Are We in a 'Sixth Great Extinction'? Maybe Not.
Last summer, a team of researchers led by Stanford University conservation biologist Paul Ehrlich released a study confirming the worst: "Earth is on the brink of a sixth mass extinction," and it's our fault. By polluting the environment and altering habitats, humans are killing off our earthly neighbors.
As disconcerting as this news is, it unfortunately came as little surprise. The notion that humans are erasing species off the face of the Earth at near unprecedented levels is a perennial story that has been blared in the media for more than two decades. In the year 2000, the United Nations' Millennium Ecosystem Assessment estimated that species are going extinct 1,000 times faster than they naturally do, and that this rate could increase to 10,000 times. These rates translate to between 17,000 and 140,000 species going extinct each year by some estimates.
As an undergraduate majoring in zoology and conservation biology at the University of Wisconsin-Madison (the institution where Aldo Leopold pioneered the field of wildlife management), these depressing numbers became engrained within my psyche, along with a desire to do something about them. That desire remains, but my acceptance of those estimates was recently shaken when I came across a simple fact. Over the last five hundred years, there have been just 875 confirmed extinctions. Why so few, when some scientists have insisted there should have been millions?
A large reason for the disparity is that we almost certainly have not come close to identifying all of the species alive on Earth. Between one and 1.5 million species have been discovered, but there may be five to 14 million in total, perhaps more. And so, conservationists assume that many of these undescribed species face similar extinction risks as a result of human activity.
This may be a flawed extrapolation, as the business of predicting species extinction is not a very certain science.
"No proven direct methods or reliable data exist for verifying extinctions," scientists Fangliang He and Stephen Hubbell noted in a paper published to the journal Nature in 2011. And the indirect method conservationists primarily use -- the species–area accumulation curve -- likely overestimates species extinction by 160 percent or more.
While their results countered the prevailing dogma that the world is undergoing a sixth mass extinction, Fangliang and Hubbell made plain that species extinction is a serious issue that must be addressed.
"Although we conclude that extinctions caused by habitat loss require greater loss of habitat than previously thought, our results must not lead to complacency about extinction due to habitat loss, which is a real and growing threat."
There's no doubt that humans have caused and are causing animals to go extinct, but to compare the current situation to previous mass extinctions is misleading. As Sarah Kaplan reported in the Washington Post last year:
The losses of the past century account for only about 1 percent of the roughly 40,000 known vertebrate species — a statistic that pales in comparison to the level of destruction seen during previous mass extinction events. Even in the least of them, between 60 and 70 percent of species were killed off. During the end-Permian event about 250 million years ago, known as “the Great Dying,” that number was more than 90 percent.
In her recent book Resurrection Science, journalist M.R. O'Connor offered a reason why the hyped notion of a sixth mass extinction persists.
"The field of conservation biology is a crisis discipline," she wrote, suggesting that the field is inclined to forecast doom and gloom in order to promote needed environmental protections.
"Conservationists themselves have said that the field breeds a culture of despair," she continued. "And at times, their pessimism threatens to undermine the cause. 'A society that is habituated to the urgency of environmental destruction by a constant stream of dire messages from scientists and the media will require bigger and bigger hits of catastrophe to be spurred into action,' wrote biologists Ronald Swaisgood and James Sheppard in 2010."
It definitely seems that those hits of catastrophe are growing more forceful. With the release of his study last year, Ehrlich issued a pressing warning, suggesting that humanity itself may even be threatened by the current mass extinction.
"We are now moving into another one of these events that could easily, easily ruin the lives of everybody on the planet," he said.