Locust (38 page)

As the surge of settlement extended westward from the Plains states, the land became increasingly dry and unwelcoming to prospective farmers. But the mountains promised riches for another kind of prospector. Mining communities sprang up along the spine of the Rockies, and with these ramshackle towns came the need for food. The miners, freighters, and merchants represented a lucrative market for farmers. The land could not produce as profusely as the tallgrass prairies to the east, but the mountain valleys had decent soils, water for irrigation, and—most important—a hungry population that had no time or inclination to grow its own crops. And at least on average,
there was far more money to be made in grain than in gold. Between 1880 and 1890 the number of farms in the eight Rocky Mountain states and territories doubled to almost 50,000. Even so, production of cereals, potatoes, and vegetables in the Boise Valley and similar communities could not keep pace with the demand, so they were forced to import food. Indeed, crop production in the Rocky Mountain states was less than half that of Kansas. From a national perspective, the farmers in the montane valleys were entirely negligible. Or so the country believed. In the end, what mattered to the breadbasket of the nation was not the pitiable amount of food grown in the Rockies. Rather, history was shaped by the ecological changes that took place as a consequence of these picturesque and profitable—but not terribly productive—farms.

 

When Larry DeBrey came into my office a few weeks after our fateful discussion regarding monarch butterflies and locusts, I could tell that he’d struck gold. He’d found a map showing the western lands under cultivation in 1880. The blotches representing croplands in the Rocky Mountain region were squarely within the Permanent Zone—the overlap was nearly perfect. The vast, windswept steppes of Idaho, Montana, and Wyoming were bare of wheat, corn, and hay fields. The pioneer farmers were courageous souls, but they were no fools. They avoided the alkali flats and sagebrush steppes, settling in the montane valleys. High above, the snowpack fed the perennial streams that cascaded down the flanks of the Rockies, joining together in the foothills to sculpt broad, sandy river courses that trapped fertile soil in their meandering bends.

But even with this coincidence of locusts and humans in time and space, the coarse scale of the map couldn’t reveal whether these ancient adversaries had actually collided. Was it possible that the locust population that gave rise to the swarms that blanketed two-thirds of a continent could be so concentrated as to succumb to the invasion of a few thousand pioneers? Surely even the most worn-out laggards of the plague from the 1870s would occupy far more land than a ragtag bunch of farmers could destroy. However, I’d learned that what seemed obvious was not always so.

It was time to let the numbers speak for themselves. How much land would the Rocky Mountain locust be likely to occupy between outbreaks? To begin, our best estimate of the peak population of the locust during a typical outbreak was somewhere around 10 trillion insects. The challenge, however, was to work backward from this population to estimate the number of individuals that would have been lying low between outbreaks. Based on the fecundity of the locust and natural mortality rates, it was safe to say that these creatures had the capacity to increase their numbers by 10- to 100-fold in each generation of an outbreak. So, let’s assume that the population had increased at these rates over the course of three generations, which was the typical period from the onset to the apex of an outbreak cycle. Given this range of reproductive rates, the result would have been 1,000 to 1 million times more locusts at the peaks than in the valleys—both mathematically and geographically. Working backward, then, we can estimate that a plague of 10 trillion locusts originated from a population of 10 million to 10 billion individuals.

From what we know of modern-day locusts, grasshoppers are a sensible surrogate for the solitary phase of
spretus
. And so, if we assume that the Rocky Mountain locust in its recession areas lived in a manner similar to that of existing rangeland grasshoppers, we can generate a reasonable approximation of the land area required to support a few million or billion creatures. On productive pastures and grasslands similar to those occupied by
spretus
in the Permanent Zone, grasshoppers typically occur at a population density of about one individual per square yard. Much higher densities are found during outbreaks, but we’re interested in the mundane times between these biological spectacles. Thus, a population of 10 million locusts would have required an area of about 2,000 acres, and a population of 10 billion locusts would have needed a couple million acres. With this line of reasoning, we can estimate that the actual habitat of the Rocky Mountain locust during its recession periods was as little as 3, and as much as 3,000, square miles. In other words, all of the individuals would have comfortably fit into a circle of land between 2 and 60 miles in diameter. Of course, this is quite a range, and we know that the montane river valleys were not laid out in neat circles, nor
were the locusts uniformly distributed. However, no matter how we parcel out the locusts and their habitats, the implications are the same—this species was squeezed into a tight ecological bottleneck.

The startling conclusion that emerges from these mathematical extrapolations is that the Rocky Mountain locust was extremely vulnerable to even small-scale human disturbances. But this inference suggests that the locust was far more concentrated than would be apparent from maps of the Permanent Zone, shown to cover parts of five states and provinces. Further ecological evidence gleaned from the historical record supports the notion that the insect was restricted to very particular conditions within the region—habitats that corresponded with the arable lands.

 

I was not the first to recognize that the immense swaths of land representing the Permanent Zone on Riley’s maps were an extremely crude depiction of the Rocky Mountain locust’s habitat. In 1959, when Ashley Gurney was struggling to explain the locust’s disappearance, he noted, “Within the [Permanent] Region,
spretus
did not breed everywhere, but instead it did so in favorable places. River bottoms, sunny slopes of uplands, and subalpine grassy areas among the mountains were considered to be favored egg-laying sites.” But Gurney was seeking an ecological change on a continental scale, so this clue was set aside without his recognizing that it was crucial to solving the mystery. Norman Criddle also harbored suspicions about the actual hideouts of the locust within its montane distribution. Although in 1917 he was still of the opinion that the creature was lurking about, it was clear that he didn’t believe that the locust would be found just anywhere in the Rockies:

Riley drew the maps of the Permanent Zone with a very broad brush, and it was not until delving into the particulars of the insect’s biology that I found the keen observations for which he was so highly acclaimed. He narrowed the ecology of the locust somewhat in describing it as “a denizen of high altitudes, breeding in the valleys, parks and plateaus of the Rocky Mountain region of Colorado, and especially Montana, Wyoming and British America.” But this panoramic allusion was too vague for a natural historian with Riley’s acute sense of ecological detail. And the more specific his description, the more intriguing this line of evidence became. His attention focused explicitly on a crucial landscape:

Riley was not the only scientist to note that the locusts were highly restricted to particular habitats. In August 1882, another renowned entomologist was leading a scientific expedition into the Permanent Zone. Lawrence Bruner, the namesake for the Rocky Mountain locust’s nearest relative,
Melanoplus bruneri,
was working his way across Montana in an effort to track down the locust. He’d already taken the time and effort to name an imposing edifice near Fort Buford “Riley Peak” in honor of his entomological colleague. As Bruner came across a remnant population of
spretus,
he noted in his journal, “We crossed a vast expanse of nearly level prairie, well grassed, with a loose, sandy, clay soil . . . admirably fitted for the rearing of locust swarms. In fact the entire country . . . is one vast hotbed, calculated to produce the largest and healthiest swarms in America [forming] cradles of the ravaging pest.”

In contrast to the effusive claims that would typify economic entomology in the later era of DDT, Bruner offered a most humble interpretation of the power of human ingenuity:

The possibility of killing the ravaging pest while it lay in its cradle—the entomological version of the mythical infanticidal Medea—never occurred to Bruner or Riley.

What did occur to Riley was that reproduction was the key to understanding the pockets of habitat that supported
spretus
in the Permanent Zone. He knew from his own observations and the reports of his contemporaries that female locusts were very particular about where they laid their eggs. In various documents he noted that oviposition was restricted to sandy and gravelly habitats, where the soils were firm but not hard-packed. The females avoided very loose soil and moist ground. Like insectan Goldilocks in search of “just right” conditions, the locusts buried their eggs in the well-drained soils of the montane valleys. The egg beds lay between the overly wet, silty streamsides and the sere, rocky hillsides. During their plundering of the prairie farms, the locusts laid their eggs in soils that mimicked the fertile river valleys of the Rockies.

The well-drained, sandy cradles in which the six-legged mothers deposited their eggs were so essential to the life of the locust that Riley argued for this being the limiting factor in the species’ ability to persist in its eastward migrations. It was clear that the Rocky Mountain locust could not indefinitely perpetuate itself in the habitats of the Midwest, but nobody knew why. After “fully digesting all the facts,” Riley concluded that the most plausible constraints were the warm, wet weather, which induced a combination of “debility, disease, and deterioration”—and the locusts’ evident “need for their native sandy
and gravelly soil.”
Spretus
was reluctant to lay its eggs in rank vegetation, hard-packed clay soils, and rich loamy fields because these nurseries produced fewer, weaker nymphs.
⁷

The Rocky Mountain locust’s initial decline had been part of the natural ebb and flow of the species. Outbreaks typically lasted for three to five years, and so the 1874-1877 plague had essentially run its course and a period of remission was to be expected. The collapse of the locust back into its Permanent Zone in the river valleys of the Rockies was ecologically unremarkable. Rather than a tidal wave of vitality washing over the continent, it had been reduced to a trickle of life restricted to its favored habitats. There were no indications that this recession was a prelude to extinction because we had not paid attention to the ecological bottleneck.

In the Rocky Mountain region, farmers and locusts crowded into the arable and irrigable lands of the broad, fertile river valleys. The opportunity for conflict over this rich habitat was undeniable. However, just because the accused and the victim were in the same place at the time of the murder does not mean that a guilty verdict is certain. A prosecutor also must demonstrate that the perpetrator had the means to dispatch the victim. Did the pioneers have the capacity to drive the locust to extinction, and what ecological weapons could have been wielded by these simple folk to slay the mightiest species on the continent?

Lawrence H. Keeley is a professor of anthropology at the University of Illinois, and his work focuses on the most human and least humane of all behaviors: warfare. His rigorous analyses of the nature and costs of waging war have yielded some surprising results. Although we might well imagine that bombers, machine guns, and other modern weapons have dramatically increased the death rate on the battlefield, Keeley paints a radically different picture in
War before Civilization: The Myth of the Peaceful Savage
. Taking part in primitive warfare is often far deadlier on a proportional basis than participation in contemporary conflict.

Perhaps the epitome of the modern battlefield was the grand campaign of the Normandy landing in World War II. Between D-Day and the liberation of Paris, a period of seventy-nine days in 1944, about 290,000 soldiers died. Guns, bayonets, grenades, artillery, tanks, bombs, mines, and the other instruments of modern warfare killed about 3,700 men during each day of this campaign. Compare these numbers to the massacres of Tutsis and Hutus in 1994. The killings began with the assassination of the presidents of Rwanda and Burundi on April 6 and continued for ninety days, ending with the capture of Kigali. Although the death toll is not fully known, between 500,000 and 1 million people died. No fewer than 5,000 lives were taken during each day of this conflict. There were no bombers, no cannons, no tanks. Yet 1 out of every 14 people in these two African nations was killed in a matter of three months. The weapons were machetes and sharp sticks, with a smattering of small arms. The capacity of humans to destroy each other—and other species—is not contingent upon advanced technologies.