Earth's ecosystems have always been changing, with species mixing and mingling thanks to
migrations, freak events, and slow creeping expansions. But since we humans arrived on
the scene, we've put things into fast-forward, moving plants, animals, and other organisms
around the globe -- whether intentionally or not.
Once in a while, one of these transplants might have an evolutionary quirk -- like deadly
poison, a lack of predators, or a voracious appetite -- that unexpectedly helps it dominate
its new territory. Earthworms, for example, were introduced into North America by European settlers
and have been inching across the continent ever since - tilling soil and improving crop
yields as they go.
But if a newcomer starts to harm us or a part of the environment we care about, we'll call
it an "invasive species." Technically-speaking, humans aren't "invasive," because we don't
usually consider ourselves an ecological threat to... ourselves.
But other species certainly are. One way they get our attention is by costing us money.
A lot of money. The US spends billions of dollars fighting invasive weeds like the pretty
but pesky yellow rocket flower that chokes fields and creeps onto golf courses and lawns.
Other invaders wreak so much havoc on fragile ecosystems that we can't help but notice even
if billions aren't at stake. For example, the yellow crazy ant - a likely native of
Southeast Asia - has been eating many beloved and endangered Australian creatures since
its arrival several decades ago. These little ants feast on nearly anything and everything
in their path - insects, amphibians, birds, small mammals, even the famous red crabs on
Australia's Christmas Island, which used to keep the island's undergrowth in check; without
them, the ecosystem has veered into chaos.
But noticing invasive species is easy - dealing with them is the real challenge.
When we brought rabbits to New Zealand for food and fur, they escaped and bred like...well...rabbits,
overrunning the country. So we introduced ferrets to control the rabbits; but they too
spread like wildfire, generally ignoring the rabbits while gobbling up rare species like
the now nearly-extinct kakapo. New Zealand is still overrun by both furry fiends, and
this is just one of many well-meaning attempts at biological control that have ended in disaster.
The best way to fight an invasion is to first arm ourselves with knowledge: Instead of trying
to kill Australia's yellow crazy ants by just spraying insecticide everywhere, we've learned
that the best approach is a year-long multi-step assault that specifically targets ant-riddled
areas at the times when the egg-laying queens are mature and susceptible to our treatments.
Then we spread two kinds of bait for the workers to bring back to the nest - one kills, and
the other sterilizes the queens. Neither poison is perfect, but executed together, they can
knock out a whopping 99 percent of the population.
But with many invasives, even 99% isn't enough, because it doesn't remove whatever advantage
gave the invaders an edge in the first place. So unless we honest-to-goodness get rid of
every reproductively active individual, eventually they'll return as vigorously as before, keeping
us constantly on our toes.
We really just need to stop moving potential troublemakers around in the first place, leaving
bunnies where they belong and making sure ants and earthworms don't stow away in ships,
cars, or excavators. But in a modern, globalized world, this is easier said than done. For
those species that we do accidentally (or intentionally) let loose only to see them
spread wildly out of control, our best hope is to learn as much as we can about their
habits and their biology - only then do we have a chance of keeping their harmful "ant"-ics
at bay.