Ecology, 89(12), 2008, pp. 3298–3305
Ó 2008 by the Ecological Society of America
ECOSYSTEM ENGINEERING BY A COLONIAL MAMMAL:
HOW PRAIRIE DOGS STRUCTURE RODENT COMMUNITIES
RON E. VANNIMWEGEN,1,4 JUSTIN KRETZER,2
AND
JACK F. CULLY, JR.1,3
1
Division of Biology, Kansas State University, Manhattan, Kansas 66506 USA
Integrated Training Area Management (ITAM), Fort Sill, Oklahoma 73503 USA
3
U.S. Geological Survey, Kansas Cooperative Fish and Wildlife Research Unit, Kansas State University,
Manhattan, Kansas 66506 USA
Reports
2
Abstract. As ecosystem engineers, prairie dogs (Cynomys spp.) physically alter their
environment, but the mechanism by which these alterations affect associated faunal
composition is not well known. We examined how rodent and vegetation communities
responded to prairie dog colonies and landcover at the Cimarron National Grassland in
southwest Kansas, USA. We trapped rodents and measured vegetation structure on and off
colonies in 2000 and 2003. We plotted two separate ordinations of trapping grids: one based
on rodent counts and a second based on vegetation variables. We regressed three factors on
each ordination: (1) colony (on-colony and off-colony), (2) cover (shortgrass and sandsage),
and (3) habitat (factorial cross of colony 3 cover). Rodent communities differed by colony but
not cover. Vegetation differed across both gradients. Rodent responses to habitat reflected
those of colony and cover, but vegetation was found to differ across cover only in the sandsage
prairie. This interaction suggested that rodent composition responded to prairie dog colonies,
but independently of vegetation differences. We conclude that burrowing and soil disturbance
are more important than vegetation cropping in structuring rodent communities.
Key words: black-tailed prairie dogs; community composition; Cynomys spp.; ecosystem engineer;
nonmetric multidimensional scaling; ordination; rodents; vegetation structure.
INTRODUCTION
Keystone species are defined by the effects they have
on neighboring biota, whereas ecosystem engineers are
defined by the mechanisms, or habitat alterations, that
cause such effects. Prairie dogs (Cynomys spp.) are
deemed keystone species because many neighboring taxa
are affected by their presence (Power et al. 1996, Kotliar
2000). They are also considered ecosystem engineers
(sensu Jones et al. 1994, 1997), because they physically
alter their habitat by cropping non-woody vegetation
(Coppock et al. 1983, Agnew et al. 1986, Whicker and
Detling 1988), disturbing soil surfaces (Bangert and
Slobodchikoff 2000), and digging extensive burrow
networks (Ceballos et al. 1999). More is known about
the effects prairie dogs have on other taxa than the
specific processes by which those effects are produced.
Prairie dogs’ keystone effects are documented at a
variety of spatial scales. Regional predators may be
more abundant at broad scales; e.g., avian raptors,
coyotes, swift foxes, badgers, and skunks are more
abundant in regions with prairie dogs than in those
without (Smith and Lomolino 2003). Arthropod diversity was higher at broad regional scales (Bangert and
Slobodchikoff 2006). Other species benefit at the local
Manuscript received 12 September 2007; revised 28 February
2008; accepted 15 July 2008. Corresponding Editor: D. R.
Strong.
4
E-mail: vanron@ksu.edu
‘‘colony’’ scale (see Plate 1). Burrowing owls, songbirds,
invertebrates, reptiles, and amphibians have higher
abundance on colonies, possibly utilizing burrows for
dens and refugia (Kotliar et al. 1999, Kretzer 1999,
Kretzer and Cully 2001a, b, Bangert and Slobodchikoff
2004, Smith and Lomolino 2004). Rodents are likely to
respond to prairie dog activities at these finer scales,
directly on colonies. Rodent abundance and diversity
varied between on and off colonies across the geographic range of black-tailed prairie dogs (Cynomys ludovicianus), and ordinations of rodent associations indicated
that species composition differed on vs. off colonies
(J. F. Cully, S. K. Collinge, W. C. Johnson, R. E.
VanNimwegen, C. Ray, B. Thiagarajan, D. B. Conlin,
and B. Holmes, unpublished manuscript).
Despite the abundant evidence of keystone effects, the
engineering question remains; i.e., how do prairie dogs
affect rodent communities? If communities are driven by
vegetation cropping, we would expect two patterns: (1)
prairie dog colonies with similar rodent communities
should also have similar vegetation and (2) if prairie dogs
do not change the vegetation from surrounding habitat,
the rodent community should not differ either. Otherwise, if burrowing and soil disturbance structure rodent
communities, we expect their composition to differ
between on and off colony sites regardless of vegetation
differences. We present analyses that test these predictions on and off prairie dog colonies in a shortgrass
prairie. By separately analyzing vegetation structure and
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3299
rodent composition, we assessed the relative importance
of prairie dogs’ grazing and burrowing activities. We also
identify those rodent species most responsible for the
patterns observed, and discuss their possible roles as
mediators in rodent communities.
METHODS
The 43 700-ha Cimarron National Grassland in
southwest Kansas, USA is composed of two primary
landcover types bisected by the Cimarron River (Fig. 1).
Here we borrow a generalized characterization of these
cover types from the ‘‘Cimarron and Comanche National
Grasslands Land Management Plan’’ (U.S. Department
of Agriculture, Department of Forestry 2007). ‘‘Sandsage prairie’’ occurs mostly south of the river and is
dominated by bunch-type grasses such as Sporobolus
cryptandrus, Eragrostis trichodes, Andropogon gerardii,
Schizachyrium scoparium, Andropogon hallii, and Panicum virgatum. Shrubs such as Artemesia filifolia, Opuntia
polyacantha, and Yucca glauca add an abundant woody
component. Primarily north of the river, ‘‘shortgrass
prairie’’ has fewer shrubs, and is dominated by sod-type
grasses such as Bouteloua gracilis, Buchloe dactyloides,
Pleuraphis jamesii, Aristida purpurea, and Pascopyrum
smithii. These two distinct cover types comprise a
vegetation gradient with the potential to shape rodent
composition via vegetation structure.
We sampled rodents and vegetation at nine sites in
2000 and six sites in 2003. Each site included a pair of
trapping grids; one located directly on a prairie dog
colony and the other placed in the surrounding offcolony vegetation, 0.5 to 2.0 km away. Both trapping
grids at any given site were located in same landcover
type; i.e., shortgrass or sandsage prairie (Fig. 1). Each
grid had 49 Sherman live traps (7.6 3 8.9 3 22.9 cm),
arranged in a 7 3 7 square with 20-m spacing. Oatmealbaited traps were set at sunset and checked each sunrise
over a period of three days. We identified captured
rodents to species level and released them with uniquely
numbered ear tags. Counts for each species were
recorded as the number of unique captures per grid
(i.e., all individuals not already tagged during those
three days). Longer trapping sessions conducted in this
grassland (.5 days, more typical of rodent studies)
usually failed to indicate more species than those we
observed in this study (R. E. VanNimwegen, unpublished
data). Trapping was analyzed only during the seasons
when concurrent vegetation was available: two seasons
in 2000 (spring and summer) and one season in 2003
(summer only).
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FIG. 1. Locations of nine study sites, numbered 21 to 29, sampled in 2000 (only sites 21–26 were sampled in 2003), each with a
pair of trapping grids; one on-colony (solid symbols) and one off-colony (open symbols). Two predominant landcover types
(‘‘shortgrass prairie’’ and ‘‘sandsage prairie’’) comprise the Cimarron National Grasslands in southwest Kansas, USA.
RON E. VANNIMWEGEN ET AL.
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Ecology, Vol. 89, No. 12
TABLE 1. Counts of seven rodent species in each of four habitats on Cimarron National Grassland in 2000 and 2003.
Habitat
On-colony shortgrass
(n ¼ 10 and 6)
On-colony sandsage
(n ¼ 8 and 6)
Off-colony shortgrass
(n ¼ 10 and 6)
Off-colony sandsage
(n ¼ 8 and 6)
Onychomys
leucogaster
Chaetodipus
hispidus
Peromyscus
maniculatus
Dipodomys
ordii
Reithrodontomys
spp.
Perognathus
flavus
Neotoma
micropus
67
7
3
0
1
3
0
41
3
7
12
4
0
1
4
20
6
0
2
4
0
24
25
20
11
9
0
1
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Note: Two trapping sessions of three nights each were summed over 18 and 12 trapping grids (n ¼ number of grids in 2000 and
2003, respectively).
At each trap, we measured five vegetation variables.
We visually estimated the percent cover of grasses, forbs,
shrubs, and bare ground within a 1-m2 quadrat placed
next to each trap. Vegetation height was measured as the
highest 5-cm section of a Robel pole with .50%
obstruction by any form of vegetation. We averaged
each variable across the 49 trap locations for each grid,
and sampled each grid once during each trapping season
as outlined above.
We used nonmetric multidimensional scaling (NMDS;
Kruskal 1964) to ordinate sample units (trapping grids)
using the function metaMDS in the vegan package
(Oksanen et al. 2008) with R for Statistical Computing
(R Development Core Team 2008). We used the BrayCurtis distance metric based on its high performance in
detecting ecological gradients in simulations (Faith et al.
1987). We applied Wisconsin double standardization to
the vegetation variables because they differed in units
(Legendre and Gallagher 2001), and rodent counts were
square-root transformed to down weight the few
exceedingly abundant species.
NMDS is a distance-based ordination technique that
graphically arranges sample units according to their
rank order of ecological distance; hence, two points
located close together on an ordination plot represent
two trapping grids with similar rodent composition or
vegetation structure. We created two ordination plots,
one based on rodent species composition and one based
on vegetation variables. Since each trapping grid could
be characterized by its landcover and/or colony status,
we fit these categorical variables to the plots using the
function envfit in the vegan package with R. We chose
three variables to represent the effects pertaining to our
hypothesis. These categories included (1) colony (oncolony and off-colony), (2) cover (shortgrass and
sandsage), and (3) habitat (colony 3 cover: on-colony
shortgrass, off-colony shortgrass, on-colony sandsage,
and off-colony sandsage). Colony represented variation
between on- and off-colony grids, cover represented
variation due to the landcover classification, and habitat
provided the factorial combinations required to detect
interactions between colony and cover. On an ordination plot, the envfit function fits a centroid to each level
of a categorical variable and calculates an R2 value as a
measure of separation among the different levels of that
variable. Additionally, a significance value for the R2
was calculated using 1000 random permutations of the
category levels. As an example, if we ordinated rodent
abundance and found a significant R2 when fitting the
colony variable, we inferred that rodent communities
differed between on and off colony grids.
In NMDS, stress is a goodness-of-fit measure to
determine the number of dimensions needed to adequately portray between-grid similarities (Kruskall
1964). Using Borg and Patrick (2005) as a guide, we
used three dimensions for the rodent data (stress ¼ 14%);
however, our figures only show the first two dimensions
because upon inspection, the third dimension did not
alter our results. Vegetation was well represented in two
dimensions (stress ¼ 11%).
RESULTS
We captured a total of seven rodent species (Table 1).
For the colony variable, rodent composition differed
between on-colony and off-colony (R2 ¼ 0.22, P , 0.001;
Fig. 2a), and so did vegetation (R2 ¼ 0.09, P ¼ 0.023; Fig.
2a). For the cover variable, rodent communities were
not different between shortgrass and sandsage (R2 ¼
0.01, P ¼ 0.780; Fig. 2b), but vegetation did differ (R2 ¼
0.36, P , 0.001; Fig. 2b).
The habitat variable revealed an interaction between
cover and colony for vegetation structure but not for
rodent composition. Rodent composition was similar
between on-colony shortgrass and on-colony sandsage,
and was similar between the off-colony shortgrass and
off-colony sandsage. All remaining combinations differed (Table 2A, Fig. 3). Vegetation structure was similar
between on-colony shortgrass and off-colony shortgrass,
but all other combinations differed (Table 2B, Fig. 3). In
other words, we detected a colony effect in rodent
composition in both landcover types, but colony only
affected vegetation structure in the sandsage landcover.
The habitat differences in vegetation structure are
easier to visualize with photographs taken from four
randomly chosen grids in our study area, each representing one level of habitat (Fig. 4). Three correlative
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3301
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FIG. 2. Nonmetric multidimensional scaling (NMDS) of trapping grids based on (a) rodent abundance and (b) vegetation
structure. Solid circles denote on-colony grids; open circles denote off-colony grids. The variable colony is plotted as centroids
(þ symbols) and 95% confidence ellipses of the mean sample score. (c and d) The same NMDS ordinations of trapping grids. The
solid circles denote sandsage landcover, and open circles denote shortgrass landcover. The variable cover is plotted as centroids
(þ symbols) and 95% confidence ellipses of the mean sample score. Confidence ellipses are for visualization purposes only; actual
significance tests are based on variable fitting functions.
relationships occurred among rodents, vegetation, and
prairie dog colonies: (1) on-colony and off-colony
rodent communities were different whether or not the
vegetation differed between the two, (2) on-colony
rodent communities were the same when the vegetation
among those colonies was different, and (3) off-colony
rodent communities did not vary across the landcover
gradient in the grassland.
RON E. VANNIMWEGEN ET AL.
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Ecology, Vol. 89, No. 12
TABLE 2. Comparisons among four levels of the categorical habitat variable regressed on nonmetric multidimensional scaling
(NMDS) ordinations for (A) small mammal communities and (B) vegetation structure in Cimarron National Grassland in 2000
and 2003.
Habitat
On-colony shortgrass
On-colony sandsage
Off-colony shortgrass
A) Rodents
On-colony shortgrass
On-colony sandsage
Off-colony shortgrass
Off-colony sandsage
0.06 (0.240)
0.28 (,0.001)
0.19 (0.001)
0.28 (,0.001)
0.25 (0.001)
0.07 (0.224)
B) Vegetation
On-colony shortgrass
On-colony sandsage
Off-colony shortgrass
Off-colony sandsage
0.24 (0.003)
0.06 (0.239)
0.67 (,0.001)
0.19 (0.024)
0.33 (0.004)
0.54 (,0.001)
Off-colony sandsage
2
Notes: Values are R with associated significance (P) in parentheses. Boldface entries indicate habitat levels with significantly
similar communities.
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DISCUSSION
Prairie dog presence affected rodent composition
throughout the Cimarron National Grassland, a pattern
consistent with several studies showing keystone relationships between prairie dogs and other taxa (Kotliar et
al. 1999, Kretzer 1999, Kretzer and Cully 2001a, b,
Smith and Lomolino 2003, 2004). Our analysis took
advantage of a grassland-scale landcover gradient and
found that both rodents and vegetation responded to
prairie dog colonies, but in differing ways. Of the
engineering activities we ascribed to prairie dogs, we
were able to separate the vegetation and burrowing
effects found on colonies.
We predicted two patterns from a vegetation-driven
engineering effect: (1) colonies with similar rodent
composition should also have similar vegetation, and
conversely, (2) in areas where prairie dogs do not change
the vegetation from surrounding habitat, the rodent
composition should not change either. Our observed
patterns deviated from those expectations. First, rodent
communities did not respond to the landcover gradient
with its pronounced vegetation differences; rodent
communities were similar on all prairie dog colonies,
FIG. 3. Nonmetric multidimensional scaling (NMDS) of trapping grid sessions based on (a) rodent abundance and (b)
vegetation structure. Four levels of the categorical variable habitat are plotted as centroids labeled with (þ) symbols and include
95% confidence ellipses of the mean sample score. Confidence ellipses are for visualization purposes only; actual significance tests
are based on variable fitting functions.
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regardless of the landcover type, and the same trend held
for rodent communities off colonies. Second, the
homogeneous vegetation in the shortgrass prairie,
regardless of prairie dog presence, contained distinctly
different rodent communities between on and off colony
sites.
A conspicuous feature of prairie dog colonies is the
contrast of their vegetation structure compared to
surrounding areas (Weltzin et al. 1997, Winter et al.
2002). In the shortgrass prairie, the vegetation did not
differ in the presence of prairie dogs visually (Fig. 4b) or
statistically (Fig. 4a). If we were to experimentally
induce a similar gradient, at least in terms of vegetation
density, we would likely mow the off-colony grids. On
this grassland, however, cattle-grazing was a common
land use, effectively inducing that homogeneity for us.
The cattle were allowed to graze equally in both
landcover types (Nancy Brewer, personal communication); however, grazing effects were different between the
two areas. In the shortgrass prairie, both cattle and
prairie dogs tend to crop the grasses uniformly low to
the ground. In contrast, sandsage vegetation differs
across the on-off prairie dog gradient, possibly due to
the greater cover of shrubs. Cattle might only consume
what is palatable; whereas prairie dogs should crop all
the vegetation they can to increase visibility (Koford
1958, Hoogland 1995). Indeed, shrub cover is lower on
prairie dog colonies in the shrub-rich sandsage prairie
(Wilcoxon W ¼ 21.5, P ¼ 0.011). Regardless of these
proposed differences in cattle and prairie dog grazing
preferences, we have shown that vegetation structure in
sandsage prairie is different between on and off colony
grids.
Turning our attention to the burrowing and soil
disturbance of prairie dogs, the impact of such effects
might be related to the dominant rodent species’ life
histories. We caught more grasshopper mice (Onychomys leucogaster) on colonies than off colonies, whereas
deer mice (Peromyscus maniculatus) were common on
both. In geographical regions where grasshopper mice
are less common, deer mice were more common on
colonies than off colonies (Thiagarajan 2006; J. F. Cully,
S. K. Collinge, W. C. Johnson, R. E. VanNimwegen, C.
Ray, B. Thiagarajan, D. B. Conlin, and B. Holmes,
unpublished manuscript). In our study area, deer mice
and other species might be displaced, possibly due to the
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FIG. 4. Photographs of trapping grids representing four levels of the habitat variable: (a) off-colony sandsage, (b) off-colony
shortgrass, (c) on-colony sandsage, and (d) on-colony shortgrass. Photos with the same number of V’s are not significantly different
with respect to vegetation; those with the same number of M’s do not differ with respect to rodents.
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Ecology, Vol. 89, No. 12
PLATE 1. A burrowing owl takes up close residence with a black-tailed prairie dog at Cimarron National Grassland in
southwest Kansas (USA). Photo credit: R. E. VanNimwegen.
carnivorous habits of grasshopper mice (Escogue 1960,
Flake 1973, Stapp 1997b, Thiagarajan 2006). Kangaroo
rats (Dipodomys ordii) were found more often on
colonies; their speed and size might allow them to
coexist with grasshopper mice (but see Rebar and
Conley 1983, Stapp 1997a). Because grasshopper mice
are highly insectivorous (Jahoda 1970, Flake 1973), the
species might associate with Coleopterans that are more
common on colonies (Kretzer 1999). Grasshopper mice
also prefer disturbed soils (Escogue 1960, Stapp 1997a)
for dust bathing. Most species caught on colonies nest
below ground, and colonies have existing tunnels with
loosened soil that is easily excavated. Of the less burrowdependent species, hispid pocket mice (Chaetodipus
hispidus) dominated the off-colony rodent communities,
and though silky pocket mice (Perognathus flavus) and
harvest mice (Reithrodontomys spp.) were uncommon
overall, they were rarely found on colonies. Deer mice
were equally abundant on and off colonies.
When we performed univariate tests for abundance on
and off colonies, only grasshopper mice and hispid
cotton mice showed significant differences, using two
sample Wilcoxon tests (W ¼ 226.00, df ¼ 46, P , 0.001
and W ¼ 10.50, df ¼ 46, P ¼ 0.001, respectively). If either
of these species was removed from the analysis,
ordination results remained unchanged, but when both
were removed the patterns dissolved. It appears that
dominant species are important in characterizing community differences, possibly causing a chain reaction in
the presence and absence of additional species.
Our observations are summarized as follows: (1)
rodent communities differed in response to prairie dog
colonies regardless of landcover type and (2) the
uniform vegetation structure in the shortgrass landcover
did not diminish this effect of colonies on rodent
communities. These observations are incongruent with
our vegetation-driven mechanism, but lend support to
our soil activity-driven mechanisms (see Introduction).
We conclude that prairie dogs’ vegetation cropping has
a minor effect, if any, whereas burrowing and soil
disturbances are the dominant engineering activity
driving rodent community structure.
ACKNOWLEDGMENTS
Funding for this report was funded by the U.S. Fish and
Wildlife Service, U.S. Forest Service, Kansas Cooperative Fish
and Wildlife Research Unit, and the Division of Biology of
Kansas State University. E. Agner, P. Lemons, D. Martin, S.
Meyers, M. Wong, J. Wertz, D. Winslow, S. Winter, and B.
Thiagarajan provided extensive field assistance and data entry.
P. Stapp, C. Paukert, and two anonymous reviewers provided
reviews and valuable comments.
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