The United States Fish and Wildlife Service has ruled that, contrary to its prior concerns, the black-tailed prairie dog is not a threatened species. The decision was made in light of a 10-state prairie dog census, which concluded that over 18 million of the furry critters currently inhabit the Western plains. How does one go about counting millions of prairie dogs?
By tallying the number of acres that play host to prairie dog colonies and then extrapolating from there. In the areas that black-tailed prairie dogs call home, population densities per acre typically range from two individuals to as many as 18. The USFWS uses these extremes to come up with an average population density of 10 black-tailed prairie dogs per acre. So, all the agency must do is find out how many acres of American land are pockmarked with black-tailed prairie dog colonies and then add a zero—which is how the USFWS came up with its rather precise-sounding estimate of 18,420,000.
The trick, then, is figuring out how much land is occupied by these burrowing rodents. This task is left up to wildlife agencies in the 10 Western states where black-tailed prairie dogs live. (It used to be 11, but the species is apparently no longer found in Arizona.) In response to a census released in 2000, which estimated the black-tailed prairie dog population at a disturbingly low 6,760,000—compared to perhaps 400 million or more around 1900—the USFWS asked its state-level counterparts to conduct more detailed surveys.
The two essential methods of prairie dog tabulation are aerial surveys and old-fashioned head counts. The former approach, which relies on remote-control planes equipped with cameras, is more accurate at identifying colonies in hard-to-reach locales. It's also less time-consuming than tramping out in the dust in search of pint-sized mammals. But field surveys may be better at sussing out colonies in settings where human development obscures small pockets of prairie dogs—say, in the suburban sprawl of Denver.
Regardless of the dedication of its practitioners, prairie dog counting remains an inexact science, and it's not altogether unfathomable that the USFWS figure is either millions too low or too high. Ranchers and farmers are likely to claim the former, as they constantly complain that voracious prairie dogs make a mess of grazing lands and crops. Environmental groups like the Center for Native Ecosystems, by contrast, have already criticized the USFWS's population estimate as Pollyannaish.