planet, a troubling statistic because most life on Earth depends on plants, Hubbell said.

Nearly 75 percent of all species thought to reside on Earth live in rain forests, which are being cut down at an alarming rate of about half a percent per year, he said.

Hubbell and He used data from the Center for Tropical Forest Science that included large areas in Asia, Africa, South America and Central America in which every tree is tagged, mapped and identified, including 4.5 million trees and 8,500 tree species — many of which are very rare.

If they go extinct, so will the animals that depend upon them.

“We need much better data on the distribution of life on Earth,” Hubbell said.

“We need to rapidly increase our understanding of where species are on the planet. We need citizens to record their local biodiversity; there are not enough scientists to gather the information. We also need much deeper thought about how we can estimate the extinction rate properly to improve the science behind conservation planning. If you don’t know what you have, it is hard to conserve it.”

Hubbell and He plan in future research to investigate more precisely how large the overestimates have been.

In the mean time, Hubbell encourages the public to spend more time enjoying nature, “especially if it’s going to be here today, gone tomorrow. If we don’t take steps to preserve animals and plants that we care about, they are going to be gone.”

“When I was a kid, I spent a lot of time doing non-macho things like collecting butterflies and turning over rocks. The only way we’re going to save nature is by making sure future generations experience nature,” he said.

“People who have never seen wild nature don’t miss it and don’t realize how impoverished their lives have become due to its loss. I worry about the loss of a conservation ethic among the public. Go to the tropics. Experience a rain forest “” while you still can.”

The study was published online May 18, 2011 in the journal Nature.

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