Study of Estimation of
Black-tailed Prairie Dog Population
at Bluff Lake Nature Center
Nancy Bernarda Pierce
Fall 2012
Department of Earth and Atmospheric Sciences
Metropolitan State University of Denver
Denver, Colorado
ABSTRACT
Black-tailed prairie dog habitat is decreasing due eradication, disease and development. In
Denver, Colorado prairie dogs living in urban fragmented areas are losing their habitat to city
expansion and development. Land pressure has pushed prairie dogs to find the nearest available
area to establish new colonies or expand existing colonies. Due to the nature of their behavior,
black-prairie dogs turn their habitats into bare soils with very low vegetation and susceptible to
erosion. Occasionally, prairie dogs move to areas that cannot afford losing vegetation due
economic value agricultural areas or environmental value which is the case of a colony in Bluff
Lake Nature Center. The black-tailed prairie dog colony at Bluff Lake Nature Center is
bordering a pristine short grass prairie, this area has a high variety of native plants, therefore is a
conservation priority for Bluff Lake Nature Center. The black-tailed prairie dog colony has been
expanding since 2009 when Stapleton housing development started. Without population control
the colony will continue to expand. No-lethal management techniques to control prairie dog
population such as visual barriers and translocation involving programs supported by the U.S.
Fish and Wildlife Service programs would be the best alternative for the Bluff Lake prairie dog
colony.
Key words: Black-tailed prairie dog, colony, Bluff Lake Nature Center, habitat, expansion,
urban, fragmentation
INTRODUCTION
Prairie dogs once inhabited hundreds of thousands of square kilometers of the Great Plains with
their habitat ranging from Canada to Mexico, their population may have numbered close to 5
billion (Morrison and Peitz, 2011). Today, the range of the prairie dog habitat has been severely
restricted by development, disease, and eradication by humans drastically reducing its original
range to approximately 7,454 square kilometers. (Magle & Crooks, 2008; USFWS, 2011).
In recent years, prairie dogs have been recognized as keystone specie of the prairie ecosystem.
However, this specie is often seen as a nuisance in agricultural and urban environments; many of
the remaining colonies are strongly affected by habitat fragmentation and urbanization (Magle &
Crooks, 2008).
The Black-tailed prairie dog (Cynomys ludovicianus) is the most common of five species:
Mexican, white-tailed, Gunnison’s and Utah prairie dog, from which the Utah and Mexican
prairie dogs are classified as threatened and endangered species. Prairie dogs reside in the shortgrass and mixed-grass prairies in eleven states in the western United States. Prairie dogs are
diurnal, colonial, burrowing rodents of the squirrel family (Hoogland, 1995). The total length of
an adult black-tailed prairie dog is approximately 14-17 inches. The weight of an adult prairie
dog ranges from approximately 1 to 3 pounds. Its appearance varies in mixed colors of brown,
black, gray, and white, with a characteristic black-tipped tail (USFWS, 2012). Prairie dogs are a
burrowing and grazing species, which influence soil mixing, decreases primary production of
plants, and increase landscape heterogeneity (Magle & Angeloni, 2011). They are also prey on a
variety of predators such as the black-footed ferret (endangered). Prairie dogs live in colonies
and within the colony they have territorial family groups called coteries composed of one
breeding male, two or three adult females, and one or two yearling of each sex. The number of
coteries in a colony depends directly on the number of adults and youngsters in the colony
(Hoogland, 1995).
Denver, Colorado is a rapidly urbanizing city located within a large short grass prairie ecosystem
with prairie dogs living in urban fragmented habitats. Urban sprawl has drastically affected
prairie dogs colonies, often exterminating all its members (when relocation is not possible) or
fragmenting the colony. When the colony is fragmented, prairie dogs try to migrate to the closest
colony or find the nearest suitable area to establish a new colony, during dispersion prairie dogs
are vulnerable to predators because warning calls are usually difficult to hear away from the
colony (Magle & Angeloni, 2011; Nistler, 2009).
LITERATURE REVIEW
The historical range of the black-tailed prairie dog
extends from south eastern Montana, southwest
North Dakota, western South Dakota, eastern
Wyoming, most of Nebraska and Kansas, eastern
Colorado, most of New Mexico, southeast Arizona,
western Oklahoma, northwest Texas, and north of
Mexico. See Figure 1, (Van Pelt, 1999).
In the state of Colorado historically black-tailed
prairie dogs inhabited approximately 30,000 km² and
now only inhabit 1,200 km² reducing prairie dogs to
Figure 1: Historical distribution of the black-tailed prairie
dog in the U.S. based on the scientific literature (Hall
1981), and the best estimate of historical range by each
of the 11 states (shaded). Source: Luce, 2003.
less than 2% of their original abundance (Johnson and Collinge, 2004).
Black-tailed prairie dog legal status varies in each state, for example while in Wyoming and
Oklahoma prairie dogs are species of special concern, in Colorado they are a small game species
allowing the purchase of a license to shoot prairie dogs as a method for population control. At
federal level, black-tailed prairie dog is classified as candidate specie (USWFS, 2012).
Black-tailed prairies dogs are adaptable to different kind of vegetation, but they have a
preference for short vegetation on open areas with less than ten percent slope. They are highly
social animals that create a unique habitat by clipping surrounding vegetation and excavating
burrow systems (see Figure 2-3). Many other wildlife species uses this habitat as shelter and
prairie dogs as food source. Burrowing owls, snakes and other small animals use the burrows
when abandoned for nesting. Predators like ferruginous hawks, red tail hawk and other raptors,
coyotes, foxes, and “the most engendered mammal in U.S., the black-footed ferret which relies
exclusively on prairie dogs for food and their burrow system for cover”. The decline of blacktailed prairie dogs affects directly to the black-footed ferret (May, 2003).
Figure 3: Black-tailed prairie dog in a burrow entrance at BLNC. Source:
Pierce, 2012
Figure 2: Social behavior.
Source: USFWS, 2012
Black-tailed prairie dogs live in groups called towns or colonies with a population density than
varies depending on area and habitat. Towns are divided into small wards that further divide into
coteries which are family units and are defended as territories. Typically a coterie is inhabit by
approximately: 1 adult male, 2 to 4 adult females and 2 to 5 yearlings less than 2 years old, and
occupy about 1 acre in size. However a coterie contains approximately 70 burrow entrances and
26 individuals (Hoogland, 1995; May 2003).
Many black-tailed prairie dog towns have an extensive underground burrow system. These
systems range from single, single-entrance systems to complex systems with multiple above
ground entrances. Tunnels generally extend three to six feet below the ground surface, and may
be 15 feet long. The complex systems have several terminal branches, turning bays and nest
chambers. Burrow systems are well ventilated. Excavated soil piled up in mounds around
burrows serve as lookout points. Usually, burrows are about 5 to 14 meters long and 2 to 3
meters deep. Burrow systems have different functions such as: refuge from predators, protection
from weather, provide flood control, reproduction, and other social activities. See Figure 4
(Gedeon, et al., 2012; Verdolin, et al., 2008).
Figure 4: Black-tailed prairie dog burrow system
High burrow entrance mounds and low vegetation facilitate detection of predators and
transmission-reception of visual signals from neighboring individuals. When a predator is
detected a danger signal is transmitted through the colony, this signal is a two-syllable bark
repeated at a rate of about 40 barks per minute. To maintain visibility near burrows, prairie dogs
remove vegetation by clipping. Clipped vegetation may be placed in underground chambers,
possibly as nest lining. Similar to grazing, burning, and other disturbances that remove mature
vegetation, clipping encourages new plant growth that is palatable to other wild herbivores and
domestic livestock (Gedeon, et al., 2012; May, 2008).
The clipping and foraging habits of black-tailed prairie dogs create a habitat of bare ground and
short, sparse vegetation which is one of the main reasons these animals are considered pests on
agricultural lands and a nuisance in urban areas.
There is not a clear agreement between scientists about the keystone role of urban prairie dogs.
Some argue that to be considered keystone specie, prairie dog colony or complex colonies must
have a minimum area of 4000 hectares to support a fully functional grassland ecosystem
(Morrison and Peitz, 2011). Other scientists state that studies of urban fragmented colonies show
that prairie dogs maintain their traditional ecological role in urban areas, although more studies
are needed to assess abundance and ecology importance to confirm a keystone status (Magle and
Crooks, 2008).
Compared with other rodents, prairie dogs reproduce slowly, one liter per year with an average
size of three to four young. However, prairie dogs can show high growth rate when density is
low within the colony. This can be evident usually after a phenomenon that significantly reduces
the number of individuals of a colony. When density depend on factors such food and space
availability are absent "prairie dog colonies can present an annual growth rate high as 2.19."
Competition for food and space occur when the young prairie dogs emerge from burrows and
food becomes limited in the center of the town forcing prairie dogs search for food in the outer
edge of the colony. Prairie dog colony expansion occurs when suitable habitat is located nearby.
When expansion is not possible, dispersal to initiate a new colony occurs, prairie dogs may
disperse from 2 to 10 kilometers. While female stay at the natal colonies, male yearlings move to
new colonies to avoid inbreeding, this practice maintains genetic variability (Nistler, 2009).
Recently, prairie dog colonies are becoming small insolated as a result of development. These
small urban patches retain genetic viability and reduce exposure to plague. These urban
fragments also allow prairie dog colonies to reach higher densities due predators decline in these
areas. This increase of density can result in a poor habitat quality where adults’ prairie dogs had
lower body mass than adults in rural colonies (Johnson and Collinge, 2004)
According with Magle & Angeloni (2010), urban prairie dog colonies can present a large
increase in density (exceeding 100 animals/ha) in comparison with unfragmented prairie dog
colonies in rural areas (10/35prairie dogs/ha). Prairie dogs in urban fragments prefer flat areas;
colonies also present higher reduction of vegetative cover that in rural areas (Magle & Crooks,
2007).
Studies in Colorado found active (occupied) black-tailed prairie dog colony size to range from
0.004 to 16.709 square kilometers with an average active colony size of 0.30351 square
kilometers. Most active colonies were in the 0.004 to 0.080937 square kilometers size category,
the second most common size category was 0.080937 to 0.40469 square kilometers. These
studies also suggest than the minimum habitat requirements are in the 0.004 to 0.080937 square
kilometers range. (May, 2003).
The most appropriate method for estimating numbers of prairie dogs depends on the study area
size, objectives, budget limitations, available equipment, and the necessary level of accuracy.
Methods for estimating colony size (number of prairie dogs that live in a colony) and colony
density (number of adults, yearlings, and juveniles per hectare) include visual counts, capturemark-recapture, and inferences from number and density of burrow-entrances. The exact way to
determine colony size and colony density is to mark all colony residents. However, catching and
marking prairie dogs is difficult and impractical in colonies with more than 100 individuals
(Hoogland, 2006).
Wildlife biologists disagree about whether the number and density of burrow entrances correlate
with the number and density of prairie dogs. Nevertheless, this method has shown effective for
small-scale research where mapping burrow entrances within the entire colony site is possible.
Furthermore, Johnson and Collinge (2004) state that colonies with more active burrow entrances
had more prairie dogs and a significant correlation between burrow density and prairie dog
density exist.
Perdue (2009) examined a prairie dog population of a colony at Buff Lake Nature Center
(BLNC) on the verge of expanding into a pristine section of high grass prairie and recommended
the relocation of the 13 individuals that formed the colony in order to preserve the ecosystem.
OBJECTIVE
The purpose of this study was to determine if the population of prairie dogs at Bluff Lake Nature
Center has increased since they arrival three years ago; and if it is necessary to implement a
population management plan.
STUDY AREA
Bluff Lake Nature Center (BLNC) is a 123 acre urban wildlife refuge located at the northeast of
Denver. Bluff Lake is home to native species of flora and fauna. Bluff Lake is also home of a
prairie dog colony that migrated into the park in 2009; this colony is located near a pristine
section of high grass prairie ecosystem. The study area of the colony is located in the southeast
of Buff Lake Nature Center, bordering Havana Way and Bluff Lake internal trail, see Figures 5
and 6.
In 1970, Bluff Lake Nature Center became part of the Stapleton airport. The land became a
“crash zone”, required at the end of all runways to shield the surrounding community from
airplane activities. For the next fifty years, Bluff Lake remained undisturbed and as a refuge for
surrounding wildlife. After Stapleton Airport closed in 1995, the Sierra Club’s successful lawsuit
against Denver’s Department of Aviation for inadequate retention ponds for ethanol glycol; and
as part of the settlement, Denver agreed to preserve the area and create the Bluff Lake Nature
Center (BLNC, 2012). In 2005, Denver Parks and Recreation requested the relocation of 60
prairie dogs to Bluff Lake Nature Center, but was denied due insurance and environmental
regulatory issues related to a capped landfill on the area. And in 2009, the first group of prairie
dogs appeared at Bluff Lake near what is considered the most pristine short grass prairie in
Denver. This area has approximately 160 species, from which 158 are native. Also, from 331
species of plants growing in Bluff Lake, 48.3% grow in this area. See Figure 8.
Figure 5: Location of Bluff Lake Nature Center
Figure 6: Map of the Study Area
Figure 7: Pristine short prairie bordering the colony
a.
b
.
Figure 8: a. and b. colony bordering housing
developing. c. effects of clipping
c.
METHODOLOGY
Bluff Lake Nature Center borders a new housing development site in Stapleton; the development
site was housing many prairie colonies that now are migrating to Bluff Lake. Since this area is
the only one in Bluff Lake with prairie dogs colonies, it was selected for the study.
The study area was designed as Bluff Lake prairie dog colony (BLPDcolony). This area is
approximately 2 hectares and encompasses one prairie dog colony divide by a secondary road
entrance to Bluff Lake Nature Center.
The field data was collected in the month of October 2012. The coordinates of burrow entrance
was recorded with a Trimble GPS and a LTI-TruePulse Laser unit. Burrows entrances where
classified as active or inactive according the presence or absence of fresh excavated soil or fresh
fecal pellets near the entrances.
The logged data was transferred to ArcMap and used to estimate the area occupied by the prairie
dogs colony by forming a polygon connecting the outermost burrow entrances. To determine the
number of burrow entrances per burrow system the Distance Band from Neighbor Count and the
Average Nearest Neighbor Summary (ArcMap Statistical analysis) was used.
To estimate the number of prairie dogs within the colony, first is necessary to obtain the number
of coteries within the colony.
The total count of active entrances will be divided by 70, assuming that on average one coterie
has 70 burrow entrances (Hoogland, 1995); this will yield the number of coteries within the
colony. Then the number of coteries will be multiplied by 26, assuming that on average one
coterie has 26 prairie dogs, this will yield an estimate number of prairie dogs within the colony.
RESULTS
The population of prairie dogs in the colony located at Bluff Lake Nature Center is increasing. A
prairie dog’s study estimated a population of 13 individuals for the same colony in 2009.
Today, the BLPD colony revealed 340 active burrow entrances, 5 coteries and approximated 130
individuals, see Table 1. According with scientific literature the average distance between
mounds is between 7.6 to 22.8 meters. The Distance Band from Neighbor analysis revealed than
the average distance between three burrow entrances is 7.8 meters which is within the ranges
mentioned before (see Table 2); therefore a burrow system has three burrow entrances.
Average Nearest Neighbor Summary proved than the burrow entrances within the study area are
random located (see Table 3 and Figure 9). Since the burrow entrances are not clustered, it was
very difficult to estimate number of prairie dogs based on the structure of a burrow system.
Therefore the population of prairie dogs was estimated based on the best scientific data available.
Table 1: Variables measured on Bluff Lake's black-tailed prairie dogs colony
Black-tailed prairie dogs colony
Active
Number of
Inactive
entrances
Total
Area
340
15
355
23,568 m²
Table 2: Average distance to a specified number of neighbors entrances (N), where: N=1, N=2 and N=3
Distance Band from Neighbor Count
Neighbor
1
Minimum distance
0.007200
Average distance
4.106462
Maximum distance
13.004044
Units: meters
2
0.606624
6.260342
14.004635
3
2.056620
7.883210
16.358521
Table 3: Average distance from each feature to its nearest neighboring entrance
Average Nearest Neighbor Summary
Study Area*
Observed Mean Distance
Expected Mean Distance
Nearest Neighbor Ratio
z-score
p-value
Pattern
44809.508453
4.106462
5.617472
0.731016
-9.695527
0.000000
clustered
23567.580083
4.106462
4.073928
1.007986
0.287857
0.773457
random
Units
meters
meters
*This calculation was done with two different measurements of the same study area. The first one is the default
value which is the area of the minimum enclosing rectangle that would encompass all the prairie dog’s burrow
entrances. The second one is the total area of the burrow entrances.
Figure 9: Average Nearest Neighbor Summary with a Study area of 23567.580083 m²
The null hypothesis states that features are randomly distributed. Given the z-score of
0.29, the pattern does not appear to be significantly different than random.
Distance method: Euclidean
DISCUSSION
At the moment the colony size is within the range of a healthy population, however, this colony
has proved that during a three years period the prairie dogs had triple their numbers each year. If
this trend continues at this rate, they will have to expand the colony for space and food, and to do
so, will move deeper into the nearest area such as the pristine short grass prairie.
According to Nistler (2009) to achieve long term control is necessary to reduce population by
90%. Although other study state than to maintain the population under 100 individuals, 55%
yearly reduction during February or March (prior to reproduction) or 77% reduction during late
summer (Crosby and Graham, 1986).
There are many methods to control prairie dogs population, but the most cost-effective and nonlethal methods that Bluff Lake Nature Center could consider are:
Visual Barriers: fences or other barriers can be used to block prairie dogs view and reduce
susceptibility of habitat expansion. Effectiveness of visual barriers depends on site/location,
materials used and maintenance. Some of the materials used are: polyethylene mesh, galvanized
roofing panels, silt fencing, pine tree, burlap, fiberglass, still panel, etc. the best material is based
on durability and low see-through visibility. This method can cost from $200 to $6,000 (Nistler,
2009).
Contraception: it is most used in urban areas or where lethal methods are not desired, A
cholesterol-inhibiting contraceptive (DiazaCon) is administered as oral bait applied to molasses
coated oats. A treatment with DiazaCon resulted in a 47% decrease in reproductive success of
adult prairie dogs in Colorado. however, there are not studies of long term behavioral effects of
contraceptives on prairie dogs. The drawback is that chemosterilants may not be available to the
public (Nistler, 2009).
Trapping and translocation: cage traps can be used to capture prairie dogs, but the process is
labor intense, time consuming and impractical on large colonies. The best time to trap is in early
spring and locating the cages with bait (oats, peanut butter and fruit) near burrow entrances. To
avoid stress should be shaded and prairie dogs should be removed promptly (Nistler, 2009).
Since is very difficult to find release sites for prairie dogs, the best option is to capture transport
and deliver prairie dogs to facilities that support the U.S. Fish and Wildlife Service National
Black-footed ferret Recovery Plan or a licensed Raptor Rehabilitation Program.
CONCLUSION
The black-tailed prairie dog colony of Bluff Lake Nature Center has increased 10 times its size
since its establishment and is threating an area considered the most pristine short grass prairie in
Denver; in order to preserve this area a management plan to control the population of prairie
dogs is needed.
Due to the short period of time this study covered, the results obtained should be considered
preliminary, and an in-depth study of the black-tailed prairie dog colony at Bluff Lake Nature
Center using both methods capture-mark-recapture and visual counts is recommended to obtain
more accurate estimates of population size and density of the colony.
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ACKNOWLEDGEMENTS
GPS equipment was loaned by Earth and Atmospheric Science Department, Metropolitan State
University of Denver.
Laser unit was loaned by Laser Technology Inc.
Special thanks:
Chris Story, Bluff lake Nature center Site Manager for sharing all his insight knowledge and
information regarding the black-tailed prairie dog colony, vegetation and fauna of Bluff Lake
Nature Center.
Dr. Jason Janke for all the help and direction during this project.