of how many die out each year.

The technique is similar to that used to figure out a country's death rate: track the number of people who die in a given population each year, scaled to that population. Mortality rates are usually calculated as the number of deaths per thousand people per year.

View Images This critically endangered male blue-knobbed curassow was photographed at the Houston Zoo. Found in the wild only in Colombia, this species' population is estimated at fewer than a thousand. Photograph by Joel Sartore, www.joelsartore.com

Applying the same statistical approach to extinction data revealed a rate of 100 to 1,000 species lost per million per year, mostly due to human-caused habitat destruction and climate change. (See: "7 Species Hit Hard by Climate Change—Including One That's Already Extinct.")

Toare, where they live, and how their populations are changing remains a key impediment."

View Images Suci, a critically endangered Sumatran rhinoceros living at the Cincinnati Zoo, died this year. Intensive poaching has left fewer than 200 in the wild. Photograph by Joel Sartore, www.joelsartore.com

People Power

To Jenny McGuire, a postdoctoral research scientist at the University of Washington's School of Environmental and Forest Sciences who wasn't involved in the study, the results aren't surprising.

Though some might quibble about the exact rate, she says, "in general scientists are in agreement that we're at a period of heightened extinction risk and rates, and that's been occurring nearly since humans have come onto the landscape."

View Images A rare chucky madtom hangs on at Conservation Fisheries, a fish-breeding center in Knoxville, Tennessee. The species'