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Locusts are an agricultural menace.
Their swarms pose a huge risk to crops on
almost every continent.
In fact, beginning in October 2019, massive
swarms have been damaging crops in
East Africa, the Middle East, and Southeast
Asia.
But notice we said almost every continent.
Antarctica is safe — that’s no surprise —
but so is North America.
But that hasn’t always been the case.
Over a century ago,
North America’s big locust swarms vanished
— and we have a few ideas about why.
First, what is a locust?
Scientifically, it’s... a grasshopper.
Most of the time.
See, the species we know as locusts usually
live as normal-looking grasshoppers,
carrying out solitary lives in the brush.
However, some species, like the desert locust,
are really sensitive about how many other
individuals are around them.
When their population swells to a certain
size, either the sight and smell of other
locusts, or the feeling of other grasshoppers
jostling against them,
triggers a change in their appearance and
behavior — sometimes within hours.
Instead of avoiding each other, the locusts
start to band together
and move as a group, flying long distances
and eating everything in sight.
And the North American species known as the
Rocky Mountain locust
could do this, too.
It’s said that a swarm could be 1.5 kilometers
high
and cover an area of 330,000 square kilometers
— roughly the size of the state of Colorado —
and could travel more than 200 kilometers
a day.
Until, all of a sudden, the locusts disappeared.
In the 1880s, the locust population crashed.
That sort of thing happens — except the
locusts failed to bounce back.
The last living specimen was collected in
Manitoba in 1902.
And for more than a century, we’ve been
trying to figure out how a species
could go from swarms the size of an entire
state...to gone without a trace.
A couple of explanations have been proposed.
Like, maybe what we’d thought of as one
species, the Rocky Mountain locust,
was really just the migratory phase of some
other species of grasshopper
that’s still around.
However, genetic testing of preserved specimens
and other analyses
have indicated that they probably were a separate
species.
Others have suggested that it might have been
related to other large-scale ecological
changes, like the near-total loss of bison.
But one idea that’s gotten a lot of attention
is a hypothesis proposed in 1990.
The idea is that despite their massive numbers,
the locusts had a critical weak point in their
life cycle.
Fieldwork done by entomologists all the way
back in the 1800s
suggests the Rocky Mountain locust had a kind
of home base.
They could survive in places outside these
zones, but they wouldn’t stay.
Even though the locusts could range across
vast distances,
they’d always return to their so-called
permanent zone along the mountains,
chiefly because that’s where they would
breed.
In particular, this region contained a number
of scattered valleys,
separated by inhospitable mountains, deserts,
or forest,
where the conditions were just right for females
to lay their eggs.
And they were very picky about exactly where:
soft soil near rivers or in floodplains.
This might have been for a couple of reasons.
Developing grasshopper eggs
need a certain amount of moisture to grow,
for instance.
Also, the area around those riverbanks was
more reliably lush and full of vegetation
compared to the more arid surroundings, meaning
plenty of food.
This pickiness isn’t unusual among locusts
today.
And it had been working for Rocky Mountain
locusts for countless generations —
until Western colonizers moved into these
valleys.
They brought with them a couple of key environmental
changes
that may have spelled doom for the locusts.
Farmers’ plows and the hooves of ranchers’
cattle
could have destroyed eggs or exposed them
to the sun and wind.
Logging, mining, and the extermination of
local beaver populations
may have increased flooding as well.
It’s unlikely that every single nest was
destroyed,
but it may have been enough to disrupt what’s
known as the metapopulation.
A metapopulation refers to a larger collection
of populations that interact.
They’re all part of the same species, but
physically separated from each other.
If each little population of locusts is like
a city or a town,
then the metapopulation would be the locust
nation in this analogy.
During the good times, if one local group
died off due to an accident or disease,
that was okay.
New locusts from other, nearby populations
would quickly move in
and repopulate the area in something called
the metapopulation rescue effect.
But this only works as long as the network
of local populations are well-connected.
The more fragmented the network becomes, the
more perilous the situation,
especially for small, picky, boom-and-bust
style organisms like locusts.
Ultimately, the farmers and ranchers probably
didn’t kill off every last locust.
But the changes they brought made sure they
couldn’t recover, either.
And then it was just a matter of time.
Within a couple of decades,
the species was gone — at least, we’re
pretty sure they are.
In the end, it’s not clear what we lost
as a result of the locusts’ extinction.
It’s been suggested that they may have had
some important ecological role,
like helping move nutrients through the environment.
There are plenty of other grasshoppers in
North America,
including some species that can swarm and
migrate, or that threaten crops,
though not nearly to the same extent.
But when we talk about the species disrupted by
the colonization of the North American West,
alongside the bison and passenger pigeon,
we might want to include the Rocky Mountain
locust.
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