Here’s a story of population growth that puts the biological aspect of human population growth into perspective:
In 1993, southern Australia was splashed with a heavy dose of rainfall that was a welcome relief to farmers. Crops thrived and livestock put on pounds of meat that were desperately needed after years of drought. Soon after, however, a plague of house mice infested the countryside. Videos of the plague show mice literally pouring out of barn doors, escaping the swift kicks of angry Australians as a swarm of gnats might avoid a swipe of a hand. The population growth of the mice was so rapid, and the grain supply relatively low, that mice began to feed on livestock. Photos show sore spots on the backs of pigs, where mice began to gnaw on living pork. With every passing day, the mice ate approximately 20% of their body weight of wheat, vegetables, and raw meat, for a total of a quarter pound of food per month on average.
After six months, the government finally stepped in with the largest pest control project to date. Risking the possibility or ecological catastrophe, rat poison was released upon the countryside from the back of fertilizer trucks and propeller planes. Within days, an estimated 100,000,000 mice had been eliminated, and total of 500,000 tons of grain had been destroyed by the mice: a tonnage great enough to feed the entire state of Utah for four years. Not including damages to private residences, the toll of the plague was estimated to be $70,000,000 in lost revenue from the crops. In the years since, scientists have been studying the population fluctuations of the Australian house mice, hoping to be able to anticipate the next infestation.
Their research has produced surprising hypotheses. Prior to the studies, it was assumed that there was a direct “if A, therefore B” relationship between the food supply and the mouse population. An increased food supply would allow mice to consume more, rear more offspring, and enjoy higher reproductive success. In essence, the restraining bar on the population would be lifted by the surplus of food, allowing the mice population to flood the Australian countryside. However, this is not what research has found.
After controlling for immigration from external populations, focal study populations of house mice were exposed to volumes of food and water that were greater than what would normally be found in the Australian outback. Scientists found that there was no statistically significant immediate response to the increased supplies. The mice enjoyed the food, and had slightly larger broods, but the brood sizes hadn’t grown enough to explain the huge explosion of rodents in 1993 (Ylönen, 2003). What was the explanation for this?
The scientists found that there appears to be a “biological memory” within the mice that can remember previous plagues for up to two years (Brown, 1999). Although the specific biochemical mechanisms are unknown, scientists hypothesize that mice inhibit spontaneous population growth by having an internal catalyst rather than an external one. If mouse populations were sparked into immediate growth by external events such as torrential rains or copious amounts of grain, the population booms would put food supplies at risk of a rapid extinction. Mice would literally starve future generations with their own short-term gluttony. An internal biochemical clock, however, allows mice populations to wait until food supplies return to sustainable levels. The plague erupts, reaches a crescendo, and the population shrinks, resetting the clock. After a few years, the ecosystem has returned to sustainable levels, food supply is high, and the mouse population can boom again.
What does this have to do with human population growth? Well, let's hope I get around to that...
