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In many ecosystems, burrowing animals act
as ecosystem engineers, physically modifying
habitat and increasing spatial heterogeneity
(Whitford and Kay 1999, Eldridge and Whit-
ford 2009, Davidson et al. 2012, Fleming et al.
2014). An ecosystem engineer is an organism
that directly or indirectly alters its surround-
ings by modifying biotic or abiotic materials,
and thus alters resource availability for other
species (Jones et al. 1994, 1997, 2010). Burrow-
ing animals aerate soils and redistribute soil
particles and nutrients by creating underground
Western North American Naturalist 76(1), © 2016, pp. 82–89
ECOSYSTEM ENGINEERING OF HARVESTER ANTS: EFFECTS
ON VEGETATION IN A SAGEBRUSH-STEPPE ECOSYSTEM
Elyce N. Gosselin1, Joseph D. Holbrook1,3, Katey Huggler1, Emily Brown1,
Kerri T. Vierling1, Robert S. Arkle2, and David S. Pilliod2
ABSTRACT.—Harvester ants are influential in many ecosystems because they distribute and consume seeds, remove
vegetation, and redistribute soil particles and nutrients. Understanding the interaction between harvester ants and plant
communities is important for management and restoration efforts, particularly in systems altered by fire and invasive
species such as the sagebrush-steppe. Our objective was to evaluate how vegetation cover changed as a function of dis-
tance from Owyhee harvester ant (Pogonomyrmex salinus) nests within a sagebrush-steppe ecosystem. We sampled 105
harvester ant nests within southern Idaho, USA, that occurred in different habitats: annual grassland, perennial grass-
land, and native shrubland. The influence of Owyhee harvester ants on vegetation was larger at the edge of ant nests,
but the relationship was inconsistent among plant species. Percent cover was positively associated with distance from
harvester ant nests for plant species that were considered undesirable food sources and were densely distributed. How-
ever, percent cover was negatively associated with distance-from-nests for patchily distributed and desirable plant
species. For some plant species, there was no change in cover associated with distance-from-nests. Total vegetation
cover was associated with distance-from-nests in the shrubland habitat but not in the 2 grasslands. The dominant plant
species in the shrubland habitat was a densely distributed shrub (winterfat, Krascheninnikovia lanata) that was defoliated
by harvester ants. Our results suggest that Owyhee harvester ants increase spatial heterogeneity in plant communities
through plant clearing, but the direction and magnitude of effect will likely be contingent on the dominant vegetation
groups. This information may inform future management and plant restoration efforts in sagebrush-steppe by directly
considering the islands of influence associated with harvester ant engineering.
RESUMEN.—Las hormigas cosechadoras tienen influencia en muchos ecosistemas, ya que distribuyen y consumen
semillas, eliminan vegetación, y redistribuyen partículas del suelo y nutrientes. Es importante entender las interac-
ciones entre las hormigas cosechadoras y las comunidades vegetales para el esfuerzo de manejo y la restauración, en
particular en los sistemas alterados por el fuego y especies invasoras, como la estepa de artemisa. Nuestro objetivo se
centró en evaluar cómo ha cambiado la cobertura vegetal en función a la distancia de los hormigueros de Pogonomyrmex
salinus en un ecosistema de estepa de artemisas. Muestreamos 105 hormigueros de hormigas cosechadoras en el sur de
Idaho, Estados Unidos. Las hormigas se distribuyen en diferentes hábitats: pastizales anuales, pastos perennes y matorrales
nativos. La influencia de las hormigas cosechadoras de Owyhee en la vegetación fue mayor en el borde de los hormi-
gueros, pero la relación fue inconsistente entre las especies de plantas. El porcentaje de cobertura se asoció positiva-
mente con la distancia a los hormigueros para las especies de plantas que se consideraban fuentes de alimento indeseable
y que estaban distribuidas densamente. Sin embargo, el porcentaje de cobertura se asoció negativamente con la distancia
a los hormigueros en especies de plantas deseables y distribuidas en parches. Para algunas especies de plantas, no hubo
cambio en la cobertura vegetal asociado con la distancia a los hormigueros. La cobertura vegetal total se asoció con la
distancia a los hormigueros en el matorral, pero no en los pastizales. Las especies vegetales dominantes en el hábitat de
matorral constituían arbustos densamente distribuidos (Krascheninnikovia lanata) que fueron defoliados por hormigas
cosechadoras. Nuestros resultados sugieren que las hormigas cosechadoras Owyhee aumentan la heterogeneidad espa-
cial en las comunidades de plantas a través de la limpieza de plantas, pero la dirección y la magnitud de su efecto es
probable que sea contingente a los grupos de vegetación dominante. Esta información puede resultar útil para futuras
tareas de manejo y restauración de plantas artemisas en la estepa de artemisas al considerar directamente las islas de
influencia asociadas a la ingeniería de la hormigas cosechadoras.
1Department of Fish and Wildlife Sciences, University of Idaho, Moscow, ID 83844-1136.
2U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Boise, ID 83706.
3Corresponding author. E-mail: jholbrook03@gmail.com
82
tunnel systems (Jones et al. 1994, Whitford and
DiMarco 1995, Eldridge and Whitford 2009).
These activities create patches of disturbed
ground that influence vegetation growth and
contribute to spatial heterogeneity in plant
communities (Whicker and Detling 1988,
Davidson et al. 2012).
Harvester ants (i.e., Pogonomyrmex and
Messor spp.) are burrowing animals that influ-
ence vegetation through ecosystem engineer-
ing and trophic pathways. Harvester ants alter
soil properties through burrowing, which in -
creases soil nutrients and water absorption on
and near nests (Mandel and Sorenson 1982,
Whitford 1988, Wagner et al. 1997, Wilby et
al. 2001, Wagner and Jones 2006, Brown et al.
2012). Consequently, vegetation abundance
generally increases near nests (e.g., Golley and
Gentry 1964, Whitford 1988, Brown et al.
2012). Additionally, harvester ants collect, con-
sume, and store seeds underground, and as a
result, move seeds throughout the landscape
(MacMahon et al.2000). Finally, harvester
ants selectively defoliate or maintain vegeta-
tion on and around nests, which influences the
plant cover and species composition of some
landscapes in fairly large proportions (Carlson
and Whitford 1991, Holbrook et al. 2015).
Thus, harvester ants may have important in -
fluences on desirable native vegetation, as well
as restoration efforts within sagebrush-steppe
ecosystems, which are considered among
the most imperiled in North America (Noss et
al. 1995, Noss and Peters 1995). Sagebrush-
steppe has de clined by 50% in the last 200
years (Knick et al. 2003, Schroeder et al. 2004)
as a result of exotic grass invasion, conifer
encroachment, human use, and altered fire
regimes (Davies et al. 2011). Vast areas of
burned or degraded sagebrush shrublands are
being restored annually (mainly through large-
scale seed sowing) in an attempt to reverse
widespread habitat loss for sagebrush-depen-
dent species, like Greater Sage-Grouse (Cen-
trocercus urophasi anus; Arkle et al. 2014).
Harvester ants have the potential to influence
restoration outcomes because of their wide-
spread distribution in arid shrublands of North
America and their removal of seeds (e.g.,
DeFalco et al. 2009, Ostoja et al. 2009, Suazo
et al. 2013) and plant material (e.g., Bucy and
Breed 2006, DeFalco et al. 2009).
The Owyhee harvester ant is a semiclaus-
tral, haplometrotic harvester ant (Anderson
and Keyel 2006) found throughout southwest-
ern Canada and the western United States
(Rust 1988). Like the nests of other members
of the genus Pogonomyrmex (MacMahon et al.
2000), nests of Owyhee harvester ants are sur-
rounded by a disk that is generally cleared of
all vegetation. Density of Owyhee harvester
ants can range up to 164 nests per hectare
(Blom et al. 1991), and foragers from closely
spaced colonies forage within nonoverlapping
boundaries to avoid encounters with neigh-
boring colonies (Howell 2015).
Our research objective was to assess the
influence of Owyhee harvester ants (hereafter,
harvester ants) on vegetation at the nest scale
(i.e., 0–3 m) within the sagebrush-steppe. We
selected 3 habitats to include in our sample,
which consisted of (1) an annual grassland site
that was dominated by a densely distributed
invasive grass, (2) a perennial grassland site
that was dominated by patchy native and
introduced bunchgrasses, and (3) a shrubland
site that was dominated by a comparatively
dense native shrub. Harvester ants remove
plants near their nests to increase soil mois-
ture and sun exposure (Wight and Nichols
1966, Bucy and Breed 2006), which are impor-
tant for colony survival (Cole 1932). However,
in some cases, harvester ants may maintain
plants on or near their nests that produce
desirable food resources (e.g., Nowak et al.
1990). Harvester ants also indirectly influence
plant species abundance and composition on
and near their nest mounds through physical
modification of soil and nutrients (Carlson and
Whitford 1991, Lei 1999). Thus, we hypothe-
sized that as vegetation density increased, we
would observe a larger reduction in vegetation
cover at distances closer to nests because of
plant removal or defoliation, but we expected
this relationship to change based on the func-
tional role of the plant to ants (e.g., seed pref-
erences). Overall, this work contributes to the
understanding of animal-mediated processes
influencing spatial heterogeneity within plant
communities in sagebrush-steppe.
STUDY AREA
Our study was conducted within the Mor-
ley Nelson Snake River Birds of Prey National
Conservation Area (BOP), a 1962-km2 region in
southwestern Idaho (lat 43.283, long −116.200).
The BOP is located in an arid (110–350 mm
2016] HARVESTER ANTS ENGINEER HABITAT 83
annual precipitation) sagebrush-steppe habitat
and is managed by the U.S. Bureau of Land
Management under a multiple-use framework.
In June and July 2014, we sampled harvester
ant nests within three 1-ha sites characterized
by annual grassland, perennial grassland, and
shrubland. Plant senescence generally oc -
curred prior to our sampling period; thus, our
results were not confounded by differences
in seasonal growth strategies of plants. The
annual grassland had coarse-silt soil and was
burned in 1996 but was not seeded postfire.
The dominant plant species was exotic Bromus
tectorum and native Sandberg bluegrass (Poa
secunda). The perennial grassland was also on
coarse-silt soil and was burned in 1996 and
seeded in 1997. This site was dominated by
P. secunda and exotic Russian wildrye (Psathy-
rostachys juncea). The shrubland had a coarse-
loam soil and had not burned in the last 30
years. The dominant plant species was native
winterfat (Krascheninnikovia lanata) and P.
secunda.
METHODS
At each of the sites, we censused and
mapped all active harvester ant nests, which
amounted to ≥30 nests per site. To measure
the nest area, we recorded 2 measurements
of the disk diameter at the nest; one in a
north–south orientation and one in an east–
west orientation. We measured distances be -
tween the nest edges, which we defined by
the widest area of bare ground where no vege-
tation was growing. We averaged the 2 diame-
ter measurements and calculated the average
nest area by applying the equation for area of
a circle (A= 0.25pd2, where d= average
nest diameter). Additionally, we calculated the
total area covered by nests within a 1-ha area
by summing nest areas.
To evaluate how plant species and total
vegetation cover changed as a function of dis-
tance from harvester ant nests, we character-
ized the vegetation at 3 distances from each
nest in each of the 4 cardinal directions. We
placed a 0.25 ×0.5-m quadrat at 0, 1.5, and
3 m from the nest edge for a total of 12
quadrats per nest (3 distances ×4 directions).
We limited our sampling extent to 3 m from
harvester ant nests because—although the
mean foraging distance of P. salinus was
found to be 8.0 m in a sagebrush-greasewood
community (Jorgensen and Porter 1982)—we
wanted to capture the major area of influence
associated with harvester ants, and foraging
decreases exponentially as distance from ant
nests increases (Crist and MacMahon 1991).
At each quadrat surrounding the nest, we
collected nadir (90°angle) photographs using
a Canon Powershot SX20 IS (12.1-megapixel
resolution) from a distance of 2 m from the
ground using a polyvinyl chloride (PVC)
monopod (see Pilliod and Arkle 2013).
We used SamplePoint 1.56 software (Booth
et al. 2006) to measure the percent cover of
each species, as well as total vegetation cover,
within photos. We cropped each photo to the
area inside of the quadrat and generated 64
computer-selected grid points. We manually
categorized each point as a plant species,
unidentifiable grasses, or extraneous material
(e.g., soil, scat, shadows, downed woody debris).
For photos from the shrubland habitat, we
categorized K. lanata either as defoliated (i.e.,
no leaves and inflorescences) or foliated to
assess how foliage state changed as a function
of distance from nest. We then used Sample-
Point to generate cover estimates for each spe -
cies category within each photo. For each nest,
we averaged these cover estimates across
the 4 photos within each distance class. Thus,
256 individual points (i.e., subsampling loca-
tions) contribute to cover estimates gener-
ated for each species at a given distance from
each nest, with a total of 80,640 points being
classified.
For statistical evaluation, we treated each
harvester ant nest as an independent sample
and summarized the data based on dominant
vegetation (i.e., habitat). We calculated 95%
confidence intervals to assess statistical differ-
ences in species and total vegetation cover
across our gradient of distance-from-nest. Only
plant species identified in ≥10% of photos in
at least one distance class (0 m, 1.5 m, 3 m)
were included in species-level analyses. To
evaluate changes in total vegetation cover (i.e.,
cover by any species), we used the average
percent cover for all detected plants, includ-
ing unidentified grasses. Finally, we assessed
differences in cover estimates of defoliated
and foliated K. lanata within each distance
class in the shrubland habitat using 95% confi-
dence intervals. All analyses were performed
in Program R (version 3.1.1) using the Rmisc
package (R Core Team 2014).
84 WESTERN NORTH AMERICAN NATURALIST [Volume 76
RESULTS
At the annual grassland, perennial grass-
land, and shrubland habitat, the densities of
active harvester ant mounds were 39, 36, and
30 nests ⋅ha−1, and the average nest areas
were 0.21, 0.30, and 0.49 m2, respectively.
Harvester ant nests comprised a total of 9, 12,
and 16 m2at the annual grassland, perennial
grassland, and shrubland, respectively.
Cover of several plant species differed sig-
nificantly as a function of distance from har-
vester ant nests. In the annual grassland, B.
tectorum cover increased and P. secunda cover
decreased significantly from the edge of the
nest to 1.5 and 3 m from the edge (Fig. 1A).
There were no significant changes in cover of
tall tumblemustard (Sisymbrium altissimum)
across distances. In the perennial grassland,
there were no significant changes in P.
secunda or P. juncea cover with distance-from-
nest (Fig. 1B). In the shrubland, we observed
a statistically significant decrease between P.
secunda at the nest edge and 3 m (a =0.10;
b with asterisk in Fig. 1C). We documented a
significant increase in K. lanata from the edge
to 1.5 and 3 m, but no significant change in
B. tectorum cover across distances (Fig. 1C).
Lastly, we observed significantly more defoli-
ated than foliated K. lanata cover, but only at
the 0-m distance class (Fig. 2).
In contrast to our species-specific results,
total vegetation cover only changed as a func-
tion of distance-from-nest in the shrubland,
where the nest edge has less total cover than
the 1.5-m and 3-m distances (Fig. 3). Although
there were significant changes in P. secunda
and B. tectorum cover in the annual grassland,
there was no significant change in total vege-
tation cover. Similarly, we observed no signifi-
cant change in vegetation cover in the perennial
grassland.
DISCUSSION
Our findings support the notion that
Owyhee harvester ants act as ecosystem engi-
neers within sagebrush-steppe habitats and
contribute to spatial heterogeneity in plant
communities through small-scale changes
magnified by the abundance of nests. We dis-
covered that nest density of harvester ants
ranged from 30 to 39 nests ⋅ha−1, with as
much as 16 m2of land denuded per hectare.
These values are lower than those reported in
some previous studies (Whitford and Bryant
1979, Carlson and Whitford 1991, MacMahon et
al. 2000), potentially indicating that harvester
2016] HARVESTER ANTS ENGINEER HABITAT 85
Fig. 1. Mean percent cover (–
+1 SE) of plant species as a
function of distance from Owyhee harvester ant (Pogono-
myrmex salinus) nests: A, annual grassland; B, perennial
grassland; C, shrubland. Plant codes: BRTE = Bromus
tectorum, POSE = Poa secunda, SIAL = Sisymbrium
altissimum, PSJU = Psathrostachys juncea, KRLA =
Krascheninnikovia lanata. Different letters denote signi -
ficant differences in cover across distance classes based on
95% confidence intervals. Note different y-axis scale in
panel B, and the asterisk in panel C indicates a difference
in POSE cover between 0 m and 3 m (a = 0.10).
ants are capable of having much greater
impacts on vegetation in other regions. Addi-
tionally, harvester ants appeared to influence
vegetation communities through selective
plant clearing, and perhaps seed selection,
that depends on species. This is consistent
with other studies that have observed har-
vester ants differentially influencing plant
species (Whitford 1988, Carlson and Whitford
1991, Brown et al. 2012). Overall, our work
informs the understanding of harvester ant–
vegetation relationships in a general sense,
as well as within sagebrush-steppe ecosystems.
The effects of harvester ants on plant com-
position depended on distance-from-nest and
plant species. In particular, the effect of ants
on specific plant species may be a result of
both the nutritional value and the density of
these plants near ant nests. Past studies indi-
cate that B. tectorum seeds have a relatively
high percentage of indigestible cell walls
(Crist and MacMahon 1992) and relatively low
calorie content per seed (Kelrick et al. 1986)
compared to seeds from native grasses, forbs,
and shrubs. Further, the long awns of B. tecto-
rum seeds might impede transport or process-
ing of the seeds (Kelrick et al. 1986, Crist and
MacMahon 1992). These attributes of B. tecto-
rum seeds could explain why harvester ants
often collect them at disproportionately low
rates and discard collected seeds in refuse
piles where germination is unlikely (Kelrick et
al. 1986, Crist and MacMahon 1992, Ostoja et
al. 2013). In the annual grassland, B. tectorum
was relatively dominant and densely distrib-
uted, and there was a significant increase in B.
tectorum cover with distance from harvester
ant nests. This was not so in the shrubland site
where B. tectorum cover was relatively low.
Dense B. tectorum might decrease soil mois-
ture and sun exposure, and because B. tecto-
rum is not a preferred nutritional resource for
harvester ants, it may be advantageous for har-
vester ants to remove this grass in areas where
it is densely distributed and near the nest.
We observed a similar relationship with
K. lanata density and removal patterns by ants.
To our knowledge, there is no literature dis-
cussing a trophic relationship between K. lanata
and harvester ants. However, harvester ants
remove leaves from shrubs near their nests,
likely to conserve soil moisture (Wight and
Nichols 1966) or to increase solar exposure
and thus increase foraging periods (Bucy and
Breed 2006). In the shrubland, we identified
an increase in K. lanata cover with distance
from harvester ant nests (where K. lanata was
relatively dense). We discovered significantly
more defoliated than foliated K. lanata at the
nest edge, but no difference between defoli-
ated and foliated K. lanata at the outer dis-
tances, suggesting defoliation by harvester
ants. Rissing (1988) found that harvester ants
(Pogonomyrmex rugosus) disproportionately
removed leaves on shrubs that were closer to
86 WESTERN NORTH AMERICAN NATURALIST [Volume 76
Fig. 2. Mean percent cover (–
+1 SE) of defoliated and
foliated Krascheninnikovia lanata (KRLA) as a function
of distance from Owyhee harvester ant (Pogonomyrmex
salinus) nests in the shrubland habitat. Different letters
denote a significant difference in cover within distance
class based on 95% confidence intervals.
Fig. 3. Total percent cover (–
+1 SE) of vegetation as a
function of distance from Owyhee harvester ant (Pogono-
myrmex salinus) nests by habitat. Different letters denote
a significant difference in cover across distance classes
based on 95% confidence intervals.
nests and observed up to 300 ants stripping
foliage off nearby shrubs. Further, recent res -
toration experiments on the BOP (our study
area) indicated that harvester ants defoliated
Wyoming big sagebrush (Artemisia tridentata)
seedlings, contributing to a nearly 50% die-off
(M. Germino personal communication). Re -
gardless of the behavioral mechanism, har-
vester ants modify shrubs and could largely
influence the success of shrubland restoration
efforts, which is concerning given the threat-
ened status of sagebrush shrublands (Noss et
al. 1995, Noss and Peters 1995) and amount
of sagebrush seeded or outplanted annually
(Arkle et al. 2014).
The behavior of leaf clipping and defolia-
tion of undesirable plants near ant nests likely
increases desirable plant species abundance
and food availability through competitive re -
lease. For instance, P. secunda (a native bunch-
grass) cover was higher in close proximity to
nests at sites where densely distributed and
undesirable plants were reduced. The increase
in P. secunda cover near nests might result
from decreased competition with other plants,
or a combination of decreased competition
along with increased soil nutrients at the nest
edge (e.g., Mandel and Sorenson 1982, Whit-
ford 1988, Wagner et al. 1997, Wilby et al.
2001, Brown et al. 2012). Nowak et al. (1990)
observed similar patterns in that Indian rice-
grass (Oryzopsis hymenoides, a native peren-
nial bunchgrass) was more abundant near har-
vester ant nests. They suggested that the most
probable mechanism was re duced competition
because of selective defoliation and removal of
other plant species by harvester ants.
Although ants reduced cover of some indi-
vidual plant species closer to nests, the per-
cent cover of S. altissimum (an introduced forb)
or P. juncea (an introduced bunchgrass) was
not influenced by distance-from-nest. The rela-
tively large size of S. altissimum and P. juncea
may have prevented harvester ants from clip-
ping and removing them, particularly if these
plants exhibited substantial growth prior to
high ant densities. This explanation, however,
seems unlikely as harvester ants are known to
influence shrubs (Lei 1999), which are often
among the largest plants in desert environ-
ments. Perhaps the density of S. altissimum
and P. juncea was not high enough to initiate a
direct removal response by harvester ants, nor
an indirect competitive release response trig-
gered by ants removing other plant species. In
addition, S. altissimum seeds are used as a food
resource by harvester ants (Schmasow 2015),
and thus may be beneficial to have on a site,
even if at low densities.
We only detected a significant effect of
distance-from-nest on total vegetation cover
in the shrubland. This pattern was similar to
results from Carlson and Whitford (1991), but
inconsistent with other studies where ant-
mediated changes in soil properties (rather
than defoliation or clearing) seemed to be the
main process influencing plant growth and
cover (Whitford 1988, Mull and MacMahon
1997, Brown et al. 2012). The subtle change in
total vegetation cover we observed was some-
what contrary to our hypothesis that harvester
ants would defoliate densely distributed vege-
tation. However, the density of vegetation at
the grassland habitats may have been low
enough that defoliation was unnecessary. Alter-
natively, selective removal of nonnatives and
indirect facilitation of desirable natives near
nests, coupled with competitive dominance by
nonnatives farther from nests (where less
defoliation or clearing occurred), may balance
total plant cover across distances. Combining
the patterns we observed among species and
total vegetation cover provides support for
the hypothesis that harvester ants selectively
defoliate undesirable plant species and indi-
rectly benefit desirable species. This pattern
was observed firsthand in the field with K.
lanata defoliation, as well as in the data with a
near doubling of P. secunda cover nearer to
nests in annual grassland and shrubland sites.
Perhaps defoliation by harvester ants is largely
driven by species-specific nutritional deci-
sions, and secondarily driven by a threshold-
dependent response to vegetation density. This
mechanism would be consistent with the pat-
terns we observed at the species and total
vegetation level, but additional research is
needed to isolate and test these mechanisms.
Alternatively, Carlson and Whitford (1991) sug-
gest that the inadvertent dispersal of seeds
near disks is possible, and this might also con-
tribute to an increase in the relative abundance
of desirable plant species near the nest edge.
CONCLUSION
Our findings provide insight concerning the
extent of ecosystem engineering by harvester
2016] HARVESTER ANTS ENGINEER HABITAT 87
ants within a sagebrush-steppe ecosystem.
However, additional work is needed within
different habitats of the Great Basin, as well
as other ecosystems, to capture the range of
variation in harvester ant engineering, which
would help identify general patterns and mech-
anisms that transcend ecological contexts. This
information could directly inform management
and restoration activities because the islands
of influence associated with harvester ants
could contribute to the success or failure of
restoration efforts (Byers et al. 2006). Incorpo-
rating engineering effects of animals within
restoration plans is particularly important to
consider when animal activities could limit
restoration efficacy through consumption of
sown seeds, stripping of shrubs, or modifica-
tion of resulting plant communities. Indeed,
characterizing spatial and temporal patterns,
as well as mechanisms associated with ecosys-
tem engineering, advances our understanding
of ecosystems, and thus aids conservation and
management efforts (Hastings et al. 2007).
ACKNOWLEDGMENTS
We gratefully acknowledge financial support
from the Doris Duke Conservation Scholars
Program Collaborative and the U.S. Geological
Survey, Idaho Cooperative Research Unit. We
sincerely thank M. Modlin for assisting with
fieldwork. We thank J. McIver, J. Rachlow,
and C. Conway for providing comments that
improved this manuscript. This is contribu-
tion 1088 of the University of Idaho Forest,
Wildlife and Range Experiment Station. Any
use of trade names is for descriptive purposes
only and does not imply endorsement by the
U.S. government.
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Received 8 May 2015
Accepted 1 December 2015
