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.
