Small Mammal Responses to the Reintroduction of Gunnison's Prairie Dog
By Betsy Black and Andrew Veselka
Abstract
The prairie dog (Cynomys sp.), due to its burrowing and grazing behavior, is widely
recognized as an ecosystem engineer throughout the North American grassland. Their
modification and regulation of habitat affects vegetation structure and a variety of species
throughout their ecosystem. In order to better understand these effects, many studies have been
conducted on the responses of small mammals to the presence of prairie dogs. However, most of
these have been conducted on the black-tailed prairie dog (Cynomys ludovicianus) and
extrapolated to the other four species in the genus. In this study, the effects of the reintroduction
of the Gunnison’s prairie dog (Cynomys gunnisoni) on the abundance and diversity small
mammal populations were surveyed. Furthermore, the density of prairie dog burrow entrances
was analyzed to determine whether or not there was a spatial correlation between burrow
location and small mammal presence. During June of 2014 small mammals were trapped on four
16 hectare plots. Two of these plots were Gunnison’s prairie dog reintroduction sites and two
were not. Additionally, traps and burrow locations were mapped to determine burrow density
surrounding successful traps. The abundance of small mammals was significantly higher for
several species as well as for the combined total in plots containing prairie dogs. Over twice the
amount of small mammals were caught compared to the same study conducted a year before, a
result possibly attributable to an increase in rainfall over the past year. The spatial analysis of
burrow location was inconclusive likely because of its small sample size.
Introduction
An ecosystem engineer is defined as an organism with behaviors that modulate the
availability of resources to other individuals; creating, maintaining, or modifying unique habitat
in the process (Jones et al 1994). Through doing this, ecosystem engineers are often essential to
the health of an ecosystem; creating net increases in habitat heterogeneity as well as in species
richness and diversity (Jones et al 1997).
A prime example of this can be found in the prairie dog (Cynomys sp.). Through their
burrowing and grazing behaviors prairie dogs alter their habitat in significant ways. Their
burrowing modifies the physical and chemical composition of soil through aeration, nutrient
redistribution, addition of organic matter, and increased water filtration (Munn 1993, MartínezEstévez et al 2013, Ceballos et al 1999.). Through such alterations as well as through direct
clipping prairie dogs directly affect vegetation structure, decreasing volume as well as cover in
some habitats (Baker et al 2013). Such modifications lead to unique habitat features that alter the
local diversity and abundance of arthropod, lizard, bird, and small mammal communities
(Davidson & Lightfoot 2007, Davidson et al 2008, Augustine & Baker 2013, VanNimwegen et
al 2008).
Since the turn of the 20th century, however, prairie dogs have been extirpated from 98%
of their historic habitat (Miller et al 1994). Due to an increased understanding of the effects that
prairie dogs have on their environments, efforts have begun to reintroduce the five species into
their native ranges. To better understand how this effort will impact ecosystems that were devoid
of prairie dogs for several decades, it is important to monitor the ecosystems where
reintroductions are currently occurring.
Small mammals are a useful surrogate in indicating the broad effects prairie dogs have on
their ecosystem. This is because small mammals are directly affected by the habitat modification
caused by prairie dogs. Many species use abandoned burrows for shelter and refuge from
predators (Agnew et al 1986). Additionally, the increase in arthropod abundance and diversity
caused by the presence of prairie dogs benefits several insectivorous species of small mammals
(Davidson & Lightfoot 2007). Due both to the fact that the majority of small mammals are
herbivorous or granivorous and that some, such as kangaroo rats, prefer more open habitat
(Nowak 1991), the alteration of vegetation structure caused by prairie dogs also influences small
mammal communities. Prairie dogs may also be a source of direct competition for certain species
(Mellink & Madrigal 1993).
Of course, the presence of prairie dogs is not expected to have the same effect for all
species of small mammal, as all differ in behavior and life history. For example, it is well
documented that the northern grasshopper mouse (Onychomys leucogaster) has a strongly
beneficial affiliation with prairie dogs, most likely due to the shelter and prey they provide
through abandoned burrows and increased arthropod abundance (Agnew et al 1986). Ord’s
kangaroo rat (Dipodomys ordii) on the other hand, has been shown on occasion to have a
negative relationship with prairie dog communities, which is hypothesized to be due to
competition for resources with prairie dogs (Mellink & Madrigal 1993). The effect that prairie
dogs have on small mammals is important because many small mammals act as primary
consumers in their environment, meaning that fluctuations in their population structure will have
significant effects on the rest of the ecosystem as well.
To better understand the effect that prairie dogs have on their environment, we took a
population survey of small mammals on plots with and without reintroduced prairie dog
communities. Our research took place on the Sevilleta National Wildlife Refuge which was
historically inhabited by Gunnison’s prairie dogs until 1936 when it became a cattle ranch and
the prairie dogs were gradually extirpated. In 1973, the land was given to the U.S. Fish and
Wildlife Service and turned into a wildlife refuge. In 1997 a prairie dog reintroduction effort
began on the refuge which has continued into the present (Davidson et al 1999).
The Gunnison’s prairie dog is a lesser studied species of the genus and much of what is
known about it was originally extrapolated from the black-tailed prairie dog, a more common
species. However, these species differ in social and feeding habits, and would thus be expected
to affect their environment in different ways.
Though many studies like ours have been conducted before, the majority have been on
black-tailed prairie dogs. Through conducting our research on Gunnison’s we hoped to
determine whether the unique characteristics of the species impact the community of small
mammals in a way not seen in black-tailed prairie dog communities. To do this, we tested 1)
whether the presence of prairie dogs affected small mammal abundance and diversity and 2)
whether proximity to a prairie dog burrow entrance would affect the likelihood of trap success.
Methods
Study Area: Our study was conducted at sites on the east side of the Sevilleta National
Wildlife Refuge in central New Mexico. Our study area, classified as short grass steppe, has a
mean annual precipitation of 243 mm; 60% of which occurs during the summer. The mean
monthly temperatures for January and July are 1.5 and 25.1 ºC respectively (Davidson &
Lightfoot 2008). Gunnison’s prairie dogs were introduced into two plots of the study site in the
summer of 2010. Other small mammal species dominant in the site include Ord's kangaroo rat
(Dipodomys ordii), the banner-tailed kangaroo rat (Dipodomys spectabilis), the silky pocket
mouse (Perognathus flavus), the southern grasshopper mouse (Onychomys arenicola), the
northern grasshopper mouse (Onychomys leucogaster), and the white-footed mouse (Peromyscus
leucopus) (Davidson Parmenter & Gosz 1999).
Experimental Design: Small mammals were trapped on four different sites on the east
side of the refuge, plots A through D (see figure 1) between June 2nd and the 28th, 2014. One
week was spent trapping at each site. Two of the plots were treatment plots where prairie dogs
had been reintroduced and the other two were control plots where they had not been
reintroduced. On each plot, 169 sherman live traps were placed 25 meters apart in 13x13 grids
and labeled using numbered pin flags. During the week spent at each of the four plots, the traps
were baited with a handful of crimped oats Monday through Thursday starting no earlier than
6:45 pm to avoid catching diurnal organisms. The traps were opened starting at 5:45am Tuesday
through Friday mornings. The traps were covered with white plastic gutters to reduce the rate at
which their temperatures increased in the daylight.
Small Mammal Sampling: Animals captured in traps were identified to species, with the
exception of the Onychomys and Peromyscus genuses, and examined for previous capture, mass,
sex, reproductive status and age. Perognathus flavus and Spermophilus spilosoma were marked
by writing a number on their belly in a colored sharpie, while all other individuals were marked
using numbered ear tags. All individuals caught were processed immediately and released at the
location of the trap. All traps were closed each morning to ensure no individuals were caught
over the course of daylight hours. In addition to the sampling data collected in June of 2014,
sampling data taken using the exact same methods was available for our analysis from June of
2013.
Burrow and Trap Mapping: To analyze spatial correlations between prairie dog burrow
entrances and trap success, we created maps of each of our two treatment plots using ArcGIS.
Using a Trimble GeoExplorer, we mapped the location of every trap for each treatment plot.
Then we did the same for every prairie dog burrow entrance. The mounds were located by
walking transects between each row of traps and locating the burrow entrances by sight. In
ArcGIS, we overlaid our burrow and trap grid maps. We made 12.5 meter buffers around each
trap and tallied how many burrows fell within that range for each trap. With that data, and our
small mammal sampling data we were able to find how many burrows fell within a 12.5 meter
radius of successful traps for each species.
Statistical Analysis: Generalized Wilcoxon tests were conducted using JMP Pro 11
software in order to compare the relative amounts of small mammals on treatment and control
plots. Data pertaining to recaptured small mammals was eliminated in order to ensure that no
individual was represented more than once. The data collected from plots A and C were
combined to represent the data collected from control plots, and the data collected from B and D
were combined to represent the data collected from treatment plots. The total captures and the
by-species total captures from both data groups were compared to determine statistical
significance. For our analysis of the spatial correlation between prairie dog burrow entrances and
trap success we combined our two treatment plots. We used two sample t tests to compare each
species’ average number of within-range-burrows to the total average number of within-rangeburrows for treatment plots. Furthermore, we conducted two sample t tests to compare the
average number of within-range-burrows for total successful traps and total unsuccessful traps.
Results
A total of 677 small mammals were captured in 2014. 240 individuals were captured in
control plots and 437 were captured in treatment plots. In comparison, 324 individuals were
captured during the same trapping session in 2013, with 138 captured in control plots and 186
captured in treatment plots. A complete breakdown of small mammal captures by species can be
seen in Table 1. In both years D. merriami were only captured in treatment plots. Of species
caught in 2014, D. merriami (p=0.0316), D. ordii (p=0.0379), Onychomys sp. (p=0.0029), and P.
flavus (p=0.0303) were all significantly more abundant on the treatment plots in comparison to
the control plots (Figure 1). In 2013 the only species to differ significantly in abundance between
the treatment and control plots were D. merriami (p=0.0303), which was more abundant on
treatment plots, and D. ordii (p=0.0299), which was more abundant on control plots.
Furthermore, the total number of small mammals captured was significantly greater on the
treatment plots in 2014 (p=0.0009), but not in 2013 (p=0.1863) (Table 2). The analysis of the
density of burrows surrounding traps successful for each species showed that prairie dog burrows
were on average most dense within 12.5 meters of traps successful for D. spectabilis and least
dense surrounding traps successful for N. micropus. However, two sample T tests run for each
species showed that no species differed significantly from the total traps in their average amount
of burrows within 12.5 meters. Additionally, there was not a significant difference between total
successful traps and total unsuccessful traps in average burrow density within a range of 12.5
meters. Thus no significant spatial correlation was found between burrow location and trap
success for a 12.5 meter buffer around traps (Figure 2).
Discussion
Small Mammals: Our results indicate that the presence of Gunnison’s prairie dogs does in
fact have an observable impact on the abundance of small mammals. Silky pocket mice,
grasshopper mice, and Ord’s and Merriam’s kangaroo rats were all found to be more abundant in
plots inhabited by prairie dogs. The presence of prairie dogs increased the total abundance of
small mammals as well. With the exception of the Ord’s kangaroo rat, all three varieties of small
mammals that increased in abundance on out treatment plots were expected to do so.
Furthermore, no species either year, with the exception of the Ord’s kangaroo rat in 2013, was
more abundant on control plots than treatment plots. We therefore conclude that the presence of
prairie dogs increases small mammal abundance. We were unable to conclude that their presence
decreases small mammal diversity as we expected.
Precipitation: The total amount of small mammals captured more than doubled in 2014
as compared with 2013; an increase from 324 to 677 captures. We believe that this is at least
partially due to precipitation, which increased from a cumulative 125.5mm between June of 2012
and May of 2013 to 310.2mm during the same months between 2013 and 2014. As studies cite
drought conditions as causal for the homogenization of vegetation structures on prairie dog
colonies and the repression of the normal unique patches of habitat created by prairie dogs, it is
likely that the variation in total rainfall was an important factor in the large increase in overall
abundance of small mammals captured (Barko et al 1999). Additionally, assuming that the
increase in rainfall amplified the unique features of the habitat on our plots by differentially
benefitting various vegetation, it can also be expected that distinct small mammal species would
differentially be affected by variation in rainfall. As seen in Table 2, two species, as well as the
total overall small mammals captured, were significantly more abundant on treatment plots than
control plots in 2014 that were not significantly different in 2013. This supports the hypothesis
that more rainfall results in more unique habitat patches created by prairie dogs and, as a result,
more unique small mammal populations on prairie dog colonies.
Ord’s Kangaroo Rat: Our findings on the abundance of the Ord’s kangaroo rat
(Dipodomys ordii) on and off treatment plots conflicted with what we expected to find. As
mentioned in our introduction, the Ord’s kangaroo rat is the only species of small mammal we
captured for which we expected a negative association with the presence of prairie dogs
(Slobodchikoff 2009, Koford 1958). There is some evidence that Ord’s kangaroo rat and prairie
dogs tend to actively avoid one another, likely due to competition for resources between the two
species. In 2013, as might have been expected, Ord’s kangaroo rats were significantly more
prevalent on control plots without introduced prairie dogs. In 2014, however, they were
significantly more abundant on treatment plots with introduced prairie dogs. A possible
explanation for this is the variation in precipitation between the two years. With less rain in
2013, and subsequently less vegetation and more homogeneity, it is likely that competition
between Ord’s kangaroo rat and prairie dogs was more extreme in 2013 than in 2014.
Furthermore, Ord’s kangaroo rats have been documented to inhabit abandoned prairie dog
burrows ((Mellink & Madrigal 1993). It is possible that, with more available resources in 2014,
the benefits of the potential shelter of abandoned prairie dog burrows outweighed the risks of
competition for Ord’s kangaroo rat.
Future Focus: As shown in table (list of 2 sample t results table), our GIS analysis of
prairie dog burrow entrance density was unable to prove any significant correlations between
burrow entrance density and trap success within a 12.5 meter radius of traps. There was some
potential for error in our burrow mapping in either malfunctions of our Trimble or
misidentification of burrows, but the most limiting factor in our spatial analysis was likely our
sample sizes (with as little as two samples for some species.) An analysis of burrow density
relative to trap success might only be practical on a much larger scale than two plots of 169
traps. Although we were unable to find the trends that we were looking for with our survey size,
we think that plotting the visible burrow entrances on a yearly basis at our two treatment plots
could provide interesting data in the long term. If the total number of prairie dog burrow
entrances is used as a measurement of prairie dog fitness, it would be interesting to see if there is
any correlation between small mammal abundance and diversity and the number of burrows at
each of our treatment plots.
Because some of the species of small mammals that we captured were insectivorous, we
think that in the future researchers should add an arthropod abundance and diversity sampling
component on treatment and control plots to this study. Prairie dog burrows provide important
habitat for many species of arthropod and even food resources for some species of ant (Davidson
& Lightfoot 2007, Mackay & Mackay 2002). Furthermore, in a larger scale GIS study, prairie
dog burrow entrance density might prove to have a significant effect on carnivorous species of
small mammals because even within 10 meters of burrows, arthropod abundance and richness, in
some studies, was up to 5 times greater on mounds than on off-mound areas (Davidson &
Lightfoot 2007.)
Acknowledgements
We would like to thank our mentor, Stephanie Baker, for teaching us everything we know
about small mammals and helping us nearly every step of the way through our analysis. We
would also like to thank Amaris Swann for giving us the opportunity to participate in the
Sevilleta LTER REU and conduct research on the Sevilleta National Wildlife Refuge. We’d like
to thank Ty Werdel for conducting the small mammal sampling of our plots in 2013 and
providing us and future small mammal sampling REU students with the beginnings of our long
term data on the small mammal abundance and diversity of the Sevilleta. Additionally, we’d like
to thank UNM biology staff, Sevilleta field crew, interns, inspirational actors and our fellow
REU students, especially Ari Anada, Faith Carney, Clint Eastwood, Ariel Elliott, Wendy Flores,
Jari Javier, Megan McClung, Cassie Miller, Roy Moger-Reischer, Rina Ouelette, Shane Piaso,
Jenny Sparks, Ben Specter, Chandra Tucker, Nathan Tutchton, and Sze Wing Yu for lending us
help with our research in numerous ways throughout this summer. We’d like to thank the
institutions which provided funding, resources, study space, and bandwidth for our research; The
National Science Foundation, The Sevilleta LTER, The University of New Mexico, and the U.S.
Fish and Wildlife Service. And finally, we’d like to thank our friends, family and truck for
lending us inspiration, motivation, and transportation the whole summer long.
Appendix
2013
Species
Treatment
2014
Control
Treatment
Control
Dipodomys ordii
5
20
36
17
Dipodomys merriami
4
0
7
0
Dipodomys spectabilis
26
15
22
22
Neotoma micropus
2
3
2
4
Onychomys sp.
17
7
66
11
Peromyscus sp.
0
4
17
5
Perognathus flavus
124
83
280
179
Spermophilus spilosoma
8
6
7
2
Table 1. Amounts of each small mammal type caught on control and treatment plots in 2014 and 2013.
Species
2013
2014
Dipodomys merriami
0.0303
0.0316
Dipodomys ordii
0.0299
0.0379
Dipodomys spectabilis
0.5935
0.3526
Neotoma micropus
0.4054
0.3485
Onychomys sp.
0.0636
0.0029
Perognathus flavus
0.0734
0.0303
Peromyscus sp.
0.1709
0.2694
Spermophilis spilosoma
0.5386
0.2954
Total Small Mammals
0.1863
0.0009
Table 2. P-value results of wilcoxon tests comparing abundance of small mammals between control
and treatment plots in 2013 and 2014. Significant p-values (bolded) indicate significant differences
in abundance between treatment and control plots.
Species
Avg. Burrows within 12.5m
Dipodomys merriami
3.1667
Dipodomys ordii
3.3200
Dipodomys spectabilis
3.4375
Neotoma micropus
0.5000
Onychomys sp.
2.8025
Peromyscus sp.
1.2000
Perognathus flavus
2.3061
Spermophilis spilosoma
2.9653
Successful Traps
2.5189
Unsuccessful Traps
2.3514
Total Traps
3.2929
Table 3. The average number of prairie dog burrow entrances within 12.5 meters of successful
traps of each species of small mammals, total successful traps, total unsuccessful traps, and total
traps.
40
35
Average Capture Rate per Night
30
25
20
control
treatment
15
10
5
0
Figure 1. Average capture rate per night of each small mammal type caught on control and treatment plots in 2014
Average number of Burrows
3.5
3
2.5
2
1.5
1
0.5
0
Figure 2. Average number of burrows within a 12.5m radius of successful traps for each species. Additionally, the
average number of burrows within a 12.5 meter range of all successful, unsuccessful, and total traps in 2014.
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