The Big Importance of Little Towns on the Prairie
Long thought to be destructive to grasslands, prairie dogs and their habits are now recognized as key to the health of the prairie ecosystem
Ever since people first set eyes on prairie dogs, the impulse to anthropomorphize has been irresistible. After all, the sociable rodents greet each other with a "kiss" (touching noses and teeth). They sound alarms when danger approaches. Sometimes they care for one another, and sometimes they are murderous. They form clans, establish boundaries, construct living quarters and live in towns. Listen to the very scientists who study the creatures: "They're just like little people," says prairie dog expert John Hoogland of the University of Maryland's Appalachian Environmental Laboratory in Frostburg. "Some are aggressive; some are meek. Some get up early, some get up late. They have individual personalities."
And another thing: Prairie dogs radically alter the habitat in and near their underground settlements, as people do with their own towns and cities. Just as we can destroy natural habitat, these sociable rodents ruin the grass for grazing animals—don't they? Haven't cattle ranchers known that to be true ever since they first set sight on the critters' towns, with their barren dirt mounds and mowed landscapes?
Whoa! Hold your horses! The notion of anthropomorphism can only go so far. For one thing, wildlife biologists have long known that prairie dogs (related to ground squirrels) and their modified habitat attract and support many other animals. And recent research has prompted a new appreciation of how important the dogs are to the whole prairie ecosystem. "Their towns are biological hotspots," says Dan Uresk, a research biologist with the U.S. Forest Service's experiment station in Rapid City, South Dakota. Not only that, it turns out that ranchers' bottom lines suffer far less from the dogs' presence than previously assumed. Under the right circumstances, cattle and prairie dogs can even mutually benefit one another.
At the beginning of the twentieth century, black-tailed prairie dogs occupied more than a hundred million acres of the Great Plains down the eastern side of the Rocky Mountains from Canada to Mexico. One town in the Texas Panhandle stretched 250 miles long and 100 miles wide and contained an estimated 400 million animals. In the decades since, ranchers and government agencies have launched massive poisoning campaigns to clear prairie dogs off rangeland. By the 1960s, the rodents occupied less than 2 percent of their original range. Yet even now, remnant populations of black-tailed prairie dogs cover nearly a million acres of prairie, many of them on public land.
Three other prairie dog species — Gunnison's, white-tailed, and the federally threatened Utah prairie dog — occur farther west. All three are far less social and have much less ecological impact than black-tails. A fifth species, the Mexican prairie dog, inhabits the highlands of central Mexico.
There is no question that black-tailed prairie dogs change their habitat. The animals clip the grasses in and around their town, eating their fill and clearing the rest so they can see one another and approaching predators. "Within a growing season, they can enter an area and make it almost like a golf course," says grassland ecologist James Detling of Colorado State University. In the center of town, this constant clipping eventually kills almost all the grass and allows broadleafed herbs and small shrubs to move in.
Out in their more recently settled suburbs, prairie dogs have a much different effect. By clipping off older grass blades and stems, they stimulate tender, nutritious new growth and actually raise the quality and diversity of the forage for other grazers (though they decrease its quantity). Cattle and bison prefer feeding on these newer parts of a prairie dog town rather than in undisturbed prairie. Studies have indicated that young bison gain significantly more weight feeding in a prairie dog town than elsewhere. But that does not mean cattle necessarily experience the same benefits from dog proximity, especially if they are packed onto a limited amount of land.
Other studies have found that prairie dog towns hold about three times the density of wildlife and nearly double the number of species as prairie without the rodents. "I never expected to find so much wildlife associated with prairie dogs," says U.S. Forest Service biologist Uresk, who has studied the animals. Vertebrate species associated with the towns number as high as 171, ranging from mice, to swift foxes, to raptors. Some animals, such as burrowing owls and prairie rattlesnakes, rely on the towns' underground architecture as shelter. A few, such as pronghorns, seek out the broadleafed herbs that grow in the heart of town. And others, such as the mountain plover—a prairie-dwelling shorebird far from any shore—favor the bare terrain where the dogs have their greatest impact.
The best known of prairie dogs' dependents is the critically endangered black-footed ferret. Ferrets live only in prairie dog burrows and eat their hosts, almost exclusively. "The ferrets couldn't exist without the prairie dogs," says ferret specialist Dean Biggins of the National Biological Survey in Fort Collins, Colorado. Biggins has estimated that towns covering fewer than 1,000 acres are too small to support even a minimal ferret population, yet few towns larger than this still exist on the plains.
As a result of that need and several other factors, including a devastating outbreak of canine distemper, ferret populations plummeted to fewer than a dozen animals as recently as 1985. Captive breeding programs have brought that number up to a few hundred today, and ferrets have been reintroduced into the wild in a few large prairie dog colonies in Montana and South Dakota.
Ever since ranchers first settled the West a century or more ago, they have viewed prairie dogs as pests. The rodents prefer to establish towns in the low grasses of heavily grazed areas. "The heavier you graze the pasture, the better the prairie dogs do," says Richard Reading, a conservation biologist with the Northern Rockies Conservation Cooperative in Jackson, Wyoming. "Prairie dogs are often a symptom of overgrazing, not its cause."
In many parts of the plains, prairie dog towns dwindle away to insignificance unless cattle or bison are present. The towns often end abruptly at fences separating grazed land from taller, ungrazed grass on the other side. In the centuries before European settlement, prairie dogs followed the bison herds, founding towns on land that had been grazed to the ground and beaten by millions of hooves. "You canjust imagine this big shifting mosaic over years and decades, of bison and prairie dog movement across the grassland," says Tim Clark, a conservation biologist at Yale University.
Oddly enough, in recent years, nature has been on the side of the ranchers in eradicating the rodents. Bubonic plague, the Black Death that terrorized Europe centuries ago, now finds prairie dog towns as ripe a breeding ground as the crowded cities of medieval Europe. Introduced to North America around the turn of the century in California, it has since spread through almost all the American west. Typically, an epidemic of plague will wipe out 90 to 95 percent of the prairie dogs in an affected colony, then move on. The survivors gradually repopulate the colony over the next few years, after which populations are dense enough again to support another epidemic. Last year, plague struck a colony of Gunnison's prairie dogs that the University of Maryland's John Hoogland had studied for seven years and a colony of Utah prairie dogs that he was just beginning to observe. "It's pretty gruesome," he says. Plague is contagious to humans, but it's easily treated with antibiotics. "I lose very little sleep worrying about myself, but I worry about the dogs a lot," Hoogland says. "When plague moves in, you can just kiss your colony goodbye."
As modern researchers re-examine the effect of prairie dogs on cattle, many are concluding that the rodents are far less detrimental to the livestock than once thought. The proper way to measure the effect of prairie dogs on cattle, contends biologist Uresk, is to compare heavily grazed prairie with and without prairie dogs. When he did that, Uresk found that prairie dogs reduced the forage available to livestock by only 4 to 8 percent.
In another study about a decade ago, Uresk and colleagues reckoned the costs and benefits of poisoning a colony near Rapid City. They found that killing the rodents and preventing them from recolonizing would cost more than would be gained in increased cattle forage.
Taxpayer-funded eradication of prairie dogs may arguably make sense as a government subsidy to ranchers on private land, Uresk says. But prairie dogs' value in boosting prairie diversity argues against wiping them out on public land. Even on private land, it could prove cheaper for the government to pay for the grass eaten by prairie dogs rather than laying out big bucks to poison them. "We're already spending that money. Why not spend it in a different way, and preserve biodiversity?" asks Reading of the Northern Rockies Conservation Cooperative.
Helping to answer such questions is relatively new territory for prairie dog researchers, most of whom have devoted far more time to understanding the rodents' complex social structure. Prairie dogs live in family groups called coteries, which usually contain one male, several females and their juvenile offspring. Daughters tend to stay at home after they mature, so a kin-group of mothers, daughters, aunts and nieces forms the foundation of each coterie. Sons, in contrast, move out to seek their own coteries, usually elsewhere in the same town.
Each coterie occupies its own territory, usually a half-acre or more, with dozens of burrow entrances. Adjacent coteries maintain stable boundaries that the United Nations might envy, with residents routinely running all trespassers off their land. "It's not at all clear to me why females don't band up and say, 'Let's push out the girls next door.' It should be a regular occurrence, but it's not," says the University of Maryland's Hoogland. "I don't know why they're so respectful of these social boundaries."
A prairie dog's year begins early, in the snows and icy winds of February, when mating season begins underground—where researchers can't see what's going on. As Hoogland can attest, figuring out the dogs' relationships from their above ground activity can take thousands of hours of observation and note-taking while huddled in a sleeping bag in an observation tower.
Most females mate with only their local male and give birth about five weeks after mating. Then an epidemic of murder and cannibalism takes place underground, wiping out two of every five litters born. Most often, Hoogland has found, the perpetrator is a female from the same coterie, but not the mother. Infanticide on such a rampant scale—especially of close relatives—is unheard of in any other animals. "It's the most remarkable discovery I've made while working on prairie dogs," says Hoogland.
But the story grows even stranger: "As far as I can tell, once the surviving pups are above ground, the females cannot discriminate among them," says Hoogland. "It's positively baffling." The same females that just a week earlier would have cannibalized their sisters' pups now nurse any pup within the coterie. "People are struck by the amicability within prairie dog colonies because they see them when they're being amicable," says Hoogland. "They don't see them in April when the winds are howling and they're killing each other's babies."
Overall, the rodents' complex and costly social system pays off, Hoogland thinks, in predator avoidance. Prairie dogs spend almost all day above ground feeding and on the alert. The appearance of a predator triggers a chorus of high-pitched "chirk" calls in nearby coteries, and the dogs take cover. In one colony, Hoogland and his field crew saw only 22 predation events in 73,000 hours of observation over 16 years. The team was keeping track of a single colony of black-tailed prairie dogs in Wind Cave National Park in South Dakota. That's the equivalent of 40 hours per week, 50 weeks per year, for more than 36 years.
Curiously, Hoogland has found that some prairie dogs regularly share in the duties of giving alarm calls, while others remain silent. In part, this is because adults with nearby relatives call more readily, presumably because they have more to protect than loners with no relatives in the area. However, even two prairie dogs in identical family situations still often differ in their willingness to call. "I have no hope of ever understanding all these things. It's just too complicated. Prairie dogs have taught me that if I can find out just the most basic patterns of social behavior, I should be satisfied," he says. In short, prairie dogs are inscrutable — just like people.
Watching Out for Public Lands
Conservation of our National Grasslands is one of the priorities of NWF's public-lands campaigns. NWF is working to establish and maintain wildlife populations on 270 million acres of federal lands.
Freelance writer Bob Holmes writes from the human colony of Santa Cruz, California.