Barnosky agreed that natural systems are resilient. "But you have to give them a chance to be resilient," he said. "Falcons can live in cities. But elephants can't."
People have been predicting disaster for centuries, including 18th century scholar Thomas Malthus and Stanford University ecologist Paul Ehrlich, who in 1968 with his wife Anne predicted famines from runaway population growth in "The Population Bomb."
Ehrlich said he was right because at least 2 billion people are malnourished.
"You'll find plenty of people who will tell you not to worry, technology will take care of it," Ehrlich said. "We'll feed, house, clothe and so on 9.5 billion people, give them happy lives with no problem at all. That's exactly the line that Anne and I got when there were 3.5 billion people on the planet. ... The answer is, they haven't done it."
Touchy strategy on growth
Reducing population growth was central to the U.S. environmental movement at its birth in 1970, spurred in part by Ehrlich's book.
Most environmental groups now steer clear of the subject.
Forced sterilizations in India in the 1970s and China's coercive one-child policy angered feminists and tainted family planning efforts.
Liberals argue that blaming environmental problems on population growth is to "blame the poor." They say the United States and other capitalist societies consume too much.
Conservatives and religious groups who oppose abortion and celebrate reproduction attack family planning at home and abroad. This summer a House Appropriations panel again slashed money for family planning aid.
Population and consumption each drive ecological damage.
"Even in poorer nations that don't have the impact that the average American has on the planet, population as it grows squeezes out other species because people need space to live, and the other species need space to live," said Jeffrey McKee, an anthropologist at Ohio State University. "At some point they come into juxtaposition, and something has to give. So far, it hasn't been us."
Plummeting fertility rates, from 4.9 births per woman in the 1960s to the current 2.6, led to the belief that worries about population were overblown.
The drop surprised demographers. Half the world -- including Japan and Western Europe but also China, Vietnam, Brazil and other emerging economies -- is below the 2.1 fertility rate needed for zero growth. The United States, the world's third-largest country behind China and India, and the only rich country still growing rapidly, recently saw its birth rate fall to 1.9.
Press coverage has stressed a "birth dearth" that threatens economic growth and elderly retirements, prompting fears that the human species could contract to 1 billion by 2300 because of a failure to reproduce.
But an important exception to falling fertility rates is sub-Saharan Africa, along with such places as Afghanistan and Yemen, where birth rates remain exceptionally high. U.N. demographers sharply raised their population projections last year, adding another billion people by century's end, to nearly 11 billion, because African fertility rates have peaked at more than five births per woman.
From now until 2050, poor countries will add the equivalent of a city of 1
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