The Critters Are Coming!

What happens when the delicate balance of nature tips in such a way that a particular animal population spikes to unsustainable levels? Pretty much what you’d expect: chaos… famine… and critters out the wazoo.

THE 48-YEAR CURSE

The wild bamboo forests in northwest India and parts of Burma are home to an odd curse: Every 48 years, like clockwork, they produce an army of hungry rats that devour the local rice crop. The phenomenon is called mautam (which translates to “bamboo death”) and is caused by the life cycle of melocanna bamboo, the local variety. The plants live for exactly 48 years, at which point entire forests die off simultaneously. But before they die, they produce a tremendous amount of seed-filled fruit. The fruit will replant the next generation of bamboo, but in the meantime, it also provides a huge increase in the amount of food available to the local black rat community.

The sudden food surplus sets off a population boom. For as long as the good times last, the rats breed continuously. It takes only about 11 weeks for the baby rats to reach maturity. That means, during the year that the forest fruits, the rat population jumps exponentially every couple of months— from as few as 100 rats per acre to as many as 12,000 per acre. And at just about the time that the rat population is hitting its peak, the bamboo fruit runs out.  
When that happens, millions of starving rats swarm the countryside, eating everything in their path… which spells disaster for the local rice farmers. In the past, without advance planning and no ability to bring in extra food from outside the region, the rat plague could lead to famine and political upheaval. As for the rats, once they’ve decimated the rice crop, they starve to death en masse. Their population numbers crash back down, but everyone knows they’ll be back… in 48 years.

THE HOUSE MOUSE INVASION

For more than a century, Australians have been at war against what they call the “mouse plague.” Once every four years on average, somewhere in Australia, vast stretches of farmland are devastated by millions of hungry mice. Why are Australian mice so hard to control? Probably because they aren’t natives— they’re an invasive species.
The mice that cause such destruction Down Under actually belong to one of the most common mouse species in the world: the house mouse. Native to Asia, these mice abandoned foraging in the wild in favor of scavenging in human settlements nearly 10,000 years ago. And as human agriculture and civilization spread across the globe, the house mice spread too. They most likely arrived in Australia as stowaways aboard the first ships that brought settlers there in the 1780s.
House mice are among the fastest breeders in the world. A female’s pregnancy lasts just 19 days and produces five to ten baby mice. Those baby mice start having their own babies when they’re only six weeks old. Oh, yeah— and females can get pregnant again just one to three days after giving birth. This means that one female mouse can produce 500 new mice in less than six months.

THE ORIGINAL AND STILL THE WORST

Desert locusts are the granddaddy of all animal plagues. Ancient Egyptians wrote about them 3,500 years ago, and they’ve been menacing much of North Africa and the Middle East ever since. For thousands of years, no one had any idea where they came from. Most years, there were no locusts at all. In a bad year, though, they showed up by the billions, in huge clouds dense enough to blot out the sun. The clouds swept across the countryside, eating every bit of vegetation in their path and leaving farm fields stripped bare. It wasn’t until the 1920s that scientists uncovered the secret of the locusts’ mysterious appearances.
It turns out that locusts are just regular grasshoppers driven crazy by overcrowding. The desert is a harsh environment, and there’s usually not enough food to support a large grasshopper population. To ensure survival of the species, female grasshoppers lay as many as 150 eggs just under the surface of loose, sandy ground. Ordinarily, not all of the eggs hatch— and not all of the ones that hatch survive. But when a particularly wet winter comes along, two things happen: First, more of the eggs hatch. And second, the extra moisture means that extra vegetation grows, providing enough food to support the extra population… at first.
Scientists aren’t exactly sure why, but overcrowded conditions cause grasshoppers to change both their appearance and their behavior. Their color morphs from green to a yellow-and-black pattern. More importantly, their personalities change— from solitary individuals to being clustered together in an organized mob that moves across the landscape as one giant, food-frenzied unit. Weird but true.

Bonus: The swarm, American-style. For the first three or four decades of settlement on the Great Plains, American farmers regularly had their crops wiped out by the Rocky Mountain locust. In 1874, a swarm of locusts estimated at a size of 198,000 square miles— about twice the size of Colorado— swept through Nebraska. By the early 1900s, the Rocky Mountain locust had disappeared from the landscape, apparently gone extinct. The only explanation scientists have come up with for why this happened is that the settlers may have plowed up the locusts’ breeding grounds without even realizing it.