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Impacts of Declining Prairie Dog Populations on Associated Wildlife Species in Boulder County

Stephen Jones of the Boulder County Audubon Society has written a compelling report documenting the impact of prairie dog habitat loss on associated species in Boulder County. This report was presented to the Boulder County Commissioners and the Parks and Open Space Advisory Committee for their consideration during the review and update of the county’s prairie dog management plan.

Impacts of Declining Prairie Dog Populations on Associated Wildlife Species in Boulder County

Stephen Jones, Boulder County Audubon Society

At least ten bird and mammal species depend directly on prairie dog colonies for their survival: ferruginous hawk, golden eagle, mountain plover, burrowing owl, horned lark, black-footed ferret, American badger, swift fox, deer mouse, and grasshopper mouse (Boulder Open Space and Mountain Parks 2011, Great Plains Restoration Council 2011). At least nine of these species occurred historically in Boulder County, and at least eight continue to occur within the county (Boulder County Comprehensive Plan, Environmental Resources Element 2011; Hallock and Jones 2011).

Some of these prairie dog-dependent species, including ferruginous hawk, golden eagle, black-footed ferret, and American badger, prey on prairie dogs. Others, including black-footed ferret, burrowing owl, and mountain plover, nest or den in and around prairie dog colonies. Below, we profile six of these species for which at least some local population data are available. Federal, State, U. S. Forest Service, and Boulder County status of each profiled species (Hallock and Jones 2010) is given in parentheses.

Ferruginous Hawk (State special concern, USFS sensitive, Boulder County watch-listed).

This largest of North American hawks preys primarily on mid-sized rodents and lagomorphs, including cottontails, prairie dogs, and jackrabbits (Bechard and Josef 1995; Cook, Cartron, and Polechla 2003). Though ferruginous hawks have not been documented nesting in Boulder County (Hallock and Jones 2010), they reside here in winter, gathering around active prairie dog colonies (Boulder County Audubon Society 1979-2011; Jones 1989; Gietzen, Jones, and McKee 1996). North American nesting populations appear to be stable (Sauer, Hines, and Fallon 2008). However, destruction of native prairies and sagebrush uplands threatens nesting populations in some areas (Bechard and Josef 1995).

Boulder County Nature Association wintering raptor surveys conducted from 1983-2011 have documented a strong correlation between numbers of prairie dogs adjacent to survey routes and numbers of ferruginous hawks observed (Boulder County Nature Association 2011; Gietzen, Jones, and McKee 1996; Jones 1989). Other recent studies have documented a similar relationship in areas of Arizona (Cook, Cartron, and Poleshla 2003).

From 1983-1995, Boulder County ferruginous hawk populations fell sharply after sylvatic plague epizootics killed large numbers of prairie dogs, then rose gradually as prairie dog numbers recovered. These trends are clearly reflected in numbers of ferruginous hawks seen along the Boulder Reservoir survey route, which was driven annually from 1983-84 to 2010-11. (Figure 1). Widespread plague epizootics struck prairie dog colonies within this area in 1987-8 and 1993-4.

Figure 1. Mean numbers of Buteos observed along Boulder Reservoir survey route, 1984-2008.

Since 1993 numbers of wintering ferruginous hawks have declined steadily and sharply throughout the county. These hawks have disappeared from areas where prairie dog colonies have been eliminated through poisoning or urban development, as well as from open space properties where prairie dog numbers have been reduced by poisoning or plague. (Boulder County Nature Association 2011, Gietzen, Jones, and McKee 1996; Figure 2).

Figure 2. Mean number of ferruginous hawks observed per km along five Boulder County wintering raptor survey routes, 1990-2011.

Since Boulder County has no incentives in place to encourage preservation of prairie dog colonies on private lands, and open lands on the plains continue to be lost to urban development, loss of prairie dog colonies on private lands is bound to continue. As remaining colonies on private lands are poisoned or plowed under, colonies on public lands will become increasingly more critical for sustaining wintering ferruginous hawk populations. The sharp decline in ferruginous hawk numbers throughout the county from 1992 to 2011 (see Figure 2) suggests that prairie dog densities have fallen below critical levels needed to sustain these open-country hawks.

Golden Eagle (Federal protected, Boulder County isolated and restricted)

Golden eagles also prey on mid-sized rodents and lagomorphs, especially cottontails, prairie dogs, ground squirrels, and jackrabbits (DeLong 2004, Jollie 1943). A 1954 Boulder County study (D’Ostilio 1954) determined that cottontails and prairie dogs were the primary prey in active golden eagle nests.

Golden eagles nest in 15-20 documented territories in the foothills and mountains of Boulder County, and nesting populations appear to be stable (Lederer and Figgs 2011). However, Boulder County Nature Association wintering raptor surveys have documented a significant decline in wintering golden eagle numbers since 2002 (Figure 3).

Figure 3. Mean number of golden eagles observed per km along five Boulder County wintering raptor survey routes, 1990-2011.

Numbers of observed golden eagles have declined sharply along three survey routes: Boulder Reservoir, Rabbit Mountain South, and Rabbit Mountain North. Along each of these routes, wintering golden eagles have abandoned areas of mixed private and public lands where prairie dog colonies have been poisoned or have disappeared as a result of plague. From 2004-2011, the number of acres of Boulder County Parks and Open Space land occupied by prairie dogs decreased from 4010 to 1523 (Boulder County Parks and Open Space 2011). We believe that this decrease, combined with the elimination of many prairie dog colonies on private land, has contributed to the decline in numbers of observed wintering golden eagles.

County staff have asserted that this ongoing decline in wintering golden eagles may result from increased territoriality of nesting pairs in Boulder County. This assertion seems flawed on two accounts. First, golden eagle nesting territories are in the foothills and mountains, whereas their wintering areas are on the plains. Second, there is no evidence that the number of nesting pairs in Boulder County has increased; the recent discovery of previously undocumented nesting territories likely stems from an increase in observer effort, particularly among City of Boulder and Boulder County Parks and open space staff (See Lederer 2011).

However, wintering hunting success of golden eagles does influence their nesting success. Steenhof et al. (1997) reported a positive correlation between prey abundance during winter and reproductive success during the following spring and summer in southwestern Idaho. In a Scottish study, winter prey abundance correlated positively with golden eagle nesting density during the following season (Watson et al. 1992).

We believe that the recent decline in wintering golden eagle numbers throughout Boulder County reflects the same prairie dog population-raptor population relationship we have documented for ferruginous hawks. Loss of prairie habitat through urban development, along with elimination and fragmentation of individual prairie dog colonies and large prairie dog colony complexes, appears to be driving wintering golden eagles out of the county.

Mountain Plover (Federal proposed threatened, State special concern, USFS sensitive, Boulder County extirpated)

Mountain plovers typically nest and forage in patches of bare ground within shortgrass prairies (Kingery 1998, Knopf 2006). Historically, intensive grazing by bison and by prairie dogs provided areas of disturbed ground where these upland shorebirds could thrive (Knopf 2008; Knowles, Stoner, and Gieb 1982). In a Montana study, mountain plovers demonstrated high fidelity to prairie dog colony nesting sites, refusing to leave the boundaries of these colonies even when chased by researchers (Knowles, Stoner, and Gieb 1982). In another Montana study, mountain plover population trends closely matched prairie dog population trends (Dinsmore, White, and Knopf 2005).

Boulder County lies on the western edge of the mountain plover’s current high plains breeding range (Kingery 1998). Only a single nesting report exists for Boulder County, from naturalist Denis Gale in 1886 (Alexander 1937). It’s possible that reduced grazing by ungulates and elimination of large prairie dog colony complexes have rendered the grasslands of Boulder County inhospitable to nesting plovers.

Nevertheless, potential nesting habitat does exist in eastern Boulder County. Creation and maintenance of extensive prairie dog colony complexes in areas such as Rabbit Mountain and the grasslands in and around Dowe Flats could eventually lead to re-establishment of nesting mountain plovers within the county.

Burrowing Owl (State threatened, USFS sensitive, Boulder County isolated and restricted).

Burrowing Owl
Photo Courtesy of Pam Wanek

In eastern Colorado, burrowing owls nest predominately in prairie dog colonies, laying their eggs in abandoned prairie dog burrows (Haug, E. A.; Milsap, B. A. and Martell, M. S. 1993; Kingery 1998). These little owls were considered “abundant” in Boulder County prior to 1910 (Henderson 1908, Betts 1913). By the mid-1980s, this species was listed as “rare and declining” in the Boulder County Comprehensive Plan, and during 2005-11, only 3-10 pairs were documented nesting each year within the county (Hallock and Jones 2011). During the 2011 nesting season, 50 volunteers surveyed nearly every Boulder County Parks and Open Space prairie dog colony at least 4 times for burrowing owls and found only 3 nesting pairs (Boulder County Parks and Open Space, unpublished data).

Productivity of Boulder County nests appears low, averaging only about 2 observed young per nest throughout the 1990s and 2000s (Hallock and Jones 2011, Jones and Mahoney 2003). This compares with mean fledge rates of 4 or more per nest in several western Great Plains study areas (Johnsgard 2002). We believe that predation by urban-adapted raptors and carnivores is the likely cause of this low productivity. Since most of our prairie dog colonies are confined by roads, subdivisions, or irrigated fields and most also contain trees or power poles that serve as perches for hunting raptors, our burrowing owls are particularly vulnerable to predation by coyotes, foxes, domestic cats and dogs, great horned owls, and prairie falcons.

If Boulder County is to serve as a viable nesting area for burrowing owls, and not as a habitat “sink” as it probably does now, we will need to create complexes of large prairie dog colonies lying within expanses of relatively natural grasslands. The Rabbit Mountain area, Rock Creek Farm Open Space west of Broomfield, and the southern grasslands/Rocky Flats area offer opportunities for re-creating sustainable burrowing owl habitat.

Black-footed Ferret (Federal endangered, State endangered, Boulder County extirpated)

Black-footed ferrets are medium-sized mustelids that once occupied much of the western plains and Intermountain West, always in association with prairie dogs (Fitzgerald, Meaney, and Armstrong 2011; U. S. Fish and Wildlife Service 2011). Boulder County lies in the heart of their historic range (U. S. Fish and Wildlife Service 2011).

Elimination of prairie dog colonies throughout the West, along with introduction of alien diseases, resulted in the apparent extinction of this species during the twentieth century (U. S. Fish and Wildlife Service 2011). In 1964 a small population was discovered in South Dakota, and a subsequent captive breeding program failed. In 1981 a second small population was discovered in Meeteetse, Wyoming. A subsequent captive breeding and reintroduction program has resulted in the current wild population of approximately 750 ferrets in two dozen relocation sites in Chihuahua, Arizona, New Mexico, Utah, Colorado, Wyoming, Kansas, South Dakota, Montana, and Saskatchewan (U. S Fish and Wildlife Service 2011; Black-footed Ferret Recovery Program website 2011).

Black-tailed Prairie Dogs

Boulder County does not currently contain sufficiently large prairie dog colony complexes to qualify as a ferret reintroduction site. In addition, the frequent incidence of sylvatic plague in Boulder County precludes reintroduction under current federal guidelines.

However, guidelines are changing; recently, the U. S. Fish and Wildlife Service has begun reviewing potential relocation sites that are just a few thousand acres in extent. The Rocky Flats/southern grasslands area of Boulder County, which is mostly protected under Boulder City, Boulder County, or federal ownership, could eventually serve as a ferret reintroduction site. Returning black-footed ferrets to Boulder County would help create natural controls on prairie dog populations, since a single adult ferret will consume as many as 100 prairie dogs per year (Fitzgerald, Meaney, and Armstrong 2011). It would also contribute substantially to recovery of a Federal endangered species.

Historically, some open space agencies have neglected or barely mentioned locally extirpated species in grassland management plans. We suggest that future Boulder County grassland plans include detailed information about the conditions that need to be created to facilitate black-footed ferret recovery.

American Badger (Boulder County special concern, endangered in Canada)

American badgers occupy a variety of habitats in Boulder County, from the plains to the Alpine (Fitzgerald, Meaney, and Armstrong 2011; Carron Meaney pers. comm.). On the plains, these badgers frequently prey on prairie dogs and sometimes den in prairie dog colonies (Goodrich and Buskirk 1998, Fitzgerald, Meaney, and Armstrong 2011). In a Wyoming study, prairie dogs occurred in 57% of stomach and fecal samples from female American badgers (Goodrich and Buskirk 1998).

Longtime Boulder County residents comment on the recent paucity of badger sightings in Boulder County (Carron Meaney pers. comm.). From 2002-11, no American badger observations were reported to the Boulder County Audubon Monthly Wildlife Inventory, while contributors reported 519 coyotes, 224 red foxes, 7 gray foxes, 33 bobcats, 2 long-tailed weasels, 1 short-tailed weasel, 2 mink, and 1 river otter (Boulder County Audubon Society 1979-2011). The Boulder County Wildlife Inventory focuses primarily on birds, but participants also report incidental observations of mammals and other wildlife. The absence of any badger reports during this 10-year period seems worthy of concern.

We recommend that Boulder County Parks and Open Space initiate a long-term study of American badger populations on the plains of Boulder County. We see their apparent decline in numbers as yet another indication that the prairie dog/grassland ecosystem that once supported a diverse assemblage of birds and mammals is collapsing in Boulder County.

Why Should We Assume Responsibility for Preserving Prairie Dog Colonies and Their Associated Wildlife Species?

We live in an urbanizing county, and we sometimes hear local wildlife managers say it’s just not possible for us to maintain suitable habitat for all the wildlife species native to our area. This responsibility, they argue, should fall on more rural areas. Unfortunately, there are few government entities in northeastern Colorado working actively to protect prairie dog habitat. In the Pawnee National Grassland, prairie dog colonies comprise less than 0.5% of the total grassland acreage (Prairie Dog Coalition 2011). Counties east of Boulder County have virtually no procedures in place to protect prairie dog habitat.

Admittedly, we are limited by the extent of our protected grasslands. But we live in one of the wealthiest counties in the world, with nearly 50% of its land protected as national forests, parks, or open space. If we can’t preserve our native fauna, who can? From the early 1980s, Boulder County has pursued a goal, reflected in the Environmental Resources Element of its Comprehensive Plan, of preserving native wildlife species within the county. We trust this effort will continue, with particular focus on prairie dog colonies and their associated wildlife populations.

We also trust that Boulder County will actively implement its authority, granted under the 1034 Local Government Land-Use Control Enabling Act, to “[protect] lands from activities which would cause immediate or foreseeable material damage to significant wildlife habitat and would endanger a wildlife species.”

Finally, we thank Boulder County Parks and Open Space staff and the Boulder County Commissioners for their longstanding support of native ecosystem restoration and conservation of native wildlife species throughout the county.

Literature Cited

Alexander, G. 1937. The birds of Boulder County, Colorado. University of Colorado Studies, 24 (2).

Bechard, M. J. and J. A. Schultz. 1995. Ferruginous hawk (Buteo regalis). In The Birds of North America, no. 172 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA and The American Ornithologist’s Union, Washington, D. C..

Betts, N. 1913. The birds of Boulder County, Colorado. University of Colorado Studies 10:177-232.

Black-footed Ferret Recovery Program website: http://www.blackfootedferret.org/

Boulder County Audubon Society. 1979-2011. Monthly wildlife inventories. Unpublished data available at www.boulderaudubon.org.

Boulder County Government. 2011. Boulder County Comprehensive Plan, Environmental Resources Element. http://www.bouldercounty.org/government/pages/bccp.aspx

Boulder County Nature Association. 2011. Wintering raptor survey route data and graphs. Unpublished data, available at www.bcna.org.

Boulder Open Space and Mountain Parks. 2011. Prairie dogs and the ecosystem. http://www.bouldercolorado.gov/index.php?option=com_content&view=article&id=1690&Itemid=1606

Cook, Rosamonde R., Jean-Luc Cartron, and Paul Polechla Jr. 2003. The importance of prairie dogs to nesting ferruginous hawks in grassland ecosystems. Wildlife Society Bulletin 31:4.

DeLong, J.P. 2004. Effects of management practice on grassland birds: golden eagle. Northern Prairie Wildlife Research Ctr., Jamestown, ND. 22 pages. http://www.npwrc.usgs.gov/resource/literatr/grassland.

Dinsmore, S. J., G. C. White, and F. L. Knopf. 2005. Mountain plover population responses to black-tailed prairie dogs in Montana. Journal of Wildlife Management 69 (4).

D’Ostilio, D. O. 1954. Nesting status and food of the golden eagle in northern Colorado. M. A. thesis, University of Colorado, Boulder.

Eads, D. A. and D. E. Biggins. 2008. Aboveground predation by an American badger (Taxidea taxus) on black-tailed prairie dogs (Cynomys ludovicianus). Western North American Naturalist 68 (3): 396-401.

Fitzgerald, J.P., Carron Meaney, and David Armstrong. 2011. Mammals of Colorado. Denver Museum of Natural History and University Press of Colorado.

Gietzen, Randal, Stephen R Jones, and Richard J McKee. Hawks, eagles, and prairie dogs: population trends of wintering raptors in Boulder County, 1983-1996. Journal of the Colorado Field Ornithologists 31:2.

Goodrich, J. M. and S. W. Buskirk. 1998. Spacing and ecology of North American badgers in a prairie dog complex. Journal of Mammalogy 79 (1): 171-79.

Great Plains Restoration Council. 2011. Prairie dogs and the ecosystem.

Hallock, Dave and Stephen Jones. 2010. Boulder County avian species of special concern. Boulder County Nature Association, PO Box 493, Boulder, CO 80306.

Haug, E. A.; Milsap, B. A. and Martell, M. S. 1993. In The Birds of North America (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA and The American Ornithologist’s Union, Washington, D. C..

Henderson, J. 1908. An annotated list of the birds of Boulder County, Colorado. University of Colorado Studies 6:220-42.

Johnsgard, Paul. 1990. Hawks, eagles, and falcons of North America. Smithsonian Institution Press, Washington, D. C.

Johnsgard, Paul. 2002. Owls of North America. Smithsonian Institution Press, Washington D. C.

Jollie, M. T. 1943. The golden eagle: its life history, behavior and ecology. PhD thesis, University of Colorado, Boulder.

Jones, Stephen. 1989. Populations and prey selection of wintering raptors in Boulder County, Colorado. Proceedings 11th North American Prairie Conference.

Jones, Stephen, and Linda Mahoney. 2003. Owls of Boulder County. Boulder County Nature Association, PO Box 493, Boulder, CO 80306.

Jones, Stephen. 2011. Boulder Reservoir 2011 Avian Species of Special Concern Monitoring Report. Boulder Parks and Recreation Department, unpublished report.

Kingery, Hugh, ed. 1998. Colorado breeding bird atlas. Colorado Bird Atlas Partnership, Denver.

Knopf, F. L. Mountain plover. 2006. In The Birds of North America (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA and The American Ornithologist’s Union, Washington, D. C..

Knopf, F. L. 2008. Mountain plover studies, Pawnee National Grassland, 1986-2007. Colorado Division of Wildlife, 6060 Broadway, Denver Colorado.

Knowles, C. J., C. J. Stoner, and S. P. Gieb. 1982. Selective use of black-tailed prairie dog towns by mountain plovers. Condor 84:71-74.

Lederer, Nancy, and Michael Figgs. 2011. Cliff-Nesting raptors in the northern Front Range: 2010 status report. Boulder County Nature Association, PO Box 493, Boulder, CO 80306.

Massachusetts Audubon Society. 2008. Managing Large Grasslands for Grassland Birds. http://www.massaudubon.org/Birds_and_Birding/grassland/large.php

Natural Resources Conservation Service. 1999. Grassland birds. Fish and Wildlife Habitat Management Leaflet, No. 8.

Prairie Dog Coalition. 2011. Life of the grasslands: Pawnee National Grassland. http://www.prairiedogcoalition.org/coalition-efforts.php

Sauer, Hines, and Fallon. 2008. The North American breeding bird survey, results and analysis, 1966-2007. Version 5.15.2008. USGS Pawtuxet Wildlife Research Center, Laurel, MD.

Steenhof, K., M. N. Kochert, and T. L. McDonald. 1997. Interactive effects of prey and weather on golden eagle reproduction. Journal of Animal Ecology 66:350-62.

U. S. Fish and Wildlife Service. 2011. Endangered species: black-footed ferret. http://www.fws.gov/mountain/prairie/species/mammals/blackfootedferret/

Watson, J., S. R. Rae, and R. Stillman. 1992. Nesting density and breeding success of golden eagles in relation to food-supply in Scotland. Journal of Animal Ecology 61:543-50.

The Board of Commissioners has scheduled a public hearing on August 28th for the review and adoption of the draft prairie dog element.