The biomass lost via the extinctions of large mammals such as mammoths and giant ground sloths during the last 50,000 years has largely been replaced by that of one species, Homo sapiens. The unprecedented success of humans is in large part possible only because people take advantage of fossil fuels, a new study suggests.

About half of the mammalian megafauna species — loosely defined as those tipping the scales at 44 kilograms or more — have died out in the past 50,000 years, says Anthony D. Barnosky, a paleoecologist at the University of California, Berkeley. That leaves only about 180 non-human mammalian species of that size on Earth today, he reports in the Aug. 12 Proceedings of the National Academy of Sciences.

Between 50,000 and 7,000 years ago, a series of megafauna extinctions swept the planet. The timing of those die-offs, particularly those in Australia (SN: 1/20/07, p. 38) and some Caribbean islands (SN: 10/29/05, p. 275) suggests that the spread of humans played a large role in the losses. Indeed, Barnosky notes, most megafauna didn’t die out until human population worldwide began to rise steeply between 15,500 and 11,500 years ago.

Near the end of the last ice age, just as the largest wave of extinctions began, humans and other large mammals were consuming most of the fruits of the planet’s natural productivity, Barnosky contends. After the other large creatures died out, humans were poised to take advantage of the newly available natural resources, but a 1,300-year-long cold spell called the Younger Dryas period intervened, dropping natural productivity and constraining megafauna population growth.

When climate warmed at the end of the Younger Dryas, about 11,500 years ago, human numbers began to swell dramatically, Barnosky says. Nevertheless, megafauna biomass, including that of humans, didn’t reach pre-extinction levels until around 400 years ago, just before the beginning of the Industrial Revolution. Since then, human population has continued to grow at an ever-increasing rate, a trend directly attributable to human use of fossil fuels, Barnosky contends.

The energy stored in fossil fuels — essentially solar energy stored by plants long ago — is supplementing that falling as sunlight on Earth’s surface today. And, Barnosky notes, fossil fuels aren’t limitless: Some projections indicate that easily recovered oil will run out in around 50 years at the current rate of use, and coal will be used up in another 2,000 years or so. If humans haven’t discovered alternative sources of energy by then, populations of all megafauna — and particularly those of humans — will crash.

Homo sapiens is fabulously successful by ecological standards, Stanford biologists Paul Ehrlich and Robert Pringle comment in the Aug 12. Proceedings of the National Academy of Sciences. Not only does the human species boast an as-yet-unchecked population growth, it has spread to all corners of the globe and vanquished many of its predators, competitors and parasites, they note. Previous studies suggest that humans alone consume almost a quarter of the planet’s natural productivity, often at the expense of other species (SN: 10/13/07, p. 235).

And biodiversity isn’t likely to improve in the near future, the researchers note. Today’s rate of species die-offs is likely thousands of times higher than long-term rates experienced during past geological ages. Not only that, they say, human dominance of many ecosystems will probably stifle the evolution of large creatures — ones that typically are highly mobile and require large habitats — for the foreseeable future. “The fate of biological diversity for the next 10 million years will almost certainly be determined during the next 50 to 100 years by the activities of a single species,” Homo sapiens, Ehrlich and Pringle propose.

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Found in: Life