May 19, 2011
The golden toad (Bufo periglenes) is one of well-over a hundred frogs that are believed to have gone extinct over the past few decades. Photo by: US Fish and Wildlife Service.
Estimating extinction rates has always been full of uncertainty. Given the lack of knowledge over how many species survive on Earth—is it 5 million or 100 million?—it is equally difficult to know how quickly species (the majority of which haven't even been described) vanish in the face of habitat loss and other impacts. Scientists have predicted that the current rate of extinction loss is between 100 to 1,000 times above the average background rate of extinctions, but the new paper says such predications are overestimations.
Paper adds controversy
The new paper, by Fangliang He and Stephen Hubbell, has aggravated the debate over just how many species are vanishing. He and Hubbell argue that the major way in which researchers estimate species extinction—the 'species-area relationship'—has overestimated extinction rates by up to 160 percent.
Native to the Indonesian island of Sulawesi, the black-crested macaque (Macaca nigra) is classified by the IUCN Red List as Critically Endangered. Photo by: Rhett A. Butler.
"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," Hubbell says, obviously wanting to make certain the media doesn't turn this into a denial of the mass extinction crisis, which experts agree is currently occurring or very near on the horizon.
But according to Hubbell and He, using species-area relationship does not come up with an accurate answer as to the rate of extinction. Species-area relationship estimates loss of species by looking at the loss of habitat. Surveying how many species are in a given area, researchers than extrapolate how many species are in the wider ecosystem. Working backwards, researchers may estimate the number of species that go extinct as the habitat is destroyed. For example if 90% of a forest is clearcut, the species-area relationship predicts about half the species in the ecosystem would vanish. These species may not vanish right away, small populations may still persist, but ecologists argue these populations suffer from 'extinction debt'. In other words, they are doomed to extinction over time, what some researchers describe as the living dead'. According to research it can take species many generations to go extinct even after a population tipping-point has been reached.
But, the paper argues that this mathematical method suffers from overestimating extinctions because most species still survive, albeit in significantly reduced populations, in the habitat remaining.
"We show that this surrogate measure is fundamentally flawed," says Hubbell, "you can't just turn [the species-area relationship] 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."
A Sumatran rhino in captivity in Borneo. The Sumatran rhino is on the edge of extinction with only 200 adult individuals estimated to survive. Photo by: Jeremy Hance.
As an example of where the species-area relationship has overblown predictions, the study points to forecasts in the early 1980s that half of the species on Earth would be extinct by 2000. However, that has not occurred among known species.
"Nothing like that has happened," Hubbell says.
Muddying the waters, of course, is the fact that while habitat loss is the greatest threat to species worldwide, other impacts, such as hunting and poaching, overconsumption, pollution, disease, and invasive species also imperil species. In addition, climate change could become a major in causing future extinctions, especially in the face of tiny populations surviving in habitat fragments.
Complicating the picture even further is conservation efforts of known species. Some species should have gone extinct, but continue to hang-on due largely to long-term and active conservation efforts. However, cryptic species—those not yet known to researchers—don't have such a luxury.
The response to Hubbell and He's paper from other experts in the field has been rapid and negative.
The Sumatran orangutan (Pongo abelii) is considered Critically Endangered with approximately 7,000 left, representing an 80% decline in 75 years. Photo by: Rhett A. Butler.
Pimm points out that the title of the Hubbell and He's paper, Species–area relationships always overestimate extinction rates from habitat loss, is undercut immediately by studies that have found that species-area relationship has resulted in correct predictions. For example, Pimm points to a study of his where the species-area relationship correctly predicted the extinction of 4.5 birds in Eastern North America. In fact, four birds have gone extinct, and one (the 0.5) is threatened with extinction.
According to Pimm, the problem lies in the paper's conflating immediate species extinction after habitat loss with eventual species extinctions over time, in other words it completely ignores the extinction debt, those living dead populations that will wink out in time.
"Imagine destruction that wipes out 95% of the habitat in an area metaphorically 'overnight'. How many species have disappeared 'the following morning'? The paper tells you. It is not many, just those wholly restricted to the 95% (and absent from the 5% where they would survive)," Pimm says, agreeing that the initial extinction rate would be low, because it would only affect endemics, i.e. those found only in the habitat destroyed. But in time, says Pimm, extinction rates would worsen even if no additional habitat loss occurs.
A leatherback sea turtle, also considered Critically Endangered, laying eggs on a beach in Suriname. Photo by Tiffany Roufs
Lian Pin Koh, a tropical ecologist at ETH Zurich, agrees with Pimm that the paper's findings are incorrect. Koh says the authors "are simply barking up the wrong tree" because they have confused how researchers estimate the number of species in an ecosystem and how they estimate extinction rates.
"The basic premise of the He and Hubbell paper is plain wrong," Koh told mongabay.com, "The species-area relationships they refer to are actually species accumulation curves—the relationship between sample size and number of species encountered. On the other hand, the species-area models that ecologists use to predict species extinctions are a different set of relationships altogether, one that is based on empirical evidence of the relationship between the size of islands and number of species inhabiting those islands."
In other words, according to Koh, the paper does not take-on the correct model.
Koh and Pimm are apart of a group of experts who are currently drafting a formal response to the Nature paper.
"The paper is a sham," Pimm continues, "it does not report extinction rates or the numbers of species that are threatened. Despite its posturing, it deals with a different issue."
A black-and-white ruffed lemur (Varecia variegata) in Madagascar. Their population has dropped by 80% in 27 years. This species is also listed as Critically Endangered. Photo by: Rhett A. Butler.
The evidence for this is undeniable: according to the IUCN Red List, the world authority on species' threat levels, 41% of the world's amphibians are threatened with extinction, 33% of cartilaginous fishes, 33% of coral reefs, 25% of mammals, 22% of reptiles, 15% of bony fishes, 14% of sea grasses, and 13% of birds. Even these are likely to be underestimates. Studies have shown that recently discovered species have a higher risk of extinction than species known for centuries. Therefore, cryptic species, those still undiscovered (which outnumber known species) are more likely to be weighted towards threatened rather than thriving.
Whatever the exact rate is at which species are going extinct, a biodiversity crisis is occurring that portends a global extinction not seen since the dinosaurs fell. Though not as important to mainstream media as Arnold Schwarzenegger's infidelity and low on the list of priorities for world leaders, ecologists—even when they disagree on the details—still agree the world is in the middle of a full-blown crisis.
"The next mass extinction may be upon us or just around the corner," says Hubbell. "There have been five mass extinctions in the history of the Earth, and we could be entering the sixth mass extinction."
CITATION: Fangliang He and Stephen P. Hubbell. Species–area relationships always overestimate extinction rates from habitat loss. Nature. Vol. 473. May 19, 2011. doi:10.1038/nature09985.
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