Feral swine are ravaging the American landscape. Did rabbits nearly doom the
Australian ecosystem? Are those cute stray cats in your neighborhood a biohazard in waiting?
All this and more as we run down the worst invasive species disasters in history.
The black rat's six distinct lineages originate from different parts of Asia,
but this particular species has hardly stayed close to its ancestral home.
Scientists estimate that black rat migration began around 20,000 years ago. The rodents reached the
Middle East first, then quickly spread into the regions of modern-day Europe. They eventually made
their way to Australia and the Americas thousands of years later by hitching rides on sailing ships
during the Age of Exploration. From there, they spread out all across these new habitats as well.
Today, black rats can be found all over every continent except Antarctica.
They can thrive in crowded cities, suburbs, farmland, and forests.
This adaptability is largely due to the fact that they eat pretty much anything.
These troublesome rodents can cause major problems because they tend to spread disease,
damage crops and infrastructure, and out-compete local indigenous species.
Today, black rats are so common in so many places that their presence is expected, and their damage
was largely done in centuries past. In some remote places, though, they remain a serious problem.
"I got this rat."
Fleas spread the bacteria responsible for the Bubonic Plague, which has caused death and
suffering many times over the course of history. It was never worse than between the years 1346 and
1353 — a time when the disease killed millions worldwide. Modern experts estimate that as much
as a third of Europe's population was killed by the plague during that period. But while
experts have long understood that the disease was transmitted via fleas, in recent years,
just what, or who, was responsible for spreading those fleas so far and wide is being reevaluated.
While the fleas that infest rats will bite human hosts if no rodents are around,
scientists have determined that rat fleas won't target humans nearly aggressively enough
to account for widespread plague outbreaks. That dubious distinction goes to the type of flea that
does target and infest humans — a pest with the common name of "human flea," appropriately enough.
It's almost certain that in the cramped and unhygienic conditions of Medieval European towns,
it was largely person-to-person contact that spread the Black Death, not rat-to-person contact.
Australia measures about 3 million square miles in area, which is about 11 times the size of Texas.
That should give you some perspective on just how rapidly wild rabbits propagated and spread
all across Australia after their ill-conceived introduction. Humans brought rabbits to Australia
and set them loose, thereby precipitating an invasive species disaster. It was 1859 when
these European wild rabbits were introduced to The Land Down Under for the purpose of sport hunting.
"Be vewy quiet! I'm hunting wabbits!"
Within the next half-century, they had spread across the entirety of the continent.
According to the National Museum of Australia, this example of proliferation was the fastest
of any introduced mammal worldwide. Today, there are an estimated 150 million wild
rabbits roaming free across the continent. That's about five feral rabbits per Aussie human being.
From that first release of wild rabbits, which involved just 13 animals and took place on
Christmas Day 1859, the effects they've had on Australia have been almost entirely ruinous.
This has included the collapse of indigenous plants, the native animals that eat those plants,
and soil erosion caused by excessive grazing. The one boon is that rabbits are a source of cheap
meat for many Aussies and were invaluable during the depression years of the 1930s.
Today, there are an estimated 200 million European starlings in North America,
and as their name suggests, they are hardly native to the Americas. They gather in flocks
that often number well into the thousands and can be found from Alaska to the south of Mexico. So,
what exactly are these birds doing here? In 1890, they were introduced to America by a
man named Eugene Schieffelin. In early March of that year, he released five dozen starlings in
New York City's Central Park as part of ongoing efforts of the American Acclimatization Society
to introduce every bird mentioned in the works of Shakespeare to the United States.
So basically, these birds were brought to America as a lark,
so to speak. But the results weren't very fun. Highly successful in their new environs,
the starlings soon multiplied and migrated, causing lots and lots of trouble. They bully other
birds by way of kicking the likes of bluebirds, flickers, and woodpeckers out of their own nests.
They can also consume entire fields of wheat. And on top of that, they transmit diseases to
all kinds of animals, including other birds and humans. Also, don’t forget there's the matter
of the mess that can be left behind when flocks start to number into the thousands.
"You have a little, on your shoulder."
America was dealt a series of blows during the Great Depression years, and no area was hit
harder than the Midwest. Already reeling from the fallout of the financial market collapse of 1929,
drought conditions in the 1930s ravaged crops across much of the so-called American Heartland.
Then in the summer of 1931, an unprecedented swarm of grasshoppers descended on farmland
from the Dakotas to Iowa, Nebraska, and beyond. The swarm in July of that year was reportedly so
thick that it blocked out the sun. It was even possible to shovel the insects with a scoop.
As a result of this wave of grasshoppers,
corn stalks were eaten to the ground, and fields were left completely bare.
The insects wreaking the havoc were technically grasshoppers,
but they had undergone a transformation into locusts. As explained by NPR,
"Most of the time, locusts exist in their 'grasshopper phase' — they lead solitary lives,
they're green and pretty unremarkable [...] But when environmental conditions are
right — usually when there's a lot of rainfall and moisture — something dramatic happens."
This dramatic "something" is a total shift of grasshopper behavior. They change from solitary
to highly social, actively seeking close proximity with other locusts
in a so-called "gregarious phase." They also physically change, growing larger, more active,
and hungrier. The locust swarms of the ‘30s were absolutely devastating. Fortunately, no such
swarm has formed in America since, though they plague many other parts of the world to this day.
Burmese pythons routinely grow to a dozen feet or so in length, with some reported at nearly
20 feet. A snake of that size can easily kill and consume other animals like deer,
panthers, immature alligators, and all sorts of species native to Southern Florida. These
fearsome snakes aren't native to that region, but they have now spread far and wide there.
The Burmese python arrived in Florida as a result of both captive snakes
escaping as well as intentional releases. According to estimates, at the very least,
they currently number in the tens of thousands. At this point, these powerful constrictors now have
a well-established breeding population in the state. According to the U.S. Geological Survey,
"The odds of eradicating an introduced population of reptiles once it has spread
across a large area are very low [...] And with the Burmese python
now distributed across more than a thousand square miles of southern
Florida [...] the chances of eliminating the snake completely from the region is low."
Hopefully, at the very least, these invasive snakes won't expand their
habitat farther up into the American mainland.
The mosquito has been called the most dangerous animal on the planet.
It's responsible for the deaths of roughly three-quarters of a million people every
year due to its uncanny ability to spread diseases like malaria and dengue fever.
Thus it was simply terrible news when the Asian Tiger Mosquito came to the U.S. in 1985.
According to the Illinois Department of Public Health, these mosquitos arrived by hitching a
ride on imported tires, with little pockets of water in the tires harboring their eggs.
In the years since, these mosquitoes have spread out across dozens of states. They're
even more dangerous than non-invasive mosquito species due to their 24-hour-a-day activity.
While many types of mosquitos will hit their peak at dusk, Asian Tiger Mosquitos feed all day long
and are also active at night. They can survive in a significant range of climates, which surely
explains how they managed to spread rapidly since they were first detected in the United States.
Snakehead fish are objectively rather terrifying. Not only do these long, lean creatures have sharp,
fang-like teeth in their jaws, but they can also breathe air with their gills, survive outside
of water for up to four days, and travel up to a quarter-mile over land by wriggling and thrashing
about. If snakeheads only existed in their native Asian and African environs, they would hardly be
worth mentioning beyond their value as a food source, regardless of how fearsome they are.
But currently, these fish are wreaking havoc on parts of the eastern American seaboard.
They have invaded the fragile waters of the Chesapeake Bay and are now spreading up and
down the coast. They've already been spotted in many freshwater areas from Virginia to Delaware.
The U.S. Geological Survey goes into detail by noting on its website:
"During all of their life stages, snakehead fish compete with native species for food and
habitat. A major concern is that snakeheads might out-compete (and eventually displace) important
native or other established predatory fish that share the same habitat. As adults, snakeheads
can be voracious predators. Should snakeheads become established in North American ecosystems,
their predatory behavior could also drastically disrupt food webs and ecological conditions,
thus forever changing native aquatic systems by modifying the array of native species."
One stray cat might look charming and harmless from a distance, but feral cats are an absolute
disaster of an invasive species when considered as a whole. According to a 2018 article in The
Times-Picayune, experts estimate that there are between 60 and 160 million feral cats in the U.S.
alone. Stray wild cats are known to kill up to 3.7 billion birds and some 6.9 million mammals
annually. They're even considered responsible for the extinction of more than 60 animal species.
Those statistics alone lay bare why wild cats are such an issue in America,
but they don't even tell the whole story. Many of the billions of animals that feral cats kill
each year are slain merely for their enjoyment. Instead of eating their prey, they leave the
dead bodies of those hapless animals behind. This can in turn serve as a food source for
other pests like rats or lead to the spread of bacteria as the discarded carcass breaks down.
And what's worse still is that many of the animals that feral cats kill would themselves
have consumed a still deadlier creature: the mosquito. Every bird or bat that a cat
kills may leave dozens or even hundreds of mosquitos alive and waiting to bite a human.
There have been wild pigs in America for almost as long as there have been European settlers.
According to the Animal and Plant Health Inspection Service of the U.S. Department of
Agriculture, free-range livestock management practices, as well as enclosure escapes,
are what led to the first establishment of feral swine populations in the country. This dates back
to the 16th century, but as recently as the 1990s, the Eurasian or Russian wild boar was introduced
into parts of the country to be used for sport hunting. Nowadays, feral swine consist of a
combination of escaped domestic pigs and Eurasian wild boars, as well as hybrids of the two.
A major problem is that these wild pigs are now experiencing explosive population growth.
They have spread out across some three dozen states, with their numbers calculated at 6 million
and growing. According to the National Invasive Species Information Center, wild hogs damage
indigenous plants and crops and compete with native species. They're also known to be able to
spread certain diseases, which can create a risk for native wild species, livestock, and humans.
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