Southwestern Association of Naturalists
Effects of Black-Tailed Prairie Dogs on Reptiles and Amphibians in Kansas Shortgrass Prairie
Author(s): Justin E. Kretzer and Jack F. Cully, Jr.
Source: The Southwestern Naturalist, Vol. 46, No. 2 (Jun., 2001), pp. 171-177
Published by: Southwestern Association of Naturalists
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THE
SOUTHWESTERN
NATURALIST46(2):171-177
46(2) :171-177
THE SOUTHWESTERN
NATURALIST
JUNE
JUNE2001
EFFECTS OF BLACK-TAILED PRAIRIE DOGS ON REPTILES AND
AMPHIBIANS IN KANSAS SHORTGRASS PRAIRIE
JUSTIN E. KRETZERANDJACKF. CULLY,JR.*
United States GeologicalSurvey-BiologicalResourcesDivision, Kansas CooperativeFish and Wildlife ResearchUnit,
Division of Biology, Kansas State University,Manhattan, KS 66506
* Correspondent:bcully@ksu.edu
ABSTRACT-Species diversity and abundance of reptiles and amphibians were measured on and
off black-tailed prairie dog (Cynomys ludovicianus) colonies to determine the extent to which herpetological species composition in a shortgrass prairie ecosystem is affected by presence of blacktailed prairie dog colonies. Ten species of reptiles and 3 species of amphibians were captured.
Total amphibian and reptile abundance did not differ between prairie dog colonies and noncolonized shortgrass prairie sites, but species composition did. Mean species richness, evenness,
and diversity for reptiles and amphibians were not different between treatments; however, diversity
of both treatments combined was considerably higher than diversity on shortgrass prairie without
prairie dogs. The mosaic pattern of prairie dog colonies enhances landscape heterogeneity and
contributes to greater reptile and amphibian diversity in the shortgrass prairie biome of western
Kansas.
RESUMEN-Se midieron la diversidad y la abundancia de los reptiles y anfibios dentro y fuera
de colonias de perritos llaneros de cola negra (Cynomys ludovicianus) para determinar hasta que
punto la presencia de las colonias influye en las especies de herpetol6gicas en un ecosistema de
hierba corta. Diez especies de reptiles y tres de anfibios fueron capturadas. La abundancia total
de 6stos animales fue igual dentro de las colonias como en los sitios no colonizados, pero la
composici6n de las especies fue diferente. El promedio de la riqueza, la diversidad, y el emparejamiento de las especies herpetol6gicas no difer6 entre los tratamientos. Sin embargo, hubo mayor
diversidad en ambos tratamientos combinados que en pradera de hierba corta sin perritos llaneros. El patr6n mosaico de las colonias de perritos llaneros aumenta la heterogeneidad del paisaje
y contribuye a una diversidad mas amplia de reptiles y anfibios en el ecosistema de hierba corta
en el oeste de Kansas.
Prior to European settlement,
prairie dogs
(Cynomys) were among the most numerous and
widespread
herbivores on North American
grasslands. However, abundance and distribution of the 5 species of prairie dogs have been
over the past century
reduced
dramatically
and Linder, 1978; Miller et al.,
(Summer
In 1902, there were an estimated
1990).
810,000 ha of black-tailed prairie dog (C. ludovicianus) colonies in Kansas (Lantz, 1903).
During the period 1990 to 1992, black-tailed
prairie dogs in Kansas covered only 18,843 ha,
a reduction of approximately 98% from the
original distribution (Vanderhoff et al., 1994).
The area represented less than 0.5% of noncultivated farmland and rangeland in Kansas
(Clements, 1990). Reduction of prairie dog
colonies has come about largely as a result of
government-supported eradication programs,
habitat destruction, and disease (Cully, 1993;
Miller et al., 1994).
Black-tailed
prairie dogs are capable of dracommunities
of
matically altering vegetation
that
1958;
(Koford,
they occupy
grasslands
Bonham and Lerwick, 1976; Coppock et al.,
1983; Agnew et al., 1986; Archer et al., 1987;
Weltzin et al., 1997). In mixed-grass prairies,
prairie dog colonies also influence species
composition and densities of mammals, birds,
arthropods, and soil nematodes (O'Meilia et
al., 1982; Ingham and Detling, 1984; Agnew et
al., 1986). Although many studies in mixedgrass prairies have identified large numbers of
animal and plant species in prairie dog colonies, there has been little work to identify the
degree to which prairie species are dependent
172
The SouthwesternNaturalist
on
presence of black-tailed prairie dogs
(Stapp, 1998; Winter, 1999). This has led to
concern that the tremendous reduction in
abundance and distribution of prairie dogs
over the last century has put a portion of the
biotic diversity of the shortgrass prairie at risk.
Grazing, fire, logging, and other ecological
disturbances alter abundance and diversity of
reptiles and amphibians in a wide variety of
habitats (Jones, 1981; Ballinger and Jones,
1985; Mushinsky, 1985; Lunney et al., 1991;
Greenberg et al., 1994; Ballinger and Watts,
1995). However, there is a paucity of information regarding herpetological communities in
shortgrass prairies, and little more than anecdotal reports and faunal surveys exist regarding response of reptiles and amphibians to
habitat modifications by prairie dogs (Klauber,
1982; Collins and Collins, 1991; Collins, 1993).
We suggest that habitat modifications caused
by black-tailed prairie dogs in shortgrass prairie will favor a different suite of reptile and
amphibian species than that found on unmodified prairie. Furthermore, the habitat mosaic
of colonies on the prairie should yield increased diversity over shortgrass prairie habitat
without prairie dogs. The purpose of this research was to determine the extent to which
herpetological species composition in a shortgrass prairie ecosystem is affected by presence
of black-tailed prairie dogs.
METHODS AND MATERIALS-Study Area-Study
sites were established on the Cimarron National
Grassland in Morton Co., Kansas (37?7'30"N,
102?00'00'W), from 28 June to 6 August 1996 and
15 April to 21 July 1997. Mean annual precipitation
(1901 to 1996) recorded at the Elkhart weather station in Morton Co. is 44.75 cm (National Weather
Service Cooperative Observer Network, M. Knapp,
pers. comm.). Precipitation varied greatly during the
2 years of this study. For the 12 months preceding
the 1996 sampling period, precipitation was 34.35
cm, 77% of the long-term mean. During the 12
months preceding the 1997 sampling, precipitation
was 67.97 cm, 150% of the long-term mean.
Two treatments were established: in 1996, sites
were selected on 5 black-tailed prairie dog colonies
and 5 areas in pastures not colonized by prairie dogs
(control). During the 1997 trapping season, 2 more
study sites were added to each treatment, for a total
of 7 study sites within each treatment. All study sites
were located north of the Cimarron River on flat
upland areas dominated by shortgrass prairie plant
species. Study sites from each treatment were inter-
vol. 46, no. 2
mixed across the landscape, and were not paired.
Maximum distance between any 2 study sites was 24
km.
Sites occupied by prairie dogs were selected because they were among the largest prairie dog colonies on the Cimarron National Grasslands; they varied from 30 to 73 ha. Similar-sized areas were selected from control pastures without prairie dogs.
Control sites were selected where vegetation was a
shortgrass community type dominated by buffalograss (Buchloe dactyloides)and blue grama (Bouteloua
gracilis). Study sites from both treatments had similar
topography, land-use history, and soil types, which
were commonly dominated by Ulysses silt loam and
Richfield silt loam types (Dickey et al., 1963). In
June 1997, plague (Yersiniapestis) was confirmed on
1 of the prairie dog colonies (Cully et al., 2000).
Adult and juvenile prairie dogs were present there
in late April, but by the end of May 1997 we were
unable to locate any prairie dogs at this study site.
Despite this circumstance sampling continued at this
site in 1997.
This study was conducted in concert with a similar
study examining influence of prairie dogs on plant
community structure and composition (Winter,
1999). We used a nonparametric Wilcoxon exact test
(c = 0.05) to statistically test for differences in canopy cover of live grass and live forbs, canopy cover
and frequency of each plant species, and percent
bare ground on and off prairie dog colonies during
the periods 1 July to 18 July 1996 and 18 June to 2
July 1997, and vegetation height and density during
27 May to 18 June 1997. Only values obtained from
study sites that corresponded between this study and
Winter's (1999) study were used to characterize vegetation differences between the two treatments.
Sampling-Reptiles and amphibians were sampled
using pitfall and funnel trap arrays. Trap arrays were
located in each study site by randomly choosing locations on aerial photograph/grid overlays. Criteria
(e.g., distance between traps; distance from study
site boundary) were not used when determining
trap locations. During summer 1996, 2 trap arrays
were randomly located in each of the 10 study sites
for a total of 20 arrays. Traps were continuously operated at each study site from 28 June through 6
August 1996. In 1997, two more study sites were added to each treatment, and the number of trap arrays
was increased to 3 arrays per study site, for a total of
42 arrays. These traps were continuously operated at
each study site from 15 April through 21 July 1997.
Trap arrays consisted of 3 5-m drift fences, arranged 120 degrees apart, each converging at a central 5-gallon pitfall trap. Funnel traps were located
at the outer ends of the drift fences. Shade boards
were used over the pitfall traps and cotton sheets
were wrapped around funnel traps to shelter cap-
June 2001
Kretzerand Cully-Effects of prairiedogs on reptiles and amphibians
173
TABLE 1-Total number of individuals captured in trap arrays in prairie dog colonies and non-colonized
control sites, Cimarron National Grasslands, Morton Co., Kansas, during the periods 28 June to 6 August
1996 and 15 April to 21 July 1997. In 1996, each treatment included 5 sites, each with 2 arrays (20 arrays
total). In 1997, each treatment included 7 sites, each containing 3 arrays (42 total arrays). One each of
Cnemidophorussexlineatus, Eumeces obsoletus,and Lampropeltistriangulum was captured in the prairie dog colonies, and Tantilla nigricepswas captured once on the control plots.
Prairie dog colonies
Species
Ambystomatigrinum
Spea bombifrons
Bufo woodhousii
Terrapeneornata
Holbrookiamaculata
Phrynosoma cornutum
Coluberconstrictor
Pituophis catenifer
Crotalus viridis
Control sites
1996
1997
Totals
1996
1997
Totals
4
6
3
1
5
0
2
2
1
3
54
0
4
11
17
8*
6
13*
7
60
3
5
16
17
10*
8
14
0
9
0
0
0
10
6
3
11*
1
41
8
0
1
21
76*
5
1
50
8
0
1
31
82*
8
2*
* Indicates a
significant difference (P < 0.05) between the control plots and the prairie dog colonies as
measured by the Wilcoxon Exact Test.
tured animals from the sun but still allow circulation
of air through the trap.
Traps were checked and captured animals were
released daily. Before release, captured lizards,
toads, and salamanders were marked by toe clipping
as described by Martof (1953), with the exception
that thumbs were not clipped on toads and long
hind toes were not clipped on lizards; snakes were
marked by belly scale clipping (Brown and Parker,
1976); and turtles were marked by notching marginal plates of the carapace (Cagle, 1939). Individuals
that were recaptured were counted only once.
Analysis-Relative abundance (number of individuals captured per 100 trap array days), species richness, Shannon's diversity index (H') and evenness
measure based on Shannon's diversity index (f)
were calculated for each study site. Data from 1996
and 1997 were analyzed separately, and mean values
were compared statistically between treatments using a nonparametric Wilcoxon exact test (oc = 0.05)
computed by SAS v. 6.11 (1996). Diversity index
equations were obtained from Brower et al. (1990).
Species that were captured only once were excluded
from analysis.
Horn's index of community similarity (Ro) was
used to determine the degree of likeness between
the two treatments. Horn's index varies from 0 to 1
with low values if species are mostly different, and a
high value when the species composition and relative abundances are similar (Horn, 1966; Brower et
al., 1990).
RESULTS-In 1996, black-tailed prairie dog
colonies differed from control sites in that they
had higher percent cover of live forbs (P =
0.03), greater percent cover and increased frequency of purple three-awn (Aristida purpurea;
P = 0.008; P = 0.04), and lower percent cover
and frequency of blue grama (P = 0.008; P =
0.008). In 1997, prairie dog colonies had lower
vegetation height (P = 0.004) and density (P
= 0.007),
greater percent cover and frequency
of purple three-awn (P = 0.002; P = 0.01) and
tumblegrass (Schedonnardus paniculatus; P =
0.04; P = 0.001),
dropseed
reduced
frequency
of sand
(Sporobolus cryptandrus; P = 0.02),
and lower percent cover of prairie coneflower
(Ratibida columnifera; P = 0.03). As a result of
prairie dog burrowing activities, large amounts
of bare soil were exposed, but total amount of
bare ground was not significantly different between prairie dog colonies and control sites
(Winter, 1999).
During 1996 and 1997, we captured 327 reptiles and amphibians
(Table 1). These included 10 species of reptiles and 3 species of amphibians. Nine reptile and 3 amphibian species
on the prairie dog colonies,
were recorded
whereas 6 reptiles and 3 amphibians
were
found on the control sites. The most common
species observed, in both prairie dog colonies
and control sites, was the plains spadefoot toad
29.7% and
(Spea bombifrons) representing
34.9% of all captures in 1996 and 1997, respectively. In 1996, 9 species were recorded on
174
vol. 46, no. 2
The SouthwesternNaturalist
prairie dog colonies, with 5 species, tiger salamander (Ambystoma tigrinum), Woodhouse's
toad (Bufo woodhousii), ornate box turtle (Terrapene ornata), lesser earless lizard (Holbrookia
maculata), and a six-lined racerunner (Cnemidophorus sexlineatus; n = 1), recorded only on
the prairie dog colonies. Five species were recorded on control sites, with one species, Texas
horned lizard (Phrynosomacornutum), recorded
only on control sites.
In 1997, 10 species were recorded on prairie
dog colonies, with 3 species, ornate box turtle
(T. ornata), Great Plains skink (Eumeces obsoletus, n = 1), and milk snake (Lampropeltistriangulum, n = 1), recorded only on prairie dog
colonies. Nine species were recorded on control sites, with 2 species, Woodhouse's toad (B.
woodhousii) and plains blackhead snake (Tantilla nigriceps,n = 1), recorded only on control
sites.
In 1996, numbers of reptiles and amphibians
captured were relatively low, so it was difficult
to detect differences in relative abundance of
individual species. Larger sample sizes and
higher capture rates in 1997 resulted in detection of statistical differences in species composition between treatments. During 1997, fewer eastern yellowbelly racers (Coluberconstrictor;
P = 0.02) occurred on prairie dog colonies
than on control sites. Woodhouse's toads,
found only on prairie dog colonies in 1996,
only occurred on control sites in 1997. Conversely, prairie rattlesnakes (Crotalus viridis)
were more abundant (P = 0.005) on prairie
dog colonies than on control sites.
During 1996, tiger salamanders and lesser
earless lizards were unique to prairie dog colonies, whereas in 1997 they were found on
both treatments. Texas horned lizards were
only found on control sites in 1996, but were
common on both treatments in 1997.
Total amphibian and reptile relative abundance was not different (P > 0.05) between
the prairie dog colonies and control sites for
either 1996 or 1997. Mean species richness,
evenness, and diversity did not differ (P >
0.05) between the prairie dog colonies and
control sites for 1996 or 1997; however, when
data from both treatments were pooled across
all study sites, total diversity was 0.85 (compared to 0.36 on prairie dog colonies and 0.33
on control sites) in 1996, and 0.72 (compared
to 0.48 and 0.47) in 1997 (Table 2). Horn's
TABLE 2-Mean species richness, evenness (f'),
and diversity(HT)in prairie dog colonies and noncolonized control sites, Cimarron National Grasslands, Morton Co., Kansas, during the periods 28
June to 6 August 1996 and 15 April to 21 July 1997.
Species that were captured only once (Cnemidophorus
sexlineatus, Eumeces obsoletus, Lampropeltistriangulum,
and Tantillanigriceps)were excluded from analysis.
Differences in mean values between the prairie dog
colonies and control sites were not significant (P >
0.05) as measured by the Wilcoxon Exact Test.
TreatPrairiedog
ments
colonies Control sites comVariable Year mean + SE mean ? SE bined
Richness 1996 2.80 ? 0.58 2.60 + 0.51
1997 4.43 + 0.75
4.29 + 0.36
Evenness 1996 0.73 + 0.18 0.71 + 0.18
1997 0.70 + 0.13
9.00
9.00
0.89
0.75 + 0.03
0.75
Diversity 1996 0.36 + 0.11 0.33 + 0.09
0.85
1997 0.48 + 0.10
0.47 + 0.04
0.72
index values of community similarity for prairie dog colonies and control sites were relatively low to moderate, with index values of 0.53
and 0.73 for 1996 and 1997, respectively.
DISCUSSION-Collins and Collins (1991) verified the occurrence of 31 species of reptiles
and amphibians on the Cimarron National
Grasslands in Morton Co., Kansas. We did not
capture any species new to the Cimarron National Grasslands; however, we did capture 3
species (E. obsoletus, C. constrictor,and L. triangulum) whose occurrence has not been documented previously on black-tailed prairie dog
colonies (Reading et al., 1989; Kotliar et al.,
1999).
Black-tailed prairie dogs create areas of habitat that are clearly different from surrounding
non-colonized shortgrass prairie of western
Kansas. It is difficult to collect sufficient numbers of reptiles and amphibians from xeric
grassland habitats to draw sure conclusions
about similarities or differences in communities that result from habitat modifications
caused by prairie dogs or other sources of disturbance. Although we captured more than
300 animals, some species were rare; 3 species
were represented by a single specimen. Despite
the modest samples of many species, we were
able to identify measurable effects of prairie
June 2001
Kretzer and Cully-Effects
of prairie dogs on reptiles and amphibians
dogs on reptile and amphibian species and
communities, and some of these effects differed between years.
The racer was the only reptile or amphibian
species we observed that appeared to be negatively influenced by black-tailed prairie dogs.
Tiger salamanders and Woodhouse's toads
were only found on prairie dog towns at the
end of a drought in 1996. Prior to sampling in
1997, fall and spring precipitation increased
more than 56% from the previous year. During
1997 sampling, tiger salamanders were found
on both treatments, whereas Woodhouse's
toads were captured only on control sites.
Plains spadefoot toads did not differentiate
shortgrass prairies altered by the presence of
prairie dogs and their burrows.
Earless lizards appeared only on prairie dog
towns in 1996, and only one was taken on a
control plot in 1997. Earless lizards prefer areas with loose soils and little or no vegetation,
and will use mammal burrows to escape extremely hot temperatures (Collins, 1993). In
Cheyenne Co., Kansas, Knight and Collins
(1977) found this lizard most abundant
around prairie dog colonies. In the Nebraska
sandhills, these lizards were associated with
blowout/washout areas, and a drastic reduction of earless lizards occurred after the elimination of cattle grazing (Ballinger and Jones,
1985; Ballinger et al., 1990; Ballinger and
Watts, 1995). Vegetation and soil disturbances,
which are typical of prairie dog colonies, appear to be critical habitat features for earless
lizards.
Texas horned lizards were not found on
prairie dog colonies in 1996. In 1997, number
of horned lizards captured was similar on and
off prairie dog colonies despite lower height
and density of the vegetation on prairie dog
colonies (Winter, 1999). Spatial distribution of
horned lizards is influenced by vegetative
structure and, to a lesser degree, by the presence of harvester ants (Pogonomyrmex).Some
studies have reported that horned lizards selected disturbed areas with less vegetative cover
(Whiting et al., 1993; Fair and Henke, 1997).
Harvester ants are the horned lizard's primary
food source, and in 1997 total density of harvester ant mounds was not different between
prairie dog colonies and non-colonized shortgrass prairie sites on the Cimarron National
Grasslands (Kretzer and Cully, 2001).
175
The eastern yellowbelly racer was the most
abundant snake trapped, with 92 individuals
captured during 1996 and 1997. The species is
common throughout grasslands of Kansas
(Collins, 1993; Fitch, 1993). Prior to this study,
only 6 specimens of racer were collected in
Morton Co., Kansas, despite a number of intensive collecting expeditions (Collins and Collins, 1991). This species was 8 times as abundant on control sites compared to prairie dog
colonies. Structural difference in vegetation
between treatments is likely responsible for the
racer being more abundant on control sites.
Fitch (1963) reported that grazed, mowed, or
burned grasslands do not provide adequate
food or shelter for the racer. Prairie dog colonies may be attractive hunting grounds for
raptors and mammalian carnivores (Campbell
and Clark, 1981; Clark et al., 1982; Sharps and
Uresk, 1990; Hoogland, 1995), and the racer
may be particularly vulnerable to predation on
prairie dog colonies that lack substantial cover.
Raptors were observed carrying snakes on 2
occasions in 1997. Of the 8 racers captured on
prairie dog colonies in 1997, 5 were captured
on the prairie dog colony site infected with a
plague epizootic where height of the vegetation was greater than that of other prairie dog
colony sites (J. E. Kretzer, pers. obser.).
Prairie rattlesnakes and bullsnakes were less
abundant than racers but similar in abundance
to each other. Bullsnakes appear to be indifferent to the presence of prairie dog colonies.
In contrast to racers, prairie rattlesnakes were
captured more often on prairie dog colonies.
The prairie rattlesnake is probably not as vulnerable to predation as the racer because of its
secretive behavior and poisonous bite. Consequently, the lack of overhead canopy cover on
prairie dog colonies may not have a negative
influence on rattlesnakes. Cox and Franklin
(1989) found that rattlesnakes in Nebraska
were commonly associated with shorter, sparser vegetation than were racers. Additionally,
rattlesnakes are more sensitive to heat than
racers (Fitch, 1963), and will use prairie dog
burrows to escape extreme daytime ambient
temperatures (Klauber, 1982; Collins, 1993).
All of the 13 species captured in this study
have been reported to use mammal burrows
and other below-ground habitats to escape extreme temperatures (Collins, 1993). Klauber
(1982) reported use of prairie dog burrows by
176
The SouthwesternNaturalist
rattlesnakes
as hibernation
dens. Perhaps a
number of species use prairie dog burrows as
hibernation
sites during winter months and
then disperse across the landscape after spring
Future research may demonstrate
emergence.
that prairie dog burrows provide critical refuge
for other species of reptiles and amphibians
on below-ground
habitats for windependent
ter survival.
Use of prairie dog altered habitat by individual species varied by year. Horn's index values
of community similarity also indicate dissimilarity between prairie dog colonies and surrounding shortgrass habitat. The combination
of prairie dog colonies and control sites resulted in considerably higher species diversity values in the shortgrass prairie ecosystem than either treatment had on its own. This comparison demonstrates
that reptile and amphibian
diversity on prairie dog colonies at the individual colony scale is similar to the shortgrass
prairie matrix. However, a mosaic patchwork of
prairie dog colonies on a shortgrass prairie matrix increases
and
landscape
heterogeneity
contributes
to greater reptile and amphibian
diversity patterns than would occur in the absence of prairie dogs.
Financial support was provided by the United
States Fish and Wildlife Service, United States Forest
Service, Kansas Cooperative Fish and Wildlife Research Unit, and Kansas State University, Division of
Biology. Field assistance and data entry were provided by E. Agner, P. Lemons, D. Martin, S. Meyers, M.
Wong,J. Wertz, D. Winslow, and S. Winter. L. Calvert
helped with the Spanish abstract. We appreciate review comments of J. Cavitt, A. Davidson, D. Kaufman, D. Margolies, D. Olson, J. Pontius, and N.
Scott, which greatly improved the manuscript.
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Submitted26January 2000. Accepted24 July 2000.
AssociateEditor was GeoffreyC. Carpenter