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This past summer Deborah and I had the pleasure of going for a walk with my daughter and her husband along a beautiful hiking trail that followed the Cache de Poudre River just west of Greeley, Colorado. There were many things to see on this hike including osprey and red-tailed hawks and old venerable cottonwood trees. We also had two energetic dogs with us that kept us on our toes. The creature, though, that most strongly captured my attention was a single prairie dog that bravely kept his body above ground to bark and whistle at us as we walked past the extensive colony of burrows in which all of his compatriots were hiding.
I hadn’t been close to a prairie dog in over 40 years. Back at Texas Tech when friends and I would go play golf on the Lubbock public golf course, prairie dogs would regularly run out onto the fairways, grab our golf balls and take them off to their burrows. The Lubbock course, according to their score cards, was the only course in the country that had prairie dogs listed as a “no stroke penalty” hazard.
Prairie dogs are ground squirrels that were once astonishingly abundant on the Great Plains of North America. Prairie dogs live in colonies (or “towns”) with clusters of complex burrows that can have 30 to 50 burrow entrances per acre. The prairie dogs in a colony have complex social behaviors and communication systems. They also have significant impacts on their habitats. Lewis and Clark described prairie dogs and their extensive colonies in one of their journal entries (September 7, 1804) when they passed through what is now northeastern Nebraska. They marveled at the depth and nature of their burrows and the elusiveness of the prairie dogs themselves. They did capture one of the prairie dogs but only after great time and effort. That prairie dog, kept as a pet, was later presented to President Jefferson as a gift.
Exactly how many prairie dogs existed in the pre-settlement Great Plains is very hard to determine. They were so abundant and so common that few early settlers took the time to even mention them let alone count them. Estimates of 80 to 100 million acres, though, are often offered for the extent of land initially occupied by prairie dog colonies. One well documented prairie dog colony in Texas covered 250 square miles up into the Texas panhandle and contained, possibly, 400 million individuals! Using present day observations of land occupied and numbers of individuals present (2.4 million acres and 24 million individual prairie dogs) we can extrapolate that could have been well over a billion prairie dogs living on the Great Plains prior to European settlement.
All five prairie dog species are only found in North America. The most abundant and most widely distributed of these species is the black tailed prairie dog (Cynomys ludoviclanus) whose range extends from southern Canada, down the Great Plains with the Rocky Mountains as the border in the west all the way to northern Mexico.
Prairie dogs live a variety of dry grassland habitats including shortgrass prairies, mixed grass prairies, sagebrush steppe, and desert grasslands. Vegetation in prairie dog inhabited grasslands is kept short (between 3 and 5 inches tall) to keep from interfering with their constant, visual monitoring for approaching predators. Prairie dogs also rely on these grasses for their food (75% of their diet are grasses), and their impacts on the quality and quantity of plants growing within the boundaries of their colonies are substantial.
Coyotes are the most significant predators of prairie dogs, but swift foxes, American badgers, golden eagles, red-tailed hawks and ferruginous hawks also readily take and eat them. A highly specialized predator of prairie dogs that actually lives within the prairie dog burrows is the black-footed ferret. This ferret is an endangered species primarily due to the great reduction in prairie dog colony sizes and distributions. The evolution of a specific predator for prairie dogs implies that prairie dogs have lived in their grassland habitats for a very long period of time.
Prairie dogs were first and foremost looked upon as pests by European settlers. They assumed that the prairie dogs ate the same grasses as their introduced cattle and sheep and were, therefore, direct competitors for a limited food resource. Massive programs of extermination involving shooting and poisoning were undertaken reducing the prairie dog distribution from the 100 million acres of the pre-settlement plains to just 364,000 acres in 1961. It was observed, though, that the quality of the rangeland suffered when the prairie dogs were excluded, and the prairie dog’s role as a keystone species became apparent.
The keystone nature of the prairie dog has both obvious and subtle aspects. One of the most obvious features of their impact on their habitats is their construction of their extensive systems of burrows. These burrows can be 4 or 5 feet deep and are typically branched into a large number of side tunnels and specialized dens (sleeping dens, defecation dens, mating and brood dens, etc.). Many other animals use these shady ground burrows for both protection and to get relief from the winter cold and summer heat on the plains. Hundreds of species of both vertebrates and invertebrates rely on prairie dog burrows as components of their preferred habitats. Some of these other species may even have tightly evolved tight, commensal connections to these burrows. The mountain plover and the burrowing owl are two bird species with very significant reliance on prairie dog tunnels.
Equally obvious is the importance of prairie dogs in the food webs of their grassland ecosystems. Their abundance, mitigated more than a little by their high levels of alertness and elusiveness, makes them a significant source of food for a large number of small to medium sized mammalian and avian predators.
The more subtle aspect of the prairie dogs’ influence on their ecosystems, though, involves a closer examination of their alleged negative impacts on their sustaining grasses. These assumptions of competition with other grazers and degradation of range quality led ranchers and sheepherders to initiate their programs of extermination. Prairie dogs, though, through their activities actually increase soil nutrient levels and water retention potentials which, in turn, lead to an accelerated rate of grass growth within the borders of their colonies. Further, the increased soil heterogeneity around and in between the burrow openings provides a more diverse growth medium for a broader range of grasses and forbs which in turn increases the diversity and ecological stability of the grassland ecosystem. Further, by keeping the grasses closely cropped around their burrows, prairie dogs keep the plants developmentally suppressed, thus maintaining a higher nutritional density in the plant tissues. Also, in many of the grasslands inhabited by prairie dogs, they actually consume plant species not favored by grazing, domesticated cows or sheep.
A telling observation concerning the increased quality of the range forage within a boundaries of a prairie dog colony is that wild bison and pronghorns and even domesticated cattle will preferentially graze in and around prairie dog burrows rather than in adjacent non-prairie dog inhabited range lands. They go where the food is better!
Prairie dogs are often in the news because of a human introduced calamity that can decimate an entire prairie dog colony. “Sylvatic plague” (plague in wild animals) is caused by the flea-borne bacterium Yersinia pestis. This bacterium is the pathogen that also causes both bubonic and pneumonic plague in humans. In the early 1900’s plague bearing rats arrived in western ports of the United States, and their fleas carrying the Yersinia bacteria, spread to wild rodent populations (including prairie dogs). Plague, then, became established in wild rodent species throughout California and Oregon and all across northern Arizona, northern New Mexico and southern Colorado.
An outbreak of plague typically kills an entire prairie dog colony. The danger of this disease spreading to humans and/or their pets via the transmitting fleas is a frequently disseminated news story in the west. This past summer, plague was detected in a prairie dog colony near Denver and news alerts quickly went out announcing the closing of the affected area to hikers and campers.
The transition of prairie dogs from pest to keystone species is still being debated, but there is no question that their influences on their grassland habitats are profound.