Wild Again. David S. Jachowski
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debate persisted despite the fact that prairie dogs, on which black-footed ferrets rely, were likely to have been one of the most abundant mammals in North America at the time. Naturalist C. Hart Merriam noted the abundance of prairie dogs when he made transcontinental train journeys across the Great Plains in the late 1800s. He wrote that “the traveler who looks out the car window by the second day west from Chicago is sure to have his attention arrested by colonies of small animals about the size of cottontail rabbits.” Merriam noticed that they had become tame to the sounds and sight of the onrushing train, staying above ground long enough for him to take notes on their colonial life and make observations on their social behavior. He reported on their elaborate burrow structure, their seasonal cycle of activities, their warning calls to others, and the species that preyed on them. Merriam reported hearing of a colony in Texas estimated to be twenty-five thousand square miles in size, and based on the density of burrow openings, he deduced that it contained 400 million prairie dogs.
At a continental scale, he postulated that prairie dogs had an inflated abundance “due to the coming of the white man” who “cultivates the soil and thus enables it to support a larger number of animals than formerly.” Although intensive grazing of the grasslands by overstocked exotic cattle created a more open prairie that might have allowed prairie dogs to increase in numbers, the species was always abundant across the Great Plains, even prior to westward invasion by the “white man.” Prairie dogs had been in the Great Plains for millions of years prior to Lewis and Clark’s expedition, with fossil records dating back to the late Pliocene epoch of 2.5–1.8 million years ago.
It was the high abundance of these uniquely New World rodents that allowed for the evolution and speciation of one of the world’s most specialized carnivores, the black-footed ferret. Similar to bison and many other New World mammals, black-footed ferrets can trace their predecessors back to Europe and Asia. The precursor to the modern black-footed ferret, a now-extinct subspecies of the steppe polecat (Mustela eversmanii berengii), followed the Bering land bridge from Asia to North America. Slowly spreading southeast from modern-day Alaska through ice-free corridors, this ancestral ferret was present in the Great Plains as early as eight hundred thousand years ago.
From the specialized beaks of Darwin’s finches to the cryptic partnerships between flowers and their uniquely dependent pollinators, ecologists have always loved to study specialization—the ability of species to adapt and form a dependence on a specific set of conditions. Over time, this new set of specialized adaptations can become so advanced that it forces a species to diverge, become unique, eventually forming a separate species. In this way, specialization itself is the foundation of biodiversity, but to be able to specialize requires three things. First, there must be an advantage to specializing, to doing one thing better than any close relative. Second, the selected advantage must be heritable and able to be passed down through generations. Finally, there must be stability in the advantage so that the plusses and minuses of reproduction and survival favor the specialist over its competition.
The prairie dog populations of the Great Plains provided the perfect medium for specialization of the black-footed ferret. They offered an abundant and stable prey source that allowed some ferrets to begin to shift their diet from a range of small rodents similar to those found in Asia and Europe to the larger prairie dogs of North America. Further, the intricate burrow systems of prairie dogs served as sufficient shelter for these prairie-dog-hunting specialists. With both food and shelter available, these early black-footed ferrets found no need to leave prairie dog colonies and interact with their ancestral predecessors, and by as early as thirty-five thousand years ago, the black-footed ferret (Mustela nigripes) was morphologically distinguishable from Mustela eversmanii berengii. Speciation had occurred, producing the highly adapted black-footed ferret that has been able to persist long after its generalist polecat predecessor had gone extinct in North America.
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As first chief of the Biological Survey (later to become the present-day U.S. Fish and Wildlife Service), Merriam focused his interest on patterns of species distributions in North America, which he mapped as “life zones” or biomes. On first observing how prairie dogs and their distribution coincided with prairie grasses but avoided belts of trees in stream valleys, he found support for his theory, musing that prairie dogs were “an important illustration of the law that in fixing the limits of distribution of animals climatic factors are even more potent than food.”
Merriam also had thoughts as an early conservationist, even contradicting the original intent of the Biological Survey, which was to provide information on birds and mammals that were agricultural pests. In his 1896 report to Congress, he opposed the “pernicious effects of laws providing bounties for the destruction of mammals and birds.” He did not, however, extend these sentiments to prairie dogs. His 1902 report entitled “The Prairie Dog of the Great Plains” reflected public pressure from the rapidly growing number of western farmers and ranchers. In this seventeen-page report, he institutionalized a view that has largely persisted since: that prairie dogs are a rangeland “scourge” that must be eradicated.
By the late 1800s farmers and ranchers had moved into the plains in such numbers that land holdings were decreasing in size, and grass losses due to prairie dog herbivory were “more keenly felt.” Merriam postulated that prairie dogs resulted in the loss of 50–75 percent of the production capacity of a piece of land for livestock (a figure widely discredited in the years to come). This assessment, combined with the view that prairie dogs were abnormally expanding their range following western settlement, led western states to develop policies and programs for the complete elimination of prairie dogs. Texas and Kansas even went so far as to adopt legislation around 1900 that stipulated fines for landowners who failed to exterminate prairie dogs on their land.
Because of the low price of strychnine and the financial repercussions of inaction, prairie dogs were removed from large portions of Texas, Kansas, and other states throughout the West. Even after millions of acres of prairie dogs on private farms and ranches had been eradicated, there were still complaints about prairie dogs on government lands where private landowners leased grazing rights. Merriam referred to this situation as a “very serious evil” for private landowners, “and one with which it is exceedingly difficult to cope.”
The federal government, with what Merriam termed “universal support,” started to investigate efficient ways to “combat the evil” on private and public lands. Prairie dog eradication changed from a backyard occupation to a federally funded campaign. The government purchased and applied poison grain laced with strychnine and provided subsidies for ranchers to do so on a colossal scale. More recently, strychnine was replaced by 1080, zinc phosphide, and Rozall, but the effect was just as lethal. After treatment, sites were revisited to ensure a complete kill. Stragglers were knocked out with targeted burrow application of bisulfide, pyrotechnic gas cartridges, cyanide flakes, or a homemade concoction called “hoky poky on cobs.”
Prairie dog poisoning became a full-time job for thousands of people. Eradication was a sure-fire campaign platform for any governor or congressman in a prairie state. Poisoning was both politically popular and well funded in order to remove an “impediment to the economic development of the west.” Between 1915 and 1965, western states eliminated more than 37 million acres of prairie dog towns, driving populations to less than 5 percent of their former range. They left only small, overlooked pockets of prairie dogs in the back corners of a few expansive ranches, public grasslands, and tribal lands. These pockets today continue to be targeted by government-sponsored poisoning campaigns, with only some small populations being managed in protected environments.
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I return to my truck and leave the prairie dogs to start their day. I find in the bottom of my backpack a granola bar, oats and other grains molded into a snack that is thankfully strychnine free, and think of C. Hart Merriam as I eat my breakfast. Reading his 1902 article, I thought that Merriam sounded interested in the ecology of prairie dogs. He seemed almost fond of prairie dogs when he referred to the black-footed ferret as “one of their [prairie dogs] most relentless and terrible enemies, and if sufficiently abundant would quickly exterminate all the inhabitants of the largest colonies.” Yet his document now can be seen as a masterful piece of political propaganda