You may have seen reports in the popular media about the rather sobering possibility that Earth may be on the brink of, or already into the leading edge of, a sixth mass extinction.

Anthony Barnosky

Those reports stem primarily from the March 3 publication in the prestigious journal Nature of a study by Berkeley paleobiologists. Comparing the species at significant risk of extinction today with the rates at which species died out in the five previous mass extinctions over the last 540 million years, the authors say we are tipping, and the next great mass extinction could be well underway in as little as 300 years. But they believe it’s not too late to save the present-day critically endangered species and stop short of the tipping point.

As detailed in Nature this month, credit for this important research goes not only to two Berkeley professors of integrative biology — principal author Anthony Barnosky and coauthor Charles Marshall — but to a coauthor team consisting of eight graduate students in integrative biology, one alumna with a recent Berkeley Ph.D., and a recent Berkeley postdoc. The study originated in a graduate seminar Barnosky organized in 2009 so biologists and paleontologists could compare, together, today’s extinction rates with the fossil record — a tricky proposition, since the fossil record, which goes back 3.5 billion years, has many gaps.

Image of a tiger behind gate
GOING — Tigers are one of Earth's most critically endangered species. Extinction of the majority of such species would indicate that the sixth mass extinction is in our near future. (Photo by Anthony Barnosky)

The team chose mammals as a starting point for their study because mammals are well studied today and are well represented in the fossil record as far back as 65 million years. They estimate that over the long haul, the extinction rate for mammals is less than two extinctions every million years, far lower than the current extinction rate for mammals. In the much more recent past — the last 500 years — at least 80 mammal species have gone extinct, disappearing from a starting total of 5,570 species.

The integrative biology graduate student coauthors on the team are Nicholas Matzke, Susumu Tomiya, Guinevere Wogan, Brian Swartz, Emily Lindsey, Kaitlin Maguire, Ben Mersey and Elizabeth Ferrer. Another team member, Jenny McGuire, completed her dissertation in 2010 and is now a postdoc with the National Evolutionary Synthesis Center at Duke University in North Carolina. Coauthor Tiago Quental, a postdoc at Berkeley while on the team, is now at the University of Sao Paulo in Brazil.

Barnosky urges, along with action to combat the loss of animal and plant specioes, similar studies of groups other than mammals to test the team’s findings.

A 20,000-year-old fossil skull
GONE — A 20,000-year-old fossil skull from the giant short-faced bear, Arctodus, which stood about six feet high at the shoulder while on all fours. This one resided in northern California; its kin lived throughout North America, but were most abundant in California. Arctodus went extinct by around 12,000 years ago, not long after humans arrived in North America. It was the largest carnivorous mammal that ever lived on this continent. Its surviving-but-threatened relative, the grizzly, once widespread in the lower 48 states, has long been extinct in California (by modern standards; the last one in this state was shot in 1922) and survives only in a small range in Yellowstone Park up through Glacier Park. (Photo by Anthony Barnosky)