We could be on the brink of a mass extinction — the Earth’s sixth — according to a paper published last year in the journal Nature. First author Anthony Barnosky, an integrative biologist at the University of California at Berkeley, says Earth has experienced five mass extinctions during the past 540 million years, and another extinction could be around the corner. During each of the five previous events, three-quarters or more of the world’s animal species died out. One of the mass extinctions — which occurred 65 million years ago — ended the dinosaurs.
Some say, however, that this isn’t much cause for alarm. Species have always come and gone over long periods of time; and given the five mass extinctions we’ve already had, it’s a natural event. But will this sixth one be a “different animal”?
The sixth is under way
Barnosky and his colleagues looked at fossil records and calculated the rate at which mammals died off in the past 65 million years. They found an average extinction rate of less than two species per million years. But in the past 500 years, a minimum of 80 of 5,570 species of mammals have gone extinct, a rate above documented rates for past mass extinctions. This means that we’re at the beginning of one.
The situation looks even more dire when all mammals currently classified as endangered or threatened by the International Union for Conservation of Nature (IUCN) are added to the count. If all of those animals disappear within a century, then 334 years from now, 75 percent of all mammal species will be gone. The team extended the same methods of analysis to amphibians, reptiles, birds, plants, mollusks and other forms of life. They found fairly consistent patterns.
H. Richard Lane, program director of the Division of Earth Sciences at the National Science Foundation, which funded this research, says that unlike the previous global mass extinctions, the sixth will be the result of climate change caused by human activities. Conservationists have warned for years that we are seeing a massive, human-caused extinction, with species from frogs to birds to tigers threatened by global warming, loss of habitat and competition for resources with introduced nonnative species.
On the upside
Others, however, point out that Barnosky’s research is based on observations from just a very few “twigs” (the IUCN’s Red List) plucked from the tree of life. And so far, only 1 to 2 percent of all species have gone extinct in the groups looked at in Barnosky’s study. Too, fossil dating is not a precise science, and the fossil records and modern data collected by conservation biologists don’t always match up well. And assuming threatened species will actually go extinct is a problem, since it’s not inevitable.
In addition, the number of unknown species may be in the millions or tens of millions — many times that of what has been discovered. That makes any percentages of how many of the world’s species are in trouble unreliable.
Still a lot to save
But co-author Charles Marshall, also an integrative biologist at UC-Berkeley and director of the university’s Museum of Paleontology, emphasized that the small number of recorded extinctions to date (1 to 2 percent) does not mean we are not in a crisis. He believes that just because the magnitude is low compared to the biggest mass extinctions Earth has seen in half a billion years doesn’t mean it isn’t significant.
Despite that, all is not gloom and doom, per the paper’s authors. It’s not too late, they believe, to save endangered animals — and stop short of the tipping point. Of course, that would require dealing with a unique mix of modern threats, including habitat fragmentation, invasive species, diseases and global warming.
Luckily, we’ve still got a lot of biota left to save.
Do you think we’re on the brink of a human-caused mass extinction? Or are such events just a cyclical part of nature? If we are at the threshold, will this mass extinction be more devastating than the ones in the past?
Feature photo: More than 80 percent of plant species in grasslands contribute to their effective functioning. Loss of biodiversity puts our environments in jeopardy. This pronghorn buck and doe were photographed in a grassland in Yellowstone National Park. ©Henry H. Holdsworth