MINNEAPOLIS / ST. PAUL -- Human population growth has turned "a very sharp corner" and is now slowing, on its way to leveling off in the next century, according to a study by University of Minnesota ecologist Clarence Lehman. He used new equations that take into account the accelerating effect population density has on per capita population growth at certain points in history--what is termed "positive feedback." The new equations show that the long-increasing human population growth rate began an abrupt decline in the 1970s, and these equations agree with recent work by demographers, Lehman said. He presented his work Monday, Aug. 6, at the Ecological Society of America meeting in Madison, Wis.
Beginning with Charles Darwin, ecological theorists have predicted that population growth will be affected by previous growth in different ways during the course of a species' existence, Lehman said. Early on, when the species is small in numbers, its population may grow rapidly as it spreads out in a new habitat and previous growth may have no effect. Later, as crowding sets in, more growth leads to depletion of resources, squabbles and other events that slow the rate of growth. What has been missing from classical ecological equations is the phenomenon of per capita birth rates rising with increasing population, said Lehman. For example, meerkats may reproduce faster when numbers are high enough to allow bands of scouts to stand watch for each other. Similar effects have happened to humans.
"As human populations grew, jobs became specialized, and this led to more comfortable lives, the rise of medicine and other things that allowed birth rates to rise faster as populations got more crowded," Lehman said. "This went on for thousands of years, but it ended in the 1970s. Now we're in a period of negative feedback. In the developed world, it's largely due to voluntary controls on birth rates. It's also due to such factors as the spread of disease associated with travel, dense concentrations of people and other aspects of modern society. Now that our growth rate is slowing, we're finally following all the classical ecological population equations, which we had flouted for millennia.
"Even so, we still are facing a near doubling of the population, and this will greatly increase the strain on our planet's ecosystems."
Equations of population ecology that include positive feedback between population density and per capita growth rate fit well with actual human population data, said Lehman. The equations predict a levelling off of world population at 10 billion-12 billion around the year 2100. The fact that the current figure of around six billion is expected to no more than double is due to the dramatic slowing in the rate of increase that has been going on for at least 25 years already. This projection is also coming from demographers.
"I work with ecological equations, but three researchers working with human demographic data just published a similar finding in Nature," Lehman said. "It appears our work corroborates each other."