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Community and Ecosystem Ecology
Ground-Dwelling Arthropod Community Response to 
Livestock Grazing: Implications for Avian Conservation
Hayes B. Goosey,1,4 Joseph T. Smith,2 Kevin M. O’Neill,3 and David E. Naugle2
1Department of Animal and Range Sciences, Montana State University, Bozeman, MT 59717, 2Wildlife Biology Program, University of 
Montana, Missoula, MT 59812, 3Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, 
MT 59717, and 4Corresponding author, e-mail: hgoosey@montana.edu
Subject Editor: Shannon Murphy
Received 14 September 2018; Editorial decision 21 May 2019 
Abstract
Terrestrial arthropods are a critical component of rangeland ecosystems that convert primary production into 
resources for higher trophic levels. During spring and summer, select arthropod taxa are the primary food 
of breeding prairie birds, of which many are imperiled in North America. Livestock grazing is globally the most 
widespread rangeland use and can affect arthropod communities directly or indirectly through herbivory. 
To examine effects of management on arthropod community structure and avian food availability, we studied 
ground-dwelling arthropods on grazed and ungrazed sagebrush rangelands of central Montana. From 2012 to 2015, 
samples were taken from lands managed as part of a rest-rotation grazing program and from idle lands where 
livestock grazing has been absent for over a decade. Bird-food arthropods were twice as prevalent in managed 
pastures despite the doubling of overall activity-density of arthropods in idle pastures. Activity-density on idled 
lands was largely driven by a tripling of detritivores and a doubling in predators. Predator community structure 
was simplified on idled lands, where Lycosid spiders increased by fivefold. In contrast, managed lands supported 
a more diverse assemblage of ground-dwelling arthropods, which may be particularly beneficial for birds in these 
landscapes if, for example, diversity promotes temporal stability in this critical food resource. Our results suggest 
that periodic disturbance may enhance arthropod diversity, and that birds may benefit from livestock grazing with 
periodic rest or deferment.
Key words:  activity-density, arthropod, livestock, rest-rotation, sage-grouse
Terrestrial arthropods are a dominant component of rangeland 
macroinvertebrate diversity (Andersen et al. 2004). As key links in 
rangeland food webs, arthropods occupy many trophic levels and 
ecological niches, provide ecosystem services (Prather et al. 2013), 
and convert primary production into resources for higher trophic lev-
els, including species of conservation concern (Gregg and Crawford 
2009, Sullins et al. 2018). Thus, changes in arthropod abundance or 
community composition may have far-reaching effects on rangeland 
ecosystems. Arthropod community response is therefore a critical 
component of understanding the influence of land use and manage-
ment interventions on rangeland ecosystems (Hoffmann 2010).
Particular groups of arthropods are key energetic resources for 
grassland and shrubland birds, many of which are imperiled in North 
America (Wiens and Rotenberry 1979, Vickery and Tubaro 1999). 
For example, spring and summer diets of threatened Mountain 
Plovers, Charadrius montanus (Townsend, Charadriiformes: 
Charadriidae), in Colorado were composed almost entirely of insects, 
mostly Coleoptera, Orthoptera, and Hymenoptera (Formicidae) 
(Baldwin 1971). Diets of nestling grassland-obligate passerines in 
Saskatchewan were dominated by Lepidopteran larvae, and mem-
bers of Lepidoptera, Orthoptera, and Coleoptera were preferentially 
selected (Maher 1979). The importance of these three orders to pas-
serine nestling diets is consistent across a range of North American 
grassland and shrubland ecosystems (Wiens and Rotenberry 1979). 
Arthropods are also important components of seasonal diets 
of declining galliform species, including lesser prairie chickens, 
Tympanuchus pallidicinctus (Ridgway, Galliformes:  Phasianidae) 
and greater sage-grouse, Centrocercus urophasianus (Bonaparte, 
Galliformes: Phasianidae); hereafter, sage-grouse. Early studies 
reported that 50–60% of the diet of young (1–4  wk) sage-grouse 
chicks was composed of arthropods, with Orthoptera, Coleoptera, 
and Hymenoptera (Formicidae) comprising the largest share of food 
items (Klebenow and Gray 1968). Lesser prairie chickens also feed 
primarily on Orthoptera and Coleoptera, though Hemiptera were fre-
quently consumed in some areas (Doerr and Guthery 1983). However, 
these early studies, which employed microhistological analysis of 
crop contents or feces, likely underestimated intake of soft-bodied 
arthropods such as Lepidopteran larvae, because these food items are 
Environmental Entomology, 48(4), 2019, 856–866
doi: 10.1093/ee/nvz074
Advance Access Publication Date: 24 June 2019
Research 
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quickly broken down following consumption (Sullins et  al. 2018). 
More recent work has linked survival of young sage-grouse chicks to 
abundance of Lepidopteran larvae (Gregg and Crawford 2009), and 
a recent diet study using DNA barcoding revealed the importance of 
Lepidopterans to lesser prairie chickens (Sullins et al. 2018). Together, 
avian dietary studies in grassland and shrubland systems indicate a 
few orders make up the bulk of arthropod food items important for 
survival.
Livestock grazing is the most widespread land use on range-
lands globally (Holechek 2011). Herbivory by livestock affects the 
composition and physical structure of plant communities (Olff and 
Ritchie 1998) which in turn, may have positive or negative effects 
on arthropods (Zhu et al. 2012). Among rangeland taxa, arthropods 
may be especially sensitive to changes in plant species composition 
(Borer et al. 2012) and vegetation structural characteristics (O’Neill 
et al. 2003, 2010). Intensive grazing results in shorter, simpler vege-
tation structure and may reduce abundance and diversity of arthro-
pods (Kruess and Tscharntke 2002, Debano 2006). Furthermore, 
because large herbivores can influence arthropods through mechan-
isms other than vegetation-mediated pathways, grazing is thought to 
have greater influence on arthropods than plant communities (Pöyry 
et al. 2006, Rickert et al. 2012).
Grazing by large herbivores can also increase quality or hetero-
geneity of arthropod habitats relative to ungrazed areas, with posi-
tive effects on arthropod abundance or diversity (Rickert et al. 2012, 
Zhu et al. 2012). Moderate grazing can increase plant aboveground 
net primary productivity (McNaughton 1979), and thereby increase 
food resources for phytophagous insects (Tscharntke and Greiler 
1995). Under light to moderate stocking rates, livestock graze 
may  selectively induce spatial heterogeneity in vegetation struc-
ture (Jerrentrup et al. 2014). Patches of reduced vegetation height 
or litter depth provide favorable microclimates for certain species 
(Bestelmeyer and Wiens 2001) and may allow some taxa to increase 
by suppressing predatory arthropods that depend on tall or complex 
vegetation structure (Farrell et  al. 2015). Seasonal or intermittent 
grazing can also induce temporal heterogeneity in vegetation, and 
evidence suggests periodic disturbance such as grazing are important 
for maintaining high biodiversity in grasslands (Allan et al. 2014). 
Price (1997) suggested that moderate levels of disturbance produce 
the greatest arthropod abundance and diversity by keeping competi-
tively dominant species in check. Thus, grazing strategies that in-
corporate temporal variation in grazing intensity may be an effective 
tool for maintaining arthropod biodiversity in managed rangelands.
One such management strategy, often advanced for a wide range 
of conservation goals, is rest-rotation grazing (Hormay 1956). 
Designed to mimic the natural patterns of herbivory of wild ungu-
lates, rest-rotation grazing involves moving livestock herds through 
multiple pastures throughout the grazing season while leaving at 
least one pasture ungrazed to allow for plant growth and reproduc-
tion. Rest-rotation grazing has been proposed for arthropod conser-
vation in temperate grasslands (Morris 1991) because the habitat 
requirements of a broader suite of families is likely to be supported 
in a landscape comprised of patches diverse in vegetation structure 
and successional stage. However, despite theoretical conservation 
benefits, little empirical evidence exists to support the use of rest-ro-
tation grazing systems to enhance rangeland arthropod communities 
(Dennis et al. 1997; but see Enri et al. 2017).
From 2012 to 2015, we investigated relative abundance and di-
versity of ground-dwelling arthropods in sagebrush habitats found in 
grazed and deferred pastures (hereafter collectively called ‘managed’) 
associated with a rest-rotation grazing system and on adjacent lands 
that have remained ungrazed for over a decade (hereafter ‘idle’). We 
focused on ground-dwelling arthropods because of their import-
ance as dietary items of shrubland and grassland birds (Sullins et al. 
2018). We hypothesized that ground-dwelling arthropod abundance 
and diversity would be higher on idle than on managed lands due to 
greater plant height and ground cover, which could provide more re-
productive, thermoregulation, and predator avoidance sites. We also 
considered the alternative hypothesis that arthropod abundance and 
diversity may be higher on managed land in response to increased 
productivity or vegetative structural heterogeneity resulting from peri-
odic and selective grazing. Because arthropod taxa and functional 
groups exhibit variable responses to grazing and associated changes 
in plant structure and microclimates (Sjödin et al. 2007), we examined 
effects of grazing and deferment on select taxa comprising the most 
important arthropod food items for grassland and shrubland birds in 
this region. However, to capture the broader ecological implications of 
the management treatments in our study area, we also tested for treat-
ment effects on ground-dwelling detritivores and predators.
Methods
Study Area
Our research area was in central Montana, north of Lavina 
(46.5176N, 108.0973W) in rolling hill terrain between eleva-
tions of 975–1,250 m.  The vegetation is intermountain basin big 
sagebrush steppe (i.e., Wyoming big sagebrush, Artemisia triden-
tata ssp. Wyomingensis (Nutt. Asterales:   Asteraceae),  and silver 
sagebrush, Artemisia cana (Pursh, Asterales:   Asteraceae))  with a 
mixed understory of perennial rhizomatous and caespitose grasses, 
including western wheatgrass (Pascopyrum smithii (Rydb., Poales:   
Poaceae)),  bluebunch wheatgrass (Pseudoroegneria spicata (Pursh, 
Poales:  Poaceae)), green needlegrass (Nassella viridula (Trin., Poales:  
Poaceae)), needle-and-thread grass (Hesperostipa comata (Trin. and 
Rupr., Poales:   Poaceae)),  and blue grama grass (Bouteloua gra-
cilis  (Kunth, Poales:   Poaceae)). Daily temperatures range between 
2.88 and 30.88°C. Annual precipitation averages 359  mm with 
the lowest during our study occurring in 2012 (265 mm) and the 
greatest in 2014 (485 mm). Commercial agriculture is the dominant 
land use with about 10% of the land used for crop production.
Concurrent with our study, research on the effect of rest-rota-
tion grazing systems on sage-grouse survival and reproduction was 
conducted by Smith et al. (2018). Rest-rotation grazing systems, ad-
ministered by the USDA Natural Resources Conservation Service 
(NRCS) through their Sage Grouse Initiative (SGI), were volun-
tarily adopted by several landowners in our study area from 2010 
to 2012. Grazing plans were generated individually for each ranch 
while adhering to the NRCS Montana Prescribed Grazing conserva-
tion practice standards. These standards included 1)  livestock util-
ization rates of ≤ 50% of current year’s growth of key forage species, 
2) duration of grazing ≤ 45 d, 3) changing timing of grazing by at 
least 20 d each year, and 4) a contingency plan outlining steps to be 
taken in exceptional circumstances such as fire or drought. Most 
landowners elected to incorporate rest into their management plans 
which provide areas of at least 5% shrub cover, a basic characteristic 
of sage-grouse nesting habitat, with ~15 mo without grazing on a ro-
tation basis. Ranches participating in SGI rotational grazing systems 
compromised the two managed land treatments in our study.
Sampling
Sampling occurred weekly in three classes of pasture: 1) deferred, 
2) grazed, and 3) idle. Grazed pastures had livestock present dur-
ing sampling. Deferred pastures were in the rest phase and had 
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been excluded from grazing since the spring/summer of the pre-
vious year. Idle pastures were located on the Lake Mason National 
Wildlife Refuge where livestock grazing has been absent for over 
a decade. Ranch grazing plans are presented in Table 1 and sam-
pling locations are presented in Fig. 1 with sampling dates, tem-
poral replicates, and total activity-density presented in Supp Table 
1 (online only). Deferred and grazed pastures were blocked so that 
both treatments were always sampled on each ranch. Three to four 
grazed and deferred pastures (experimental unit) were sampled 
each year, 2012–2014. Idle pasture sampling was conducted on the 
1,245 ha Lake Mason National Wildlife Refuge in 2014 and 2015. 
Grazed and deferred pastures were sampled 2012–2014 while idle 
land was sampled in 2014 and 2015. Each year, one sampling lo-
cation was used in each grazed and deferred pasture while three 
locations were used on idle land. All locations were established 
in sage-grouse nesting habitat comprised of structural sagebrush 
with an understory of native grasses and forbs providing food and 
concealment along with additional insect food resources (Connelly 
et al. 2011).
Vegetation Characteristics
Weekly and at all sampling locations, we placed a 0.5 m2 metal ring 
(n = 10) 2 m apart along a random compass bearing to visually esti-
mated percent bare ground at each location. Also, weekly at an add-
itional 10 random places located no more than 10 m from the pitfall 
trap transect, the height of the nearest live perennial native grass 
(excluding inflorescence) and at an additional five random places the 
height of the nearest live sagebrush (including inflorescence) were 
measured.
Arthropod Sampling
Activity-density was estimated weekly with 10 pitfall traps per rep-
licate. Each trap was two, nine-cm diameter; 0.5-liter plastic cups 
Table 1.  Pasture grazing information obtained from landowners enrolled in the NRCS sage-grouse initiative rest-rotation grazing program 
during 2012–2014 sampling north of Lavina, MT
Year Dates grazed Days grazed # Head Animal type Pasture size
2012 1 June – 16 July 45 81 Cow/calf pair 259
 1 June – 4 July 33 150 Cow/calf pair 260
 6 June – 4 July 45 93 Yearlings 262
2013 9 May – 1 June 24 90 Cow/calf pair 613
 1 May – 5 June 36 225 Cow/calf pair 668
 1 June – 15 July 45 93 Yearlings 405
2014 1 May – 5 June 36 100 Cow/calf pair 445
 21 May – 9 June 20 100 Cow/calf pair 260
 1 May – 15 June 45 100 Yearlings 262
 7 June – 8 July 31 164 Cow/calf pair 520
Fig. 1. Sampling locations in deferred, grazed, and idle pastures in Musselshell and Golden Valley counties, central Montana, USA, during the 2012–2015 field 
seasons.  The upper right corner inset represents a closer view of idle land sampling locations on the Lake Mason National Wildlife Refuge.
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(Solo Cup Company, Lake Forest, IL) stacked together and dug into 
the ground so that the top cup was flush with the soil surface. Traps 
were spaced 1 m apart on a linear transect determined by a random 
compass bearing, were filled one-third full of propylene glycol-based 
antifreeze (Arctic Ban, Camco Manufacturing Inc., Greensboro, NC), 
and remained open the entire time between weekly collections. Each 
trap was fitted with a 25-cm-diameter clear plastic cover suspended on 
three 10-cm bolts with a minimum 2-cm gap between the soil surface 
and the cover rim. Traps contents were placed in 11.5 × 23 cm bags 
(Whirl-Pak, Nasco Inc., Fort Atkinson, WI), the antifreeze was replen-
ished, and samples were returned to the Montana State University for 
identification according to Triplehorn and Johnson (2005).
Pitfall traps are limited in that insect activity and density are 
confounded (Topping and Sunderland 1992) and so values obtained 
from our pitfall trapping were treated as indices of ‘activity-density’ 
(Kromp 1989). Capture rates may also reflect arthropod habitat 
selection in addition to differences in population size (Woodcock 
2005) because individuals were free to disperse among locations. 
Despite its limitations, pitfall trapping is a widely used method to 
sample ground-dwelling arthropods (Spence and Niemelä 1994, 
Zou et al. 2012). Furthermore, activity-density, as opposed to true 
density, may be a more meaningful metric of arthropod availability 
to feeding birds because complex habitats often provide refugia for 
prey thus reducing capture efficiency (Gotceitas and Colgan 1989). 
Increased bare ground, for example, positively affects carabid beetle 
activity-density (Thomas et  al. 2006) and is strongly selected by 
ground-foraging insectivorous birds (Schaub et al. 2010).
Statistical Analysis
Bare ground (%) and grass and sagebrush heights (cm) were aver-
aged for each sampling location and treatment differences were es-
timated with Tukey’s Honest Significant Difference Test, assuming 
normal error distribution using the Proc GLM procedures of SAS 
(SAS 2008).
Over 4 yr of study, 26 locations were sampled (Supp Table 1 
[online only]). Treatment × year interactions were determined using 
the Proc GLM procedures of SAS (SAS 2008). For mixed model stat-
istical analyses, arthropod counts were summed for each location 
across sampling weeks into arthropod Families and also into two 
functional groups of predators and detritivores. An additional group 
termed ‘Food Arthropods’ was also summed to represent food items 
of sage-grouse as determined by Klebenow and Gray (1968) and 
Sullins et al. (2018). Predators: Coleoptera: Carabidae, Coccinellidae 
Histeridae; Araneae: Lycosidae, Gnaphosidae, Philodromidae, 
Salticidae, Thomisidae, Hahniidae, Linyphiidae, Liocranidae, 
Dictynidae, Amaurobiidae, and Pisauridae. Detritivores: Coleoptera: 
Tenebrionidae, Scarabaeidae, Dermestidae, Nitidulidae. Food 
Arthropods: Coleoptera: Carabidae, Tenebrionidae, Scarabaeidae, 
Chrysomelidae, Curculionidae, Coccinellidae; Lepidoptera: 
Arctiidae, Noctuidae, Saturniidae, Pieridae; Hymenoptera: 
Formicidae; Orthoptera: Gryllidae, Acrididae, Tettigoniidae.
Summed sampling location counts were analyzed with a gener-
alized linear mixed model using the Proc GLIMMIX procedures of 
SAS version 9.2 with sampling year as a random variable. Summed 
counts were fit to a negative binomial distribution and standard-
ized using an offset to account for the variable number of sampling 
weeks among sites and years with the marginal likelihood approxi-
mated using Laplace’s method. Including vegetation characteristics 
as covariates did not uniformly improve AIC values for all compari-
sons so for model simplicity and consistency, we dropped these vari-
ables from further consideration. Differences among treatment least 
squared means were calculated using the least significant difference 
(LSD) test. Contrasts, using the same model, were also estimated for 
functional and food arthropod groups to compare managed (i.e., the 
weighted average of deferred and grazed data) and idle pastures to 
assess land management strategies in our study that do and do not 
incorporate livestock grazing (Proc GLIMMIX, SAS 2008).
Simpson’s 1-D Family-level diversity was estimated for each rep-
licate using Past v. 3.19 (Hammer et al. 2001) and these values were 
subjected to the Proc MIXED, LSD procedures of SAS (SAS 2008) 
with fixed treatment and random year assuming a normal error dis-
tribution to test for treatment differences.
Table 2.  Mean bare ground, grass height, and sagebrush height ± SEM from Deferred, Grazed, and Idle locations during 2012–2015 field 
seasons and the study average of the same metrics collected north of Lavina, MT
Sample location metrics
Year Location Bareground (%) Grass (cm) Sagebrush (cm)
2012 Deferred 29.12 ± 4.40 19.98 ± 2.00 36.15 ± 3.84
 Grazed 36.10 ± 8.63 16.99 ± 1.59 38.82 ± 4.37
 Idlea --- --- ---
2013 Deferred 36.04 ± 4.69 24.76 ± 3.70 42.41 ± 4.64
 Grazed 53.38 ± 8.01 17.98 ± 3.56 32.88 ± 5.74
 Idlea --- --- ---
2014 Deferred 37.38 ± 10.16 21.34 ± 6.78 34.11 ± 6.78
 Grazed 42.32 ± 6.32 19.04 ± 1.58 34.06 ± 3.43
 Idle 10.83 ± 4.18 22.89 ± 3.69 37.68 ± 6.08
2015 Idle 8.33 ± 3.48 21.18 ± 2.43 37.40 ± 5.26
Study Averageb     
 Deferred 34.18 ± 2.56a 22.02 ± 1.42 37.56 ± 2.49
 Grazed 43.93 ± 5.05a 18.08 ± 0.59 35.25 ± 1.81
 Idle 9.58 ± 1.25b 22.04 ± 0.85 37.54 ± 0.14
aNot sampled during the 2012 and 2013 field seasons.
bStudy average means in columns followed by different letter groupings statistically differ (α = 0.05); Tukey’s Honest Significant Difference Test (Proc GLM, SAS 
Institute 2008) where all comparisons df = 2,20.
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Results
Vegetation
Bare ground was highest in grazed pastures (F  = 18.48; df = 2,7; 
P  =  0.0049). Grass height tended to differ among treatments 
(F = 4.84; df = 2,7; P = 0.0674) with the shortest grass in grazed pas-
tures. Sagebrush height did not differ among treatments (F = 0.43; 
df = 2,7; P = 0.6713; Table 2).
Arthropods
Over 37,000 arthropod specimens were collected and identified 
from 54 families during 4 yr of study. Diversity analyses included 
counts from all 54 Families while functional group, food group, and 
Family-level analyses were conducted on counts from select taxa of 
32 Families. Analysis of variance indicated no treatment × year inter-
actions for total arthropods (F = 2.10; df = 2,25; P = 0.155), food 
arthropods (F = 2.18; df = 2,25; P = 0.142), predators (F = 0.59; 
df=2,25; P = 0.566), or detritivores (F = 0.01; df = 2,25; P = 0.993), 
nor were interactions detected at the Family level (all Family-level 
interactions were: F ≥ 3.33; df = 2,25; P ≥ 0.059); therefore, data 
were combined across years for analyses.
Weekly Activity-Density
Contrast analyses indicated that total arthropod activity-density 
was twice as high on idle compared to managed land (F = 69.55; 
df = 1,20; P < 0.001). Arthropod pasture level activity-density was 
greatest in idle compared to grazed and deferred, which did not 
differ from each other (Fig. 2a; Table 3).
Food Arthropods
Contrast analyses indicated that food arthropod activity-density 
was twice as high on managed compared to idle land (F = 53.60; 
df = 1,20; P < 0.001). Pasture level activity-density was also lowest 
in idle compared to deferred and grazed, which did not differ (Fig. 
2b; Table 3).
Activity-density of Coleoptera represented the largest portion of 
food items in deferred (33.7%), grazed (26.4%), and idle (29.5%) 
pastures (Table 4). Ranked second was Lepidoptera larvae in de-
ferred (25.5%) and grazed (18.5%) and Orthoptera in idle pastures 
(27.7%) (Table 5). Ranked third was Orthoptera activity-density in 
deferred (8.8%) and grazed (12.5%) and Lepidoptera in idle pastures 
(2.1%) (Table 5). Among Coleoptera, food arthropod activity-den-
sity of Carabidae and Chrysomelidae did not differ among deferred, 
grazed, and idle pastures (Table 4). Tenebrionidae, Coccinellidae, 
and Curculionidae activity-density differed with the largest weekly 
catch recorded in deferred and grazed pastures. Scarabaeidae activ-
ity-density was greatest in idle when compared to deferred and 
grazed pastures (Table 4). Gryllidae, Acrididae, and Tettigoniidae 
treatment activity-density did not differ but Formicidae was greatest 
in idle compared to deferred and grazed pastures (Table 4).
Predators
Contrast analyses indicated that predator activity-density was twice 
as high on idle compared to managed land (F = 63.17; df = 1,20; 
P < 0.001) and that treatment activity-density was greatest in idle 
compared to grazed and deferred pastures, which did not differ (Fig. 
2c; Table 3). Predator activity-density in idle, compared to grazed 
and deferred pastures, was dominated by an approximate fivefold in-
crease in Lycosidae. A total of 11 Araneae Families were captured in 
deferred and grazed compared to only four in Idle pastures (Table 4).
Detritivores
Contrast analyses indicated that Detritivore activity-density was 
three times greater on idle compared to managed land (F = 43.40; 
Deferred Grazed Idle
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Fig. 2. Deferred, grazed, and idle pasture activity-density for a) all arthropods, b) food arthropods, c) predators, and d) detritivores where bars represent 
normal distribution weekly catch least squared means and error bars represent the standard error. Deferred and grazed pastures are associated with livestock 
grazing while idle land has not be grazed by livestock in over a decade. Sampling was conducted during the 2012–2015 field seasons north of Lavina, MT. Bars 
with different letters differ (α = 0.05); A generalized linear mixed model with random year and a negative binomial distribution were fit to count data offset by 
sampling weeks; LSD (Proc GLIMMIX, SAS Institute 2008) where all comparisons df = 2,20.
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df  =  1,20; P  <  0.001) and that pasture level activity-density was 
greatest in idle compared to deferred and grazed, which did not 
differ (Fig. 2d; Table 3).
Captures of Tenebrionidae, Nitidulidae, and Scarabaeidae rep-
resented over 95% of total activity-density in deferred and grazed 
pastures. Nitidulidae, Histeridae, and Scarabaeidae represented 
approximately 94% of the activity-density on idle land (Table 4). 
Activity-density of dung-feeding Scarabaeidae was greatest in idle 
compared to grazed and deferred pastures (Table 4) and was dom-
inated by four dung feeding beetles, Rhyssemus germanus (L., 
Coleoptera:   Scarabaeidae), Canthon pilularius (L., Coleoptera:   
Scarabaeidae), Onthophagus nuchicornis (L., Coleoptera:   
Scarabaeidae), and Aphodius distinctus (Müller, Coleoptera:   
Scarabaeidae). Scarabaeidae activity-density was 4.4 and 5.4 times 
greater in idle than in deferred and grazed pastures, respectively 
(Table 4).
Family-Level Diversity
Total activity-density diversity, measured with Simpson’s 1-D, was 
greater in grazed than idle pastures (F = 2.72; df = 2,20; P = 0.012) 
but did not differ from deferred which did not differ from idle. Food 
arthropods were most diverse in deferred and grazed, and least di-
verse in idle pastures (Table 5).
Discussion
Differences in arthropod community structure among treatments 
mirrored patterns in vegetation structure, with the largest contrasts 
between managed and idle land. Although total arthropod activi-
ty-density was greatest on idle land, this pattern masked important 
variation in responses of arthropod taxa to long-term absence of 
grazing. Greater activity-density on idle land was driven by a tri-
pling of detritivores and a doubling of predators, primarily Lycosid 
spiders. In contrast, food arthropod activity density was two times 
greater and Family-level diversity slightly greater on managed land. 
Arthropod diversity may be particularly important to birds if it pro-
motes temporal stability in this critical food resource (Sullins et al. 
2018). In total, we observed 9 of 14 food arthropod Families at their 
lowest activity-density on idle land, but low statistical power likely 
limited our ability to detect significant reductions of some families. 
Interestingly, most of these Families achieved their highest activ-
ity-density in deferred pastures, suggesting they may benefit from 
structural changes in vegetation (Pöyry et al. 2006, Fritch et al. 2017) 
afforded by periodic rest from grazing, for example, via increased 
reproduction or overwintering survival. However, long-term absence 
of grazing appears to have altered the structure of the arthropod 
community such that these benefits were negated. Notably, the 
activity-density of lepidopteran larvae, which are especially impor-
tant prey for a variety of birds including greater sage-grouse (Gregg 
and Crawford 2009), was lowest on idle land. Other studies have 
reported that low (Lyons et al. 2017) and intermediate levels (Kaltas 
et al. 2013) of grazing can increase both abundance and species rich-
ness of carabid beetles, an avian food source.
In contrast to other studies (Jepson-Innes and Bock 1989, O’Neill 
et  al. 2010), our results suggest that Orthopteran activity-density 
is unchanged in grazed areas. However, our sampling method may 
have obscured negative effects of grazing on Orthopteran activ-
ity-density if reduced vegetation structure on managed land posi-
tively biased capture rates relative to the more structurally complex 
idle treatment (Schirmel et al. 2009). Activity-density of Formicidae 
was greatest on idle land, and may contrast with other studies, sug-
gesting ants respond more strongly to differences in soil and vegeta-
tion type (not measured in our study) than grazing (Hoffmann 2010). 
Though we did not include Araneae among avian food focal taxa, 
their importance to avian diets is debatable and may vary by Family 
(Gunnarsson 2007). In several studies, spiders were fed to nestlings 
of grassland and shrubland passerines (Wiens and Rotenberry 1979, 
Petersen and Best 1986); however, these studies do not report the 
particular Families represented in diets. Lycosids are sit-and-pursue 
predators, which quickly stalk and ambush prey (Dondale 2005) in 
the lower vegetative canopy and on the ground (Schmitz and Suttle 
2001), which may make Lycosids a particularly challenging prey 
for birds. That Lycosids did not appear in the diets of lesser prairie 
chickens despite the common appearance of other Araneae Families 
suggests they may not be important foods for precocial young of 
grouse (Sullins et al. 2018). Regardless of whether Araneae are ex-
cluded outright from food arthropods or only non-Lycosid spiders 
are included, the relative trends in activity-density of food arthro-
pods among treatments remains the same.
The most notable consequence of long-term idling was reduced 
bare ground, likely from increased litter cover, as grass and shrub 
cover were similar among treatments. Differences among treat-
ments are consistent with Smith et  al. (2018), who observed in-
creased litter cover and reduced bare ground on the Lake Mason 
National Wildlife Refuge. These effects of grazing exclusion on vege-
tation structure are generally consistent with findings in other North 
American grassland or shrubland ecosystems where researchers have 
documented greater litter depth (Naeth et al. 1991) cover (Adler and 
Table 3.  Treatment least squared means ± SE of 2012–2015 weekly catch, food arthropods, and the functional groups of predators and 
detritivores collected from pitfall traps located in deferred, grazed, and idle pastures north of Lavina, MT
Treatment Defer vs. graze Defer vs. idle Graze vs. idle
 Deferred Grazed Idle t P t P t P
Weekly catch 151.46 ± 24.86b 139.25 ± 24.86b 287.25 ± 30.15a 1.16 0.261 5.07 <0.001 4.36 <0.001
Predatorsa 53.11 ± 9.01b 50.02 ± 9.01b 127.20 ± 12.28a 0.66 0.514 7.24 <0.001 7.85 <0.001
Detritivoresb 12.24 ± 6.09b 7.56 ± 6.09b 40.55 ± 7.16a 1.33 0.199 5.72 <0.001 6.70 <0.001
Food Arthropodsc 113.14 ± 33.03a 105.38 ± 33.03a 46.05 ± 35.48b 0.91 0.373 3.70  0.001 4.40 <0.001
Least squared means in rows followed by different letter groupings statistically differ (α = 0.05); LSD (Proc GLIMMIX, SAS Institute 2008) where all compari-
sons df=2,20.
aPredator functional group is the summed total of Coleoptera: Carabidae, Coccinellidae, Histeridae; Araneae: Lycosidae, Dictynidae, Amaurobiidae, and 
Pisauridae.
bDetritivores is the summed total of Coleoptera: Tenebrionidae, Scarabaeidae, Dermestidae, Nitidulidae.
cFood Arthropods is the summed total of Coleoptera: Carabidae, Tenebrionidae, Scarabaeidae, Chrysomelidae, Curculionidae, Coccinellidae; Lepidoptera: 
Arctiidae, Noctuidae, Saturniidae, Pieridae; Hymenoptera: Formicidae; Orthoptera: Gryllidae, Acrididae, Tettigoniidae.
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Lauenroth 2000) or biomass (Willms et al. 2002, Farrell et al. 2015) 
and reduced bare ground cover (Naeth et  al. 1991, Yeo 2005)  in 
ungrazed compared to grazed areas (but see Davies et al. 2018).
The reduction of bare ground from litter accumulation is 
reflected in higher activity-density of predators on idle land. This 
finding agrees with previous work showing predatory arthropods 
respond positively to vegetation and detritus (Langellotto and 
Denno 2004) and are more abundant without grazing (Farrell 
et  al. 2015, Fritch et  al. 2017). Predator families differed in their 
response to treatments; the fivefold higher activity-density of 
Lycosidae on idle land was accompanied by a reduction in most 
other predatory Families. Notably, 9 of 13 predatory Families 
Table 4.  Arthropod family least squared means ± SE of weekly activity-density during 2012–2015 field seasons from pitfall traps located in 
deferred, grazed, and idle pastures north of Lavina, MT
Treatment Defer vs. graze Defer vs. idle Graze vs. idle
 Deferred Grazed Idle t P t P t P
Coleoptera          
Carabidae 12.25 ± 5.25 12.28 ± 5.25 0.55 ± 6.11 0.10 0.921 1.24 0.228 1.32 0.203
Tenebrionidae 11.12 ± 2.86a 6.29 ± 2.86ab 0.56 ± 3.26b 2.00 0.060 2.69 0.019 1.35 0.191
Scarabaeidae 3.25 ± 2.18b 2.64 ± 2.18b 14.33 ± 2.12a 1.09 0.287 4.70 <0.001 5.66 <0.001
Chrysomelidae 2.09 ± 0.79 1.64 ± 0.79 −0.37 ± 0.89 0.63 0.533 0.24 0.816 0.12 0.908
Curculionidae 4.90 ± 1.73a 3.46 ± 1.73a −0.99 ± 2.12b 0.90 0.378 2.47 0.023 2.19 0.041
Coccinellidae 1.57 ± 0.69 1.43 ± 0.69 −0.48 ± 0.70 0.05 0.963 0.01 0.989 0.01 0.989
Histeridae 0.50 ± 8.98b 0.65 ± 8.98b 39.16 ± 9.72a 0.79 0.443 6.70 <0.001 5.77 <0.001
Dermestidae 0.42 ± 0.60b 0.16 ± 0.60b 5.50 ± 0.65a 1.26 0.227 5.38 <0.001 5.86 <0.001
Meloidae 0.35 ± 2.77b 1.84 ± 2.77b 21.08 ± 2.99a 1.28 0.216 6.66 <0.001 6.17 <0.001
Silphidae 1.62 ± 3.33b 2.86 ± 3.33b 46.00 ± 4.30a 1.67 0.110 6.75 <0.001 5.13 <0.001
Nitidulidae −1.46 ± 3.96b −0.14 ± 3.96b 12.84 ± 4.00a 0.69 0.501 3.35 0.004 2.80 0.014
Elateridae 15.57 ± 6.33 4.68 ± 6.32 5.00 ± 7.56 1.35 0.191 0.66 0.5193 1.72 0.102
Melyridae 1.78 ± 3.77 7.96 ± 3.77 −1.14 ± 3.90 2.47 0.026 0.01 0.994 0.01 0.993
Lepidoptera          
Noctuidae 4.18 ± 2.63a 5.24 ± 2.63a −2.46 ± 2.88b 0.60 0.555 2.74 0.012 3.12 0.005
Arctiidae 24.49 + 4.79a 12.84 ± 4.79a −0.82 ± 6.03b 0.83 0.416 3.14 0.005 2.48 0.022
Saturniidae 0.12 ± 0.10 0.36 ± 0.10 0.01 ± 0.13 1.68 0.109 0.00 0.998 0.00 0.998
Pieridae 0.01 ± 0.69 1.08 ± 0.69 1.11 ± 0.89 0.00 0.998 0.00 0.998 3.02 0.007
Hymenoptera          
Formicidae 39.96 ± 15.49b 47.00 ± 15.49b 32.62 ± 17.66a 0.17 0.863 2.82 0.011 3.14 0.005
Orthoptera          
Gryllidae 5.49 ± 4.40 8.82 ± 4.41 2.93 ± 5.10 0.85 0.403 0.86 0.400 1.50 0.142
Acrididae 4.58 ± 2.67 4.37 ± 2.67 7.96 ± 3.26 0.24 0.811 0.56 0.560 0.76 0.455
Tettigoniidae 0.04 ± 0.03 0.03 ± 0.03 0.01 ± 0.04 0.26 0.7984 0.00 0.998 0.00 0.998
Araneae          
Lycosidae 15.44 ± 4.72b 14.76 ± 4.72b 76.84 ± 5.99a 0.11 0.913 8.56 <0.001 8.92 <0.001
Gnaphosidae 15.76 ± 3.45a 15.24 ± 3.45a 5.73 ± 4.07b 0.24 0.759 2.43 0.023 2.29 0.032
Philodromidae 4.49 ± 0.91 3.04 ± 0.91 1.74 ± 1.11 1.62 0.114 1.01 0.325 2.03 0.056
Salticidae 2.83 ± 0.55a 1.95 ± 0.55ab 0.68 ± 0.67b 1.26 0.220 2.30 0.031 1.50 0.146
Thomisidae 1.52 ± 0.40 1.72 ± 0.40 −0.09 ± 0.51 0.24 0.816 0.01 0.993 0.01 0.993
Hahniidae 0.58 ± 0.42 0.68 ± 0.42 −0.15 ± 0.53 0.19 0.848 0.00 0.996 0.00 0.996
Linyphiidae 0.87 ± 0.34 0.94 ± 0.34 0.19 + 0.34 0.13 0.900 0.00 0.998 0.00 0.998
Liocranidae 0.89 ± 0.28 0.93 ± 0.28 0.13 ± 0.29 0.27 0.794 0.00 0.999 0.00 0.997
Dictynidae 0.21 ± 0.16 0.25 ± 0.16 0.08 ± 0.17 0.20 0.847 0.00 0.998 0.00 0.998
Amaurobiidae 0.32 ± 0.14 0.25 ± 0.10 0.01 ± 0.10 0.42 0.677 0.00 0.995 0.00 0.995
Pisauridae 0.28 ± 0.34 0.47 ± 0.34 0.29 ± 0.34 0.99 0.336 0.01 0.995 0.01 0.995
Least squared means in rows followed by different letter groupings statistically differ (α = 0.05); A generalized linear mixed model with a random year effect and 
a negative binomial error distribution was fit to count data offset by sampling weeks; LSD (Proc GLIMMIX, SAS Institute 2008) where all comparisons df = 2,20.
Table 5.  Least squared means ± SE of Simpson’s 1-D for Total Diversity and Food Arthropod Diversity calculated from 2012 to 2015 pitfall 
traps located in deferred, grazed, and idle pastures north of Lavina, MT
Simpson’s 1-Da Defer vs. graze Defer vs. idle Graze vs. idle
 Deferred Grazed Idle t P t P t P
Total Diversity 0.88 ± 0.02ab 0.90 ± 0.02a 0.81 ± 0.03b 0.87 0.392 1.96 0.062 2.72 0.012
Food Arthropod Diversity 0.71 ± 0.03a 0.70 ± 0.03a 0.59 ± 0.04b 0.23 0.821 2.07 0.050 1.87 0.075
Least squared means in rows followed by different letter groupings statistically differ (α = 0.05); A generalized linear mixed model with a random year effect 
and a negative binomial error distribution was fit to count data offset by sampling weeks; LSD (Proc MIXED, SAS Institute 2008) where all comparisons df=2,20.
aSimpson’s 1-D calculated from 54 families.
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were never captured on idle land. North American Lycosids are, 
with the exception of Sosippus spp., (Araneae:   Lycosidae), non-
web building predators that stalk and ambush prey and may 
therefore benefit from elevated levels of detritus, which provides 
abundant cover. Increased habitat complexity is also thought to 
decrease intraguild predation and cannibalism (Denno et al. 2005). 
Although increases in Lycosid spider activity-density may indicate 
reduced cannibalism, our observation of reduced Family-level di-
versity on idle land suggests Lycosids, where they achieve extreme 
dominance, may exert strong top-down effects on arthropod com-
munities including exclusion of other predatory taxa, perhaps 
through intraguild predation.
The accumulation of litter is also reflected in higher activity-den-
sity of all detritivores except Tenebrionidae on idle land. The op-
posite response of Tenebrionidae suggests that long-term grazing 
exclusion does not consistently benefit species belonging to this 
family. Similarly, Newbold et  al. (2014) reported from the short 
grass steppe higher species richness and greater abundances of cer-
tain Tenebrionidae species in long-term grazing exclosures, but also 
reported that some species increased with grazing.
Unexpectedly, we recorded lower activity-density of Scarabaeidae, 
most of which were dung beetles, in subfamilies Aphodiinae (Leach) 
and Scarabaeinae (Larreille), on managed land despite presumably 
greater dung food resources. Moderate livestock grazing typically 
benefits dung beetle populations (Lobo et  al. 2006, Verdu et  al. 
2007). Dung beetles opportunistically feed on detritus (Holter et al. 
2002), which was abundant on idle land; however, detritus would 
only benefit adults as their young are provisioned with dung (Scholtz 
et  al. 2009a). An alternative explanation for reduced dung beetle 
activity-density on managed land is the treatment of livestock with 
veterinary parasiticides. Residues of these parasiticides pass from the 
animal to the pasture via dung and lethal to sublethal effects can 
manifest in dung colonizing species (Wall and Benyon 2012).
We hypothesize high plant detritus may also explain high activ-
ity-density of Dermestidae and Nitidulidae on idle land, whereas 
high activity-density of Histeridae may be a response to abundant 
eggs and larvae. Dermestidae, Nitidulidae, and Histeridae are diverse 
families of beetles associated with plant detritus, souring plant fluids, 
dung, and carrion. Dermestidae and Nitidulidae primarily consume 
these items while Histeridae mainly prey upon the smaller insects 
and eggs living in these materials (van Emden 2013). Silphidae are 
primarily necrophagous with adults of some species feeding on 
plant detritus or dung (Anderson and Peck 1985). Approximately 
98% of Silphidae in our study were identified as Nicrophorus 
spp. (Coleoptera: Silphidae), which are carrion obligates of small 
mammal carcasses. Therefore, higher activity-density of Silphidae 
suggests small mammals may be more abundant on idle land. Small 
mammals, especially microtine rodents, are often positively associ-
ated with vegetation biomass and commonly respond negatively to 
large mammal grazing (Grant et al. 1982, Warren and Frisina 2013). 
Given that Dermestidae and Nitidulidae are common catches in car-
rion beetle traps (Shubeck 1976) we also speculate their presence on 
idle land could be in part due to a higher small mammal population 
providing an additional food source.
Pitfall trapping may be subject to bias if treatments differ in 
structural complexity (Melbourne 1999); however, inference in 
that study was limited to ants (Hymenoptera: Formicidae). We 
documented differences in bare ground among treatments, sug-
gesting greater cover of bare ground on managed lands may have 
been partly responsible for higher activity-density of some taxa. 
However, total activity-density was generally highest in deferred 
pastures, which had less bare ground than grazed pastures. We 
therefore doubt that the confounding effect of habitat structure 
on arthropod activity-density substantially affected our inference. 
Despite its limitations, pitfall trapping remains a very suitable 
method to sample mobile ground-dwelling arthropods (Taboada 
et al. 2010, Oxbrough et al. 2012), many of which are food items 
for birds.
Pitfall trapping may not accurately reflect abundance of cer-
tain vegetation-dwelling taxa such as grasshoppers (Orthoptera: 
Acrididae) across gradients in vegetation structure (Schirmel et al. 
2009). However, our use of pitfall traps reflects our objective to 
characterize relative availability of arthropods for consumption by 
sage-grouse chicks, which are incapable of flight. Given this phys-
ical constraint, only those arthropods located on or very near to 
the soil surface are available for consumption. Thus, although we 
cannot infer treatment effects on abundance of grasshoppers, activ-
ity-density measured by pitfall trapping remains a useful index of 
food availability for ground-foraging birds. Furthermore, Schmitz 
(2005) reports from a caged field study on behavioral changes in 
Melanoplus spp. grasshoppers, a genus common to our study area, 
in response to various spider hunting modes. The presence of a cano-
py-dwelling nursery web spider (Pisaurina spp., Araneae: Pisauridae) 
caused grasshopper prey to move to locations closer to or on the 
soil surface. In contrast, when a ground-dwelling Lycosid spider 
(Rabidosa spp., Araneae: Lycosidae) was introduced, grasshoppers 
moved from the soil surface to higher vegetation. As the most abun-
dant predator on idle lands by a large margin, Lycosid spiders may 
elucidate an additional indirect reduction of available chick food on 
the soil surface regardless of whether a reduction in absolute abun-
dance occurred.
Sampling of Lake Mason National Wildlife Refuge was added 
opportunistically during the study as new project partners brought 
new questions and resources. The idle treatment therefore lacked the 
Fig. 3. Yearly degree-day accumulations (Y-axis) calculated by the single-
sine method (Coop 2002) from weather station HORSE THIEF HORM8 RAWS 
(46.4256N; 108.6742W; elevation 1,061 m) using 0° C lower and 35° C upper 
thresholds versus ordinal date (X-axis). The vertical shaded bar represents 
the range of ordinal dates where our sample occurred during the 2012–2015 
field seasons and displays virtual analogues accumulations of degree-days 
between 2015 where Idle was sampled but not Managed and 2012 where 
Managed was sampled but not Idle and also between 2013 and 2014. This 
indicates that the inferences we have drawn from these data are appropriate 
in relation to the discussed arthropod taxa.
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replication and dispersion of our grazed and deferred treatments and 
was sampled in a different, but overlapping, set of years. We included 
these samples in spite of these caveats because it was the only nearby, 
ecologically similar land where livestock grazing had been absent 
for a long period and therefore presented a unique opportunity to 
measure the effects of livestock exclusion on arthropods. Sampling 
in different years has the potential to confound our inference 
regarding treatment differences if nonoverlapping years differed 
with respect to degree-day accumulation (Coop 2002), the primary 
environmental factor determining arthropod emergence and activ-
ity. However, degree-day accumulation in 2015 (idle but no man-
aged land sampling) and 2012 (managed but no idle land sampling) 
were virtual analogues of each other (Fig. 3) as were 2013 and 2014 
when all treatments were sampled. This alleviated our initial con-
cern that treatment and year effects could be strongly confounded. 
Furthermore, our inclusion of a random effect for year should help 
ensure that comparisons reflect treatment differences rather than 
annual effects. Nevertheless, additional replication would help clar-
ify the generality of our findings with respect to the effect of grazing 
exclusion on arthropod communities in sagebrush rangelands.
Periodic deferment or rest are commonly prescribed to achieve 
desired conservation outcomes on rangelands and our findings sug-
gest several arthropod taxa comprising important food resources for 
shrubland and grassland birds may benefit from this practice. In this 
grazing-adapted ecosystem, however, long-term absence of grazing 
or other disturbance dramatically altered the structure of the vege-
tative and arthropod communities, ultimately resulting in reduced 
availability of the arthropod taxa most important to shrubland and 
grassland avifauna.
Livestock grazing, by periodically removing aboveground plant 
biomass and reducing litter accumulation, may indirectly suppresses 
predatory arthropods, such as Lycosidae, thus numerically releasing 
populations of prey taxa (Price et al. 2011) some of which are crit-
ical food items for birds. Thus, when viewed from the perspective of 
energy availability for higher trophic levels, managed grazing may 
serve a valuable conservation function. Dung-feeding Scarabaeidae 
were a notable exception and if their reduced activity-density is a re-
sponse to livestock parasiticide use, we would expect that mitigation 
of this source of mortality or morbidity would further increase this 
food source on managed lands. Conservation of dung beetles due to 
their contributions to functioning ecosystems needs to become more 
of a focal point for land management due to their innate characteris-
tics as bio-indicators (Scholtz et al. 2009b) and their quick response 
to vegetative changes (Tocco et al. 2013). Given the importance of 
all arthropod taxa, not only as critical avian food but also as vital 
components of healthy ecosystems, further studies into the effects of 
land use on food web processes are needed for conservation of native 
species and native landscapes.
Supplementary Data
Supplementary data are available at Environmental Entomology 
online.
Acknowledgments
The authors are appreciative for the constructive comments from two an-
onymous reviewers which lead to considerable improvements to the manu-
script. We also thank Marni G. Rolston and Greg D. Johnson for their help 
and use of lab space and equipment for sample processing and Emmett Wester 
for his help with field sampling and sample preparation. We also thank the 
participating landowners who graciously allowed us access to their property 
to complete this project. Funding for this project was provided in part by the 
USDA Conservation Effects Assessment Project and the Pittman-Robertson 
Wildlife Restoration Act through the United States Department of the Interior, 
Fish and Wildlife Service.
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