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Go to Brilliant.org/scishow to check out their course on Solar Energy. [♪ INTRO]. 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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