Locust (28 page)

 

A century after the massive slaughter of bison on the Great Plains had begun, the fate of these animals was circumstantially linked to that of the Rocky Mountain locust. The cause-and-effect relationship was
rather counterintuitive, but the modus operandi was not entirely absurd. In 1954, Irving Cantrall and Frank Young proposed that the extirpation of bison had been the cause of the locust’s outbreaks. They reasoned that when the bison were removed from the land a change took place in the vegetation that was favorable for the Rocky Mountain locust. This alteration of the prairie accounted for the locust’s massive outbreaks in the 1860s and ’70s. Thus, Cantrall and Young maintained, the population explosion of the locusts was the consequence of a profoundly disturbed and out-of-kilter ecosystem. In rather short order, ecological succession restored the balance of nature, and the habitat recovered. They argued that with this reordering of nature, the conditions again became unfavorable for
spretus
and it disappeared.

The core of Cantrall and Young’s notion involved the overlap of bison and locusts in three ways: space (both species existed on immense scales extending from the Rocky Mountains to the Mississippi River), time (the extermination of bison preceded the irruption of locusts), and ecology (locust plagues being evidence of a perturbed and dysfunctional ecosystem arising in the absence of bison). In terms of spatial overlap, their argument was compelling. Brett’s alfalfa hypothesis might have been dismissed for being absurdly patchy and localized, but eliminating tens of millions of bison surely had sweeping effects across the grassland ecosystem where the locusts had flourished. However, the temporal sequence of events was problematic. In criticizing the “bison hypothesis,” Ashley Gurney pointed out that outbreaks of
spretus
had preceded the great bison hunts. Records of phenomenal swarms reached back into the 1830s, when bison were still abundant across the Great Plains. Thus, he argued, the outbreaks of the Rocky Mountain locust could not have been stimulated by changes in the landscape resulting from the elimination of bison.

The ecological foundation of the “bison hypothesis” also relied on the notion that insect outbreaks were the consequence of a disturbed system. Indeed, this idea continues to pervade our understanding of ecology. We seem to believe that, as with a well-behaved child or a good worker, species should refrain from extreme outbursts. The concept of a Golden Mean or “everything in moderation” is rooted in the ancient Greeks, the progenitors of Western culture. This idealization
of the world has been variously manifested throughout history, and it formed a pillar of Charles Darwin’s evolutionary theory. Darwinian gradualism as an explanation for continuous modification of species was derived from geological uniformitarianism—the notion that the physical forces in the world were constant. This position emerged both from empirical evidence and as a reaction to the Church’s reliance on catastrophes to explain the history of the earth. The Victorian interpretation of moderation in the natural world reflected the ideal emotional state—a balance of tranquil stability. The same concept was embodied in the ecological principle of succession, in which disturbed ecosystems return in an orderly manner to their original condition. And the ideal of equilibrium has lived on in our perception that an outbreak or crash of a population is an unnatural aberration, an indication of a troubled species.

But the leitmotif of the Rocky Mountain locust was its phenomenal flights of reproductive fancy. Manic swarms swept over the plains only to subsequently collapse into pockets of exhausted survivors. Evidence that this pattern was natural and normal came from Indian accounts, which revealed that locust outbreaks had happened well before European alterations of the western landscape. The Indians had witnessed what scientists were at pains to deny.

In recent years, the emerging field of complexity is finding that sudden catastrophic changes may be inherent in some systems, including populations. My own work in the field of catastrophe theory suggests that modern grasshopper outbreaks may be precisely such systems. Their erratic dynamics are entirely normal, although we can exacerbate the outbreaks by mismanagement of the rangeland. We’ve even found evidence that grasshopper populations exhibit a phenomenon called
self-organized criticality,
in which they naturally develop to the point where outbreaks and crashes are triggered by their own biology. And so, impetuous, even explosive, population dynamics do not require anthropogenic disturbance, nor do they reflect dysfunctionality.

Of course, people, species, and ecosystems can manifest extreme dynamics during times of trouble. But we are alarmed by nonconformity because of our self-interest in having a predictable world, our social intolerance of radicalism, our economic objective of slow but
steady growth, and our Protestant ideal of moderation. Sometimes the outburst of joy from a child, the cry of anguish from a neighbor, or the outpouring of life by a species does not need to be “fixed,” controlled, or managed but understood, accepted, and honored. And so, Cantrall and Young’s attempt to explain locust outbreaks in terms of a degraded ecosystem was ultimately founded on a false premise deeply ingrained in our society—the presumption that sudden change is unnatural.

The next theory explaining the locust’s disappearance would avoid this pitfall by embracing the notion of natural, even rather sudden, change. In an attempt to link the most widely known dramatic and large-scale alterations of nature with the decline of the locust, a new suspect was found.

Amid the current debate regarding the rate and causes of global warming is a central point of consensus: Dramatic changes in the climate can occur naturally. Although there is no doubt that human activities can alter the weather on various scales, we also recognize that droughts, hurricanes, and ice ages unfold without our prompting. The Little Ice Age between 1450 and 1890 saw temperatures one to two degrees colder than those of modern times. Such a change might not seem very dramatic, but consider the alarm associated with a similar rise in temperature being forecast in the course of the next century.

The Little Ice Age was felt around the world, although more keenly in the Northern Hemisphere. In Europe, glaciers began to advance menacingly from the Alps, slowly swallowing entire villages and farms. The people of Chamonix, at the base of Mount Blanc, were terrified that their town would be engulfed, and so they appealed to the Church. The bishop of Geneva—perhaps aware that locusts had been repulsed by holy edicts—performed a rite of exorcism at the toe of the glacier. And the ice retreated, but only for a few years. Fishermen in Iceland battled against sea ice that choked the North Atlantic, and the famished island dwindled to half its previous population. By 1370 not a single human survived on Greenland, which was cut off
from seagoing travel for more than a century. In southern China, citrus groves that had flourished for hundreds of years were decimated by cold. In the United States, New Yorkers walked across the frozen harbor between Manhattan and Staten Island during the winter. The Little Ice Age nearly turned the course of American history, as the horrific winter at Valley Forge almost fatally crippled George Washington’s forces. The coolest periods of the Little Ice Age were during the seventeenth and nineteenth centuries—1816 was called “the year without a summer.”

Modern-day entomologists are predicting that the ongoing warming trend will cause both the expansion of subtropical disease vectors into our otherwise healthy temperate zones and the loss of alpine species that can’t migrate northward for lack of movement corridors. The possibility—to some, a near certainty—that climate change will cause the loss of species suggests that such a lethal process might account for past extinctions. Could climate changes at the end of the Little Ice Age explain the disappearance of the Rocky Mountain locust?

 

In the 1950s, Ashley Gurney rejected the alfalfa and bison theories of the Rocky Mountain locust’s demise as lacking the necessary scale and the requisite timing, respectively. By the process of elimination he could find only one factor that had the ecological potential and spatial scope to do in the locust. Although not entirely satisfied, Gurney was left favoring climate as the key suspect. In hindsight we can see that there were two problems—one in practice, the other in principle—with the “weather theory.”

The practical limitation to establishing a link between the decline of the locust and potentially abnormal weather events in the late 1800s was the paucity of data concerning temperature and precipitation during this period. However, circumstantial evidence was most intriguing in some instances. The outbreak in the 1870s was certainly quashed in many midwestern locales by an unusually wet spring in 1877. However, the locusts never persisted for very long in the Midwest, so this decline could hardly have portended the extinction of the species across its entire range. Fortunately, Riley’s reports during the locust’s decline included some local records from within the Permanent Zone
of the locust. But the available information, although somewhat limited in the number of sites, provides no support for there having been a remarkable change in temperature or precipitation. For example, the spring weather in Havre and Missoula, Montana, during the outbreak years of the 1870s was quite similar to the annual mean temperature and total precipitation between 1880 and 1900. There were tremendous blizzards in the late 1880s in Montana, but there were also deadly winters in the western states in the early 1870s. So if the locusts survived the winter storms of 1871 and 1872 to reach plague proportions just five years later, then surely the harsh winters of the 1880s should not have been lethal. And this reasoning leads to the greater problem with blaming the weather for the locust’s demise.

For a climate change to drive a species to extinction, the scale and severity of the conditions must be extreme, functionally unprecedented within the history of the species. The Little Ice Age reached one of its peaks in the nineteenth century, and there were certainly some severe winters and cool summers during this time. However, the same can be said of the seventeenth century—and the locusts obviously survived these conditions. It may well be the case that the woolly mammoths and cave bears of the Pleistocene declined with the end of the Ice Age some 12,000 years ago, but such dramatic events unfolded across millennia or centuries, not in a matter of two or three decades. Although climate change had the spatial scale that entomologists sought for a viable suspect in the death of the Rocky Mountain locust, the severity and duration of weather changes in the waning years of the Little Ice Age were simply insufficient to have extinguished the life of this species.

If anything, the generally warmer and drier conditions that began to prevail at the end of the 1800s should have favored the locust. Riley had noted that the population explosions of
spretus
were due to “the fact that the western climate is more subject to excessive droughts, which cut off the supply of nourishment at a time when the insects are acquiring wings, and thus oblige them to migrate.” Like locusts from other temperate climes, the Rocky Mountain locust entered the migratory phase when dry conditions forced the nymphs to crowd into a shrinking habitat. Suitable vegetation in the Permanent Zone would have been found along watercourses and drainages. Like bone-chilled
campers huddled around a dying fire, these locusts were crammed into the bottom lands that provided edible food. In mid-August, I’ve seen rangeland where there are perhaps 5 or 10 grasshoppers per square yard on the hilltops and more than a hundred per square yard seething in the vales, where a bit of green vegetation lingers. For the locusts a bit of localized adversity engendered continental opportunity. By swarming into new habitats, the Rocky Mountain locust would have survived the worst that the weather had to offer in the late 1800s.

Perhaps these shortcomings in the weather theory should have been apparent to Gurney, but entomologists were clinging to a model dictating that some large-scale factor could account for the locust’s extinction. In his groundbreaking book,
The Structure of Scientific Revolutions
, Thomas Kuhn sagely pointed out that scientists are loathe to abandon a paradigm, even given apparently fatal flaws, until a viable alternative can be identified. Balancing on a teetering bridge is preferable to falling into a yawning canyon. For Gurney and others, even a weak argument within the accepted context of the science was preferable to admitting defeat. And so, the search for a more robust large-scale ecological factor became imperative.

What entomological detectives needed was an ecological suspect with a more direct link to the victim, something that was intimately connected to the time and place of the murder. The most obvious suspect in light of their well-documented effects on the western landscape and prairie ecology—bison hunters—had an alibi. They were not on the scene when the locust was flourishing, so they could not be implicated in its subsequent demise. But by the late 1960s ecologists were becoming cognizant of the intimate yet often inconspicuous connections among species—even organisms that appeared to have little in common. So, what if we were missing some critical ecological link in our reconstruction of the events leading up the locust’s demise? Could there yet be a way of tying the mass killing of bison to the end of the locusts?

Fifteen years after Cantrall and Young published their theory linking the bison’s demise to locust outbreaks, Paul Riegert, a brilliant Canadian entomologist, inverted the bison-locust equation and arrived at a
most enticing theory. If, as Gurney had pointed out, the locusts and bison were busy swarming and stampeding over the continent in earlier times, then perhaps the decline of the two creatures was linked after all. But rather than the loss of the bison being favorable, albeit temporarily, for the locust—as Cantrall and Young hypothesized—Riegert advocated precisely the opposite consequence. He suggested that the changes in the landscape following the extirpation of bison were detrimental, over the long term, for the locust.