Signs of Fall 9: Prairie Dogs (from Pests to Keystone Species)!

Prairie dog town in Wyoming. Photo by W. lemay. Witkimedia Commons

(Click on the folowing link to listen to an audio version of this blog …. Prairie dogs from pests to keystone species

There are patches of open land all around us here in Northern Colorado that are full of prairie dog burrows. They stretch along our hiking and biking trails, they shadow the roads and the highways all the way down into Denver, and they fill-in great squares of acres that are probably fated to become more condos or apartment buildings.

Prairie dogs are ground squirrels that were once astonishingly abundant out in 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 their journals (September 7, 1804) when they passed through what is now northeastern Nebraska. They marveled at the depth and nature of their burrows and their elusiveness. 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.

Original distributionof black-tailed prairie dog. Map by Dankk2. Wikimedia Commons.

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.

Black-tailed prairie dog. Photo by J. Ravi. Wikimeida Commons

There are five species of prairie dogs, and they are all found only in North America. The most abundant and most widely distributed of these five 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 dominated by buffalo grass, mixed grass prairies, sagebrush steppe, and desert grasslands. Vegetation in these prairie dog inhabited grasslands is kept short (between 3 and 5 inches tall) to keep from interfering with the prairie dogs’ constant, visual monitoring of possible, 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.

Coyote hunting in prairie dig town. Photo by L. Sith. Wikimedia Commons

Prairie dogs are a common prey item for many predators. Coyotes are without a doubt the most significant predator of prairie dogs, but swift foxes, American badgers, golden eagles, red-tailed hawks and ferruginous hawks also readily take and eat prairie dogs. A highly specialized predator of prairie dogs that actually lives within their burrows is the black-footed ferret. This ferret is an endangered species due to the great reduction in prairie dog colony sizes and distributions. The evolution of a specific predator for prairies 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 the settlers’ 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 10 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 of 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 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.

Native thickspiked wheat grass (Elymas lancelatus) growing in prairie dog town. Photo by M. Lavin. Wikimedia Commons

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. The 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. And, by keeping the grasses closely cropped around their burrows, prairie dogs keep the plants developmentally suppressed and, thus, help to maintain a higher nutritional density in the plant tissues. Further, in many of the grasslands, the prairie dogs 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 rangelands. They go where the food is better!

Black-tailed prarie dog. Photo by Asiir. Wikimedia Commons

Prairie dogs are often in the news because of a human introduced calamity that can decimate an entire prairie dog colony. Sylvatic plague is the disease in wild animals caused by the flea-borne bacterium Yersinia pestis. This bacterium 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 plague bacteria, spread to wild rodent populations (including prairie dogs). Plague, then, became established in these wild rodent species throughout California and Oregon and all across northern Arizona, northern New Mexico and southern Colorado.

Outbreaks of plague typically kill entire prairie dog colonies. The danger of this disease (via the transmitting fleas) of spreading to humans and/or their pets is a frequently disseminated news story in the west. Three summers ago, 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.