i 

Bison Grazing Ecology at the Rocky Mountain Arsenal 

National Wildlife Refuge, Colorado

Open-File Report 2013–1112

U.S. Department of the Interior

U.S. Geological Survey
ii

Cover photographs: Close-up profile of buffalo bull, courtesy of Ryan Hagerty, U.S. Fish and Wildlife Service; 

Grazing bison, courtesy of Steve Hillebrand, U.S. Fish and Wildlife Service.
i 

Bison Grazing Ecology at the Rocky Mountain Arsenal 

National Wildlife Refuge, Colorado

By Stephen Germaine, Linda C. Zeigenfuss, and Kathryn A. Schoenecker

Open-File Report 2013–1112

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013 

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Suggested citation:

Germaine, Stephen, Zeigenfuss, L.C., and Schoenecker, K.A., 2013, Bison grazing ecology at the Rocky

Mountain Arsenal National Wildlife Refuge, Colorado: U.S. Geological Survey Open-File Report 2013–1112, 

20 p., http://pubs.usgs.gov/of/2013/1112/.

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iii

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 1

Study Area..................................................................................................................................................................... 3

Methods......................................................................................................................................................................... 3

Mapping Pasture Vegetation...................................................................................................................................... 3

Vegetation Productivity, Utilization, and Species Composition................................................................................... 4

Bison Habitat Selection.............................................................................................................................................. 7

Statistical Analysis ..................................................................................................................................................... 7

Results........................................................................................................................................................................... 9

Vegetation Productivity, Utilization, and Species Composition................................................................................... 9

2007 ....................................................................................................................................................................... 9

2010 ..................................................................................................................................................................... 11

Bison Habitat Selection............................................................................................................................................ 17

Discussion ................................................................................................................................................................... 18

References Cited......................................................................................................................................................... 20
iv

Figures

1. Rocky Mountain Arsenal National Wildlife Refuge (NWR) 2007 vegetation map and locations of 2007 

and 2010 (July sampling plots shown; September sampling plots not shown) U.S. Geological Survey 

vegetation sampling sites .......................................................................................................................................... 5

2. Total herbaceous standing crop (kilograms per hectare; mean ±1 standard error) estimates for each 

of four vegetation cover types defined for the June and September, 2007 exclosure-cage sampling sessions 

at Rocky Mountain Arsenal National Wildlife Refuge, Colorado .............................................................................. 10

3. Percent of total herbaceous standing crop comprised of cheatgrass in each of four vegetation cover 

types on Rocky Mountain Arsenal National Wildlife Refuge bison pasture as defined for sampling in June 

and September 2007 ............................................................................................................................................... 11

4. Location data gathered from one cow bison fitted with a GPS radio-collar at Rocky Mountain Arsenal 

National Wildlife Refuge, Colorado, March 3, 2008, through April 10, 2008 ............................................................ 17

Tables

1. Mann-Whitney U test statistics for assessing effects of herbicide applications on plant biomass in 

kilograms per hectare (kg/ha) in each of five vegetation functional groups during July and September 

2010 at Rocky Mountain Arsenal National Wildlife Refuge, Colorado ....................................................................... 8

2. Consumption (kg/ha [kilograms per hectare]) and offtake (in percent) values for four vegetation 

cover types defined for June and September 2007 sampling dates at Rocky Mountain Arsenal National 

Wildlife Refuge, Colorado. Data are mean ±1 standard error (s.e.)......................................................................... 10

3. Standing crop (kilograms per hectare [kg/ha]); median and 90-percent confidence interval (CI) of 

vegetation in the Rocky Mountain Arsenal bison pasture, July and September 2010. Non-overlapping 

confidence intervals indicate differences between groups....................................................................................... 13

4. Percent consumption (offtake) of vegetation by bison and prairie dogs (median and 90-percent 

confidence interval [CI]) in the Rocky Mountain Arsenal bison pasture, July and September 2010. 

Non-overlapping confidence intervals indicate differences between groups............................................................ 15
v 

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

NOTE TO USGS USERS: Use of hectare (ha) as an alternative name for square hectometer (hm2

) is restricted to the 

measurement of small land or water areas. 

Abbreviations Used in This Report

FWS U.S. Fish and Wildlife Service

GIS geographic information system

GPS global positioning system 

LSD least significant difference

NCSG native cool season graminoids 

NNCSG non-native cool season graminoids 

NWR National Wildlife Refuge

NWSG native warm season graminoids 

RMA Rocky Mountain Arsenal 

USGS U.S. Geological Survey 
1 

Bison Grazing Ecology at the Rocky Mountain Arsenal 

National Wildlife Refuge, Colorado

By Stephen Germaine, Linda C. Zeigenfuss, and Kathryn A. Schoenecker

Abstract

The Rocky Mountain Arsenal (RMA) National Wildlife Refuge reintroduced bison to a small 

pasture in 2007. Refuge managers needed information on the effects of bison grazing on vegetation 

communities in the bison pasture as well as information on how bison might affect other management 

priorities at RMA. In particular, RMA managers were interested in bison grazing effects on vegetation 

productivity, amount of vegetation utilization by bison, and habitat selection by bison to inform RMA 

herd managers and for potential expansion of bison range on the refuge. In 2007, U.S. Geological 

Survey (USGS) designed a study to investigate bison grazing effects through measurement of vegetation 

in the 600-hectare enclosure where the bison are currently pastured. This research was a collaborative 

effort between USGS and RMA refuge staff and had active field components in 2007 and 2010. We 

found that the effects and intensity of bison grazing on vegetation in the RMA bison pasture is linked to 

prairie dog presence. Where both species were present, they were removing a significant amount of 

biomass compared to areas where only bison were present. Also, prairie dogs appeared to enhance the 

greater production of native forbs, but we were not able to identify the mechanism for this increased 

production. We were not able, however, to generate an accurate vegetation map for the bison pasture, 

and this limited our ability to achieve the level of statistical precision necessary to identify grazing 

impacts and habitat selection of bison. 

Introduction

The Rocky Mountain Arsenal (RMA) National Wildlife Refuge (NWR) encompasses 6,070 

hectare (ha) of shortgrass prairie approximately 16 kilometers (km) northeast of the center of Denver, 

Colorado. Historically, RMA was native shortgrass prairie; however, between 1880 and 1942, these 

lands were grazed by domestic cattle and converted to croplands and non-native grasses. The former 

farmland was bought in 1942 by the U.S. Army to establish the Rocky Mountain Arsenal, a chemical 

weapons factory. During World War II and the Cold War of the 1950s, the U.S. Army produced 

chemical weapons at the site and leased portions of the facility to private companies that produced 

agricultural pesticides. Years of chemical production left the industrial core of the site contaminated, but 

deer (Odocoileus virginianus), prairie dogs (Cynomys ludovicianus), coyotes (Canis latrans), eagles, 

and many species of hawks, owls, and other birds thrived in the surrounding fields, grasslands, and 

woodlots that had been protected from 40 years of urban sprawl and development. Extensive cleanup of 

the site began in the 1980s under an Environmental Protection Agency Superfund designation. 

In 1992, Congress designated the site as a future national wildlife refuge. Under the National 

Wildlife Refuge System Improvement Act of 1997, national wildlife refuges were tasked with 

maintaining biological integrity, diversity, and environmental health of the areas they protected. Under 
2 

these mandates, RMA identified restoration of native shortgrass prairie vegetation as a primary 

management objective. Removal of contaminated soils was begun, followed by restoration of native 

vegetation; both activities continued until completion in 2010. Between 2004 and 2006, 5,059 ha of 

RMA land was transferred from the U.S. Army to the U.S. Fish and Wildlife Service (FWS) to establish 

the Rocky Mountain Arsenal National Wildlife Refuge (RMA NWR). Cleanup activities by the Army 

and private companies were completed in 2010 and at that time an additional 1,012 ha was transferred to 

FWS making the RMA NWR the largest urban wildlife refuge in the United States (see U.S. Army 

Rocky Mountain Arsenal Web site at http://www.rma.army.mil/site/sitefrm.html). 

In 2007, the U.S. Fish and Wildlife Service translocated 16 American bison (Bison bison) from 

the National Bison Range, in Montana, to the RMA NWR, and later that year three calves were born. 

The next year, two yearling bulls from Sully’s Hill National Game Preserve in North Dakota were 

introduced to the herd. Seven bison calves were born at RMA during spring and summer 2008, bringing 

herd size to 28 bison. Through additional translocations and on-site reproduction, herd size increased to 

45 by July 2010. All bison are individually identified with microchips. They are currently pastured in a 

607-ha enclosure in the northwestern portion of RMA. The refuge is planning to develop facilities to 

conduct annual roundups in an effort to manage the population within carrying capacity and complete 

herd-health monitoring in future years (Dratch and Gogan, 2010). 

Bison can influence the composition and ecological function of prairie vegetation (Knapp and 

others, 1999), and it is not yet clear how they may affect vegetation restoration efforts at RMA. 

Therefore, monitoring the effects of bison on native shortgrass prairie restoration efforts became a 

necessary component of refuge management at RMA. The influence of bison on grassland bird species 

diversity and abundance is also unclear because breeding bird densities may either decrease or increase

in the presence of large grazers, depending on the bird species (Fontaine and others, 2004; Smith and 

Lomolino, 2004; Lueders and others, 2006). To help gather relevant information, the U.S. Geological 

Survey (USGS) and RMA staff developed a plan (K. Schoenecker, L. Zeigenfuss, T. Wright, and S. 

Skipper, written commun., January 7, 2008) to identify the effects of bison grazing on the vegetation of 

the RMA bison pasture in order to assess potential conflicts with other refuge management objectives. 

Of particular interest were the potential impacts of bison herbivory on nesting grassland bird habitat 

including plant species diversity; shrub-habitat structure and extent; vegetation productivity; and 

invasion of exotic species such as cheatgrass (Bromus tectorum) and crested wheatgrass (Agropyron 

cristatum) into native grasslands, particularly those recently restored to a mix of native sand prairie 

grasses. 

RMA managers asked USGS researchers to address three specific objectives at the outset of this 

study in 2007: (1) identification of the vegetation communities used most by bison in the pasture, (2) 

measurement of vegetation utilization by bison in each community, and (3) interpretation of how these 

changes might affect the breeding songbird community. These objectives were further refined to include 

measurement of changes to vegetation composition and quantification of productivity as well as

utilization rates among vegetation cover types (S. Germaine, USGS, unpub. data, June 3, 2009). In early 

2010, refuge managers requested that the study objectives be again changed to: (1) determining annual 

net-pasture-forage productivity and (2) measurement of vegetation functional-group responses to 

grazing (S. Berendzen, U.S. Fish and Wildlife Service, oral commun., January 28, 2010). This report 

details the methods and data collected during the 4-year effort (2007–2010) and identifies additional 

data and materials that RMA NWR staff will need if they wish to quantify bison herbivory effects at 

RMA in the future. 
3 

Study Area

RMA comprises 6,070 ha on the plains of eastern Colorado in the Denver suburb of Commerce 

City. Approximately 3,845 ha of the refuge are currently suitable for bison grazing with the potential to 

support 250 animals (T. Ronning, U.S. Fish and Wildlife Service, oral commun., March 15, 2009).

Since translocation to RMA, the bison have been pastured in a 607-ha enclosure in sections 27, 28, 33, 

and 34, T. 25 N., R. 67 W. in the west-central part of the refuge. At the time bison were reintroduced, 

vegetation in the pasture was believed to be composed of the following: approximately 227 ha of 

restored sand prairie (a mix of blue grama [Bouteloua gracilis], sand dropseed [Sporobolus 

cryptandrus], sandreed [Calamovilfa longifolia], and switchgrass [Panicum virgatum]); 157 ha of sand 

dropseed; 48 ha of needle-and-thread (Stipa spp.); 23.5 ha of blue grama; 21 ha of western wheatgrass 

(Pascopyrum smithii); 36 ha dominated by weedy forbs (Kochia spp. and Russian thistle [Salsola spp.]) 

associated with black-tailed prairie dog (C. ludovicianus) towns; 44 ha of yucca (Yucca glauca); 10 ha 

of mixed shrub (rubber rabbitbrush [Ericameria nauseosa], winterfat [Krascheninnikovia lanata], 

fringed sage [Artemisia frigida], and green sagebrush [Artemisia dracunculus]); and 23.5 ha of crested 

wheatgrass (Agropyron cristatum). Tree groves and roadways covered 1.2 ha, and 15.8 ha and were not 

accounted for. Areas of non-native vegetation in the pasture were being restored to native-dominated 

communities. 

Methods

Mapping Pasture Vegetation

We generated a pasture vegetation map in 2007 using color-infrared aerial-photographic imagery 

collected on July 11, 2003. This map was developed to provide quantitative baseline vegetation data in 

support of project objectives. From the remote imagery, we interpreted pasture vegetation into nominal 

cover types based on dominant grass/herb species present and then mapped patches of each cover type 

into a geographic information system (GIS). We then ground-truthed section 34 of the pasture 

vegetation map by visually identifying the dominant vegetation species present at each of 20 randomly 

located points. Error rates indicated that the initial mapping effort did a poor job of classifying 

vegetation cover in section 34 (T. Wright, U.S. Fish and Wildlife Service, oral commun., February 

2008). 

We conducted a second round of in-field vegetation mapping in section 34 during summer 2008 

and reclassified vegetation as native shortgrass, native midgrass, or exotic species-dominated and 

further defined each of these as cool-season or warm-season dominated. During this exercise, covertype assignments were made on hardcopy maps while traversing parallel transects separated by 

approximately 30 m. Assignments were not tied to global positioning system (GPS) coordinates, no 

additional error checking was conducted within section 34, and the vegetation map of the pasture 

outside of section 34 was not error-checked in either this or the initial (2007) mapping effort. 

Recognizing that a vegetation map containing known levels of error (that is, an error-checked 

map) was necessary for accurately estimating vegetation productivity and utilization in the pasture, in 

February 2010, we began a pilot effort to map pasture vegetation using software designed to aid the 

process of interpreting features present on remotely sensed imagery. Using Envi Feature Extraction 

software (ITT Visual Information Solutions, 2008), USGS staff performed a computer-aided 

segmentation of sections 27 and 28 of the pasture from a color-infrared aerial photograph dated June 27, 

2009. The segmentation identified six distinct vegetation types and identified boundaries of all patches 
4 

that exceeded 0.40 ha. Immediately following the segmentation, we conducted site visits to identify the 

dominant plant species present at 260 points that were systematically located throughout each of the six 

vegetation types. This data was then transferred to RMA personnel to be used to produce a revised 

pasture vegetation map. 

Vegetation Productivity, Utilization, and Species Composition

In 2007, we used two methods to evaluate the impacts of bison on pasture vegetation: exclosure 

cages and vegetation transects. First, based on the priorities identified by RMA biologists and using the 

2007 pasture vegetation maps, five vegetation types were identified: restored sand prairie, blue grama 

grasslands, needle-and-thread grasslands, western wheatgrass grasslands, and sand dropseed grasslands. 

These vegetation types were (1) considered to be representative of the majority of forage types in the 

bison pasture, (2) specifically informative for grassland birds that may experience habitat impacts from 

the reintroduction of bison, (3) found on both the pasture and other areas of the refuge outside of the 

pasture fence for potential control-impact comparison, and (4) important types for shortgrass prairie in 

general or had been restored to shortgrass prairie. However, we sampled only three of the vegetation 

types (blue grama, sand dropseed, and needle-and-thread) plus crested wheatgrass, which was added as 

another type during sampling. Sixteen sites were sampled—five in blue grama grasslands, five in 

needle-and-thread grasslands, four in crested wheatgrass communities, and two in sand dropseed 

grasslands (these last two sites were not recorded on a GPS and thus not mapped; fig. 1). Within each 

vegetation type, we randomly located clusters of 3–4 exclosure cages at each site. Each cage protected a 

1-m2

 area from bison grazing allowing us to estimate annual aboveground herbaceous productivity and 

compare bison forage utilization among vegetation types. Cages were installed in early spring, and 

vegetation was sampled in June. Cages were then randomly relocated and sampled again in September

2007. 

During March and Sept. 2007, we also sampled vegetation using a 50-m fixed-point line-transect 

methodology to determine plant species composition, density, and diversity at the 16 study sites. At 

each 1-m increment along transects, we tallied frequency of occurrence of bare soil, litter, rock, and 

each grass, forb, shrub, and tree species at points 1 m to each side of the transect centerline. These data 

were entered into an RMA database and transformed into percent cover for comparisons among 

vegetation types and years. From this data, a variety of comparisons are made possible including meanpercent cover of each species or functional group; relative live-percent cover; frequency that each

species, cover type, or functional group occurs at each site; percent total cover that is live vegetation; 

and percent total cover composed of native vegetation. This method had been used historically at RMA 

NWR, so results should have been directly comparable with previously collected data. 

After 2007, we recognized that prairie dogs occupied roughly half of the pasture area making 

necessary a second type of cage that would exclude both prairie dogs and bison. Therefore, in May 

2010, we identified all areas potentially occupied by prairie dogs by buffering the 2009 GIS pasture 

prairie dog map outward by 100 m to account for expansion that might have occurred since then. We 

then deployed one cage of each type at 41 randomly located prairie dog sites and one bison exclosure 

cage at each of 39 randomly located non-prairie dog sites. Eighty new random plot locations were 

generated after the first sampling, and cages were moved to those locations in preparation for the late 

summer sampling session. Concurrent with vegetation sampling during both sessions, we tallied the 

number of active prairie dog burrows within 10 m and 25 m of each site center to determine whether 

prairie dogs were actually present or not. In instances where prairie dogs were present at plots that had 

been misclassified as bison-only plots, the data were discarded because no prairie dog exclusion cage 
5 

was installed at the site, and the full suite of grazing effects could not be represented from these 

particular sites. In instances where prairie dogs were absent from plots that had been misclassified as 

having them present, data from the bison-prairie dog exclosure cages were discarded, whereas data from 

the bison exclosure and the non-exclosed plot were retained and reclassified as a bison-only plot. 

Due to the high error rates present in the 2007 pasture vegetation map, sample sites still could 

not be partitioned accurately by vegetation community in 2010. The study was further confounded by 

multiple herbicide treatments that had been broadly applied to some areas of the pasture to eliminate 

invasive forb species in 2010, but personnel associated with sample-site selection and vegetation 

sampling were not made aware of the herbicide applications until after sampling was well underway. 

Only general areas treated with herbicide were identified by RMA personnel; based on this information, 

we had to make a determination as to whether an individual sampling location had likely been subjected 

to herbicide treatment.

Figure 1. Rocky Mountain Arsenal National Wildlife Refuge (NWR) 2007 vegetation map and locations of 2007 

and 2010 U.S. Geological Survey vegetation sampling sites. T. 25 N., R. 67 W. July sampling plots shown; 

September sampling plots not shown. 
6 

In both 2007 and 2010, we clipped all grasses, forbs, and sub-shrubs at ground level in a 

circular, 0.25-m2 plot located inside each cage and another plot placed 5 m north of the bison exclosure. 

Sites were sampled in late June 2007 and mid-July 2010 and again in September in both years. In 2007, 

we sorted vegetation by species to afford measures of seasonal peak-standing crop (the current year’s 

growth). In 2010, RMA staff revised the nominal plant groupings, and vegetation was sorted into five 

major plant functional groups—native forbs, non-native forbs, native cool season graminoids (NCSG), 

non-native cool season graminoids (NNCSG), and native warm season graminoids (NWSG). Vegetation 

samples were bagged, labeled, oven dried at 55 ο

C for ≥48 hours, and then weighed. After each 

sampling event, we randomly relocated cages for the next sampling visit.

Vegetation productivity (B) was calculated as the standing crop inside the most exclusive cage 

present at each plot (that is, bison and prairie dog exclosures at all prairie dog-occupied sites and bison 

exclosures at non-prairie dog sites) at the time of sampling. Since the cages were not sampled on a 

monthly basis throughout the growing season, aboveground net primary productivity could not be 

estimated. If no biomass was collected for a particular functional group in a clipped quadrat, then a 

value of zero was entered for that group. 

We determined utilization and offtake (see below) of herbaceous forage following Bonham 

(1989). In areas where only bison were present, utilization of forage by bison was estimated as the 

difference between standing crop inside the bison exclosure cages versus standing crop outside the 

exclosure cages (equation 1). In areas where both bison and prairie dogs occurred, utilization of forage 

by bison was defined as the difference in standing crop inside the bison exclosure cage versus in the 

unexclosed plot (equation 1), whereas prairie dog utilization was defined as the difference in standing 

crop inside the bison-prairie dog exclosure versus inside of the bison exclosure cage (equation 2).

Ubison = Pbison exclosure–Puncaged (1) 

Uprairie dogs = Pbison-prairie dog exclosure–Pbison exclosure (2) 

Here Ubison and Uprairie dogs are amounts of forage used by bison and prairie dogs, respectively; 

Pbison exclosure is the amount of biomass inside the bison exclosure cage; Pbison-prairie dog exclosure is the 

amount of biomass inside the bison-prairie dog exclosure cage; and Puncaged is the amount of biomass in 

paired uncaged plot. 

Percent offtake was the preferred variable for comparing utilization between various vegetation 

types or other groupings because it scales the measure of consumption of forage to the total biomass 

present in a particular vegetation type or individual sampling site. Percent offtake by bison and prairie 

dogs for the growing season was calculated as the percentage of production that was consumed 

(utilized) following equation 3. 

Ox = (Ux÷ Bungrazed) ×100 (3) 
7 

Here Ox = percent offtake of standing crop by ungulate species x; Ux = utilization by ungulate species x; 

and Bungrazed = ungrazed standing crop within the most exclusive cage. 

In cases where no biomass was measured for one of the five major functional groups inside the 

ungrazed cage, offtake could not be determined (due to division by zero), and this cage was eliminated 

from offtake calculations. 

Bison Habitat Selection 

To assess habitat use by bison, we attached a GPS radio collar (one NorthStar Science and 

Technology, King George, Virginia, and one Habit Research, Victoria, British Columbia, Canada) to 

each of two adult cow bison in March 2008. A positional fix was collected by each collar every 4 hours 

with an accuracy of ±5 m. The Habit collar stored data on-board, whereas the NorthStar collar 

transmitted real time location data over the Globalstar satellite network to NorthStar’s Sensorlink Web

site. 

Statistical Analysis

All statistical analyses were performed using Systat version 11 or SAS version 9.2 statistical 

analysis software. In 2007, we tested for differences in productivity and offtake among vegetation types 

using Fisher’s least significant difference (LSD) multiple comparison test (SAS, 2008). 

Preliminary analysis of the 2010 data using Shapiro-Wilk W tests (Systat, 2007) revealed that 

the distributions of productivity and offtake data from all five vegetation classes were non-normal (P < 

0.05). We assumed a priori differences in sites based on the presence of prairie dogs due to site 

disturbance from burrows and therefore analyzed areas with prairie dogs separately from sites where 

prairie dogs were absent. We then assessed whether there were differences between areas that received 

herbicide applications versus those that did not, based on results of the Mann-Whitney tests (table 1). 

Based on these preliminary investigations, we determined that comparisons of 90-percent confidence 

intervals around the median (Wilkinson and Engelman, 2007) would be more revealing than rank-based 

nonparametric analyses as to the nature of differences in productivity and offtake. 
8 

Table 1. Mann-Whitney U test statistics for assessing effects of herbicide applications on plant biomass in 

kilograms per hectare (kg/ha) in each of five vegetation functional groups during July and September 2010 at 

Rocky Mountain Arsenal National Wildlife Refuge, Colorado.

[NCSG = native cool season graminoids; NNCSG = non-native cool season graminoids; NWSG = native warm season 

graminoids; n = sample size; P = probability of obtaining the observed test value] 

Vegetation functional 

group

Standing crop (kg/ha)

Herbicide treatment No herbicide treatment

MannWhitney U PMedian (25th–75th

percentile) n

Median (25th–75th

percentile) n

Prairie Dogs Absent

July

Native forbs 11 (0–207) 26 12 (0–126) 24 606 0.912

Non-native forbs 11 (0–354) 26 0 (0–1) 24 478 0.004*

NCSG 212 (0–1,040) 26 28 (0–285) 24 545 0.183

NNCSG 73 (0–961) 26 566 (19–1,410) 24 671 0.249

NWSG 115 (15–658) 26 345 (58–608) 24 630 0.726

Total 2,481 (1,573–2,841) 26 1,462 (1,078–2,228) 24 507 0.042*

September

Native forbs 0 (0–104) 25 3 (0–40) 22 534 0.884

Non-native forbs 0 (0–90) 25 0 (0–0) 22 443 0.025*

NCSG 191 (0–1,086) 25 0 (0–85) 22 451 0.068

NNCSG 8 (0–44) 25 760 (27–960) 22 668 0.003*

NWSG 0 (0–115) 25 329 (117–640) 22 702 0.000*

Total 1,200 (839–1,997) 25 1,558 (1,023–2,009) 22 578 0.286

Prairie Dogs Present

July

Native forbs 43 (0–384) 19 311 (113–833) 10 201 0.019*

Non-native forbs 800 (239–1,817) 19 39 (0–124) 10 98 0.018*

NCSG 7 (0–310) 19 130 (0–347) 10 163 0.540

NNCSG 9 (0–112) 19 1 (0–39) 10 156 0.794

NWSG 64 (12–648) 19 41 (13–148) 10 129 0.334

Total 1,738 (1,205–3,127) 19 1,232 (1,020–1,496) 10 111 0.74

September

Native forbs 0 (0–118) 19 86 (0–422) 8 130 0.311

Non-native forbs 804 (4–3,440) 19 0 (0–1,132) 8 72 0.035*

NCSG 0 (0–410) 19 0 (0–816)) 8 110 0.882

NNCSG 0 (0–0) 19 0 (0–60) 8 124 0.389

NWSG 17 (0–146) 19 270 (25–407) 8 142 0.107

Total 1,800 (857–3,479) 19 1,618 (1,147–1,844) 8 99 0.490

*significant at alpha = 0.05. 
9 

Results

Vegetation Productivity, Utilization, and Species Composition

2007

During the early growing period (Feb.–June), measured productivity was marginally greater in 

crested wheatgrass than in needle-and-thread communities (Fisher’s LSD = 0.049) and did not differ 

among any other vegetation cover types (fig. 2). During the late season (June–Sept.), standing crop was 

greater for crested wheatgrass than for either blue grama (Fisher’s LSD = 0.020) or needle-and-thread

(Fisher’s LSD = 0.003) and did not differ among any other vegetation types. 

Offtake data were difficult to interpret due to a number of samples (43 out of 103) where 

measured productivity was greater outside the grazing cages than inside. When averaged for all samples, 

offtake estimates were negative for two of the four measured vegetation types in June and all vegetation 

types in September, suggesting that greater productivity occurred outside the grazing exclosures than 

inside, despite grazing. Measured offtake did not differ among cover types during either the early or late 

season (Fisher’s LSD >0.05 in all cases; table 2). 

Cheatgrass was a large component of the herbaceous vegetation in blue grama and sand 

dropseed vegetation throughout the growing season (fig. 3). Offtake of cheatgrass ranged from 14

percent in blue grama sites to 80 percent in crested wheatgrass sites. Grazing by bison reduced 

cheatgrass biomass early in the growing season and reduced cheatgrass biomass 25–57 percent by the 

end of the growing season in all vegetation types. 

Pooling early and late season vegetation transect data, native species composed 64.1 percent of 

all vegetation cover in 2007. Organic litter composed 28.0 percent of the ground cover, whereas 8.7 

percent of the area sampled was bare soil. Species richness was greatest in blue grama communities (24 

species observed) and least in sand dropseed communities (7 species observed). Needle-and-thread and 

crested wheatgrass communities had 14 species each. Cover of non-native species was highest (43-

percent mean cover) on the sand dropseed sites with most of this cover contributed by crested 

wheatgrass (32-percent mean cover). Mean cover of non-natives was 27 percent in blue grama sites, 21 

percent in crested wheatgrass sites, and 17 percent in needle-and-thread sites. Cheatgrass composed 9 

percent of cover in sand dropseed communities, 14 percent in needle-and-thread, and 17 percent in blue 

grama communities, but only 1.7 percent in crested wheatgrass communities.
10

Figure 2. Total herbaceous standing crop (kg/ha [kilograms per hectare]; mean ±1 standard error) estimates for 

each of four vegetation cover types defined for the June and September 2007 exclosure-cage sampling 

sessions at Rocky Mountain Arsenal National Wildlife Refuge, Colorado. 

Table 2. Consumption (kg/ha [kilograms per hectare]) and offtake (in percent) values for four vegetation cover

types defined for June and September 2007 sampling dates at Rocky Mountain Arsenal National Wildlife 

Refuge, Colorado. Data are mean ±1 standard error (s.e.). 

Vegetation type

Consumption (kg/ha) Offtake (percent)

June September June September

n mean ± s.e. mean ± s.e. mean ± s.e. mean ± s.e.

Blue grama 5 317.2 ± 129.6 210.7 ± 330.3 13.3 ± 8.5 -16.4 ± 43.4

Crested wheatgrass 4 193.4 ± 173.9 -143.7 ± 585.2 -1,383.4 ± 1,379.8 -97.1 ± 81.3

Sand dropseed 2 -481.5 ± 130.5 175.6 ± 70.8 -15.0 ± 5.1 -6.9 ± 19.4

Needle-and-thread 5 118.7 ± 200.9 -77.5 ± 318.4 1.8 ± 11.5 -0.6 ± 15.1
11

Figure 3. Percent of total herbaceous standing crop comprised of cheatgrass in each of four vegetation cover 

types on Rocky Mountain Arsenal National Wildlife Refuge bison pasture as defined for sampling in June and 

September 2007.

2010

Very few differences were found in the 2010 standing crop based on overlap in 90-percent

confidence intervals about the median (tables 3 and 4). Median total herbaceous standing crop values 

were nearly identical inside (July median = 1,462 kg/ha, September median = 1,375 kg/ha) and outside 

(July median = 1,378 kg/ha, September median = 1,479 kg/ha) of bison-exclusion grazing cages in areas 

with no prairie dogs in both July and September. Prairie dogs appeared to exhibit little influence on 

graminoids or total standing crop during the early growing season (July sampling period), but ungrazed 

plots did appear to have greater biomass of native forbs when prairie dogs were present (median = 312 

kg/ha) compared to areas where prairie dogs were absent (median = 12 kg/ha). However, during the late 

growing season (September sampling period), biomass of non-native cool season graminoids (NNCSG) 

was greater in areas where prairie dogs were absent (median = 760 kg/ha) than where they were present 

(median = 0 kg/ha). In areas where prairie dogs were present, total standing crop was greater (as 

evidenced by non-overlapping 90-percent confidence intervals around the median) in plots inside bison 

and prairie dog exclusion cages (median = 1,680 kg/ha) compared to plots outside cages (median = 812 

kg/ha), indicating that together, bison and prairie dogs were removing a significant amount of biomass 

(table 3). 
12

Offtake again had a high number of negative values (84 of 216 cages sampled, table 4). In July 

and September in areas containing only bison, confidence intervals were too wide to discern differences 

in offtake among most of the plant functional groups. However, in both July and September, offtake of 

native forbs by bison was greater in areas where prairie dogs were absent (July median = 87-percent

biomass consumed, September median = 100-percent biomass consumed) compared with bison offtake 

in areas where prairie dogs were present (July median = 20-percent biomass consumed, September 

median = 0-percent biomass consumed, table 4). This is likely because prairie dogs competed with bison 

for native forbs. In July, prairie dog offtake was much greater on cool season graminoids (both NCSG 

and NNCSG) than bison offtake in areas where prairie dogs were present. Though bison offtake 

appeared much higher on these functional groups in areas where prairie dogs were absent, confidence 

intervals were so wide that such differences could not be conclusively determined. In September total 

offtake was lower when prairie dogs were absent (median = 5 percent of biomass consumed) than in 

areas where both prairie dogs and bison grazed (median = 48 percent of biomass consumed). In areas 

where prairie dogs were present, offtake of total biomass by prairie dogs (median = 40 percent of 

biomass consumed) was greater than offtake of total biomass by bison (median = 9 percent of biomass 

consumed). 

Despite trends toward differences in certain plant groups based on herbicide treatment, the only 

significant differences of plant functional groups were found in September standing crop of native warm 

season graminoids (NWSG) and non-native cool season graminoids (NNCSG) and only in areas where 

prairie dogs were absent. In these sites herbicide-treated caged plots had lower biomass than untreated 

caged plots for these two functional groups (table 3). Overlapping confidence intervals were found 

between all other tested comparisons indicating a small measurable effect of herbicide treatment at these 

sites.
13

Table 3. Standing crop (kilograms per hectare [kg/ha]); median and 90-percent confidence interval (CI) of vegetation in the Rocky Mountain Arsenal 

bison pasture, July and September 2010. Non-overlapping confidence intervals indicate differences between groups.

[(H) denotes herbicide treatment; NCSG = native cool season graminoids; NNCSG = non-native cool season graminoids; NWSG = native warm season 

graminoids; n = number of samples] 

Standing crop (kg/ha)

Ungrazed Grazed only by prairie dogs Prairie dog and bison grazing

Plant Group n Median 90 % CI n Median 90 % CI n Median 90 % CI

July—Prairie dogs absent

Native forbs 50 12 0–60 . . . 50 0 0–16

Non-native forbs 24 0 0–0 . . . 24 0 0–0

Non-native forbs (H) 26 11 0–230 . . . 26 46 2–328

NCSG 50 116 26–275 . . . 50 11 0–304

NNCSG 50 260 59–621 . . . 50 79 21–364

NWSG 50 330 106–491 . . . 50 281 168–356

Total 24 1,462 1,104–2,006 . . . 24 1,378 826–1,879

Total (H) 26 2,481 1,922–2,643 . . . 26 1,698 1,268–2,221

July—Prairie dogs present

Native forbs 10 312 113–1,100 10 139 79–1,620 10 164 91–661

Native forbs (H) 19 43 0–235 19 66 6–357 19 137 8–320

Non-native forbs 10 39 0–1,280 10 0 0–1,760 10 1 0–404

Non-native forbs (H) 19 800 266–1,424 19 740 484–1,055 19 560 221–1300

NCSG 29 20 0–241 29 17 1–55 29 27 0–67

NNCSG 29 1 0–66 29 0 0–16 29 0 0–12

NWSG 29 49 24–200 29 62 9–133 29 55 16–82

Total 29 1,495 1,284–1,890 29 1,419 1,188–2,013 29 1,350 1,102–1,556

September—Prairie dogs absent

Native forbs 47 1 0–13 . . . 47 0 0–9

Non-native forbs 22 0 0–0 . . . 22 0 0–2

Non-native forbs (H) 25 0 0–48 . . . 25 3 0–84

NCSG 47 0 0–185 . . . 47 0 0–236

NNCSG 22 760 35–892 . . . 22 202 6–1,120
14

Table 3. Standing crop (kilograms per hectare [kg/ha]); median and 90-percent confidence interval (CI) of vegetation in the Rocky Mountain Arsenal 

bison pasture, July and September 2010. Non-overlapping confidence intervals indicate differences between groups.—Continued

[(H) denotes herbicide treatment; NCSG = native cool season graminoids; NNCSG = non-native cool season graminoids; NWSG = native warm season 

graminoids; n = number of samples]

NNCSG (H) 25 8 1–25 . . . 25 15 0–180

NWSG 22 329 158–566 . . . 22 298 199–610

NWSG (H) 25 0 0–50 . . . 25 0 0–234

Total 47 1,375 1,089–1,832 . . . 47 1,479 1,174–1,727

September—Prairie dogs present

Native forbs 27 16 0–118 27 0 0–40 27 0 0–15

Non-native forbs 8 0 0–1,600 8 76 0–880 8 45 0–130

Non-native forbs (H) 19 804 17–2,115 19 348 73–1,260 19 269 62–812

NCSG 27 0 0–253 27 0 0–263 27 0 0–160

NNCSG 27 0 0–0 27 0 0–1 27 0 0–20

NWSG 27 38 11–217 27 8 0–104 27 33 0–153

Total 27 1,680 1,416–2,384 27 1,260 1,024–1,528 27 812 633–1,386
15

Table 4. Percent consumption (offtake) of vegetation by bison and prairie dogs (median and 90-percent confidence interval [CI]) in the 

Rocky Mountain Arsenal bison pasture, July and September 2010. Non-overlapping confidence intervals indicate differences 

between groups.

[(H) denotes herbicide treatment; NCSG = native cool season graminoids; NNCSG = non-native cool season graminoids; NWSG = native warm season 

graminoids] 

Offtake (percent)

Total offtake Bison offtake Prairie dog offtake

n Median 90 % CI n Median 90 % CI n Median 90% CI

July—Prairie dogs absent

Native Forb 30 87 53–100 30 87 53–100 . . .

Non-native forb 6 -6 -28–100 6 -6 -28–100 . . .

Non-native forb (H) 16 27 -105–77 16 27 -105–77 . . .

NCSG 32 41 -32–100 32 41 -32–100 . . .

NNCSG 39 68 -6–85 39 68 -6–85 . . .

NWSG 43 19 -43–53 43 19 -43–53 . . .

Total 23 32 -34–40 23 32 -34–40 . . .

Total (H) 26 13 -20–43 26 13 -20–43 . . .

July—Prairie dogs present

Native Forb 10 64 -37–93 10 20 -16–54 10 43 -131–88

Native forb (H) 13 30 -277–75 13 0 -34–57 13 -3 -632–76

Non-native forb 5 66 -278–100 5 37 -378–348 5 29 -248–100

Non-native forb (H) 18 29 -7–86 18 17 -13–43 18 6 -20–31

NCSG 18 52 -12–100 18 3 -30–25 18 68 30–95

NNCSG 17 91 35–100 17 1 0–12 17 85 71–100

NWSG 24 44 2.4–72 24 14 -17–34 24 50 -37–73

Total 29 18.2 4.0–43 29 12 -7–24 29 24.2 -18–39

September—Prairie dogs absent 

Native forb 25 100 32–100 25 100 32–100 . . .

Non-native forb 3 85 -467–95 3 85 -467–95 . . .

Non-native forb (H) 11 83 -48–99 11 83 -48–99 . . .

NCSG 20 29 5–100 20 29 5–100 . . .

NNCSG 19 28 -54–97 19 28 -54–97 . . .

NNCSG (H) 17 55 -17–100 17 55 -17–100 . . .

NWSG 20 5 -45–81 20 5 -45–81 . . .

NWSG (H) 8 74 -122–100 8 74 -122–100 . . .

Total 43 5 -12–15 43 5 -12–15 . . .
16

Table 4. Percent consumption (offtake) of vegetation by bison and prairie dogs (median and 90-percent confidence interval [CI]) in the 

Rocky Mountain Arsenal bison pasture, July and September 2010. Non-overlapping confidence intervals indicate differences 

between groups.—Continued

[(H) denotes herbicide treatment; NCSG = native cool season graminoids; NNCSG = non-native cool season graminoids; NWSG = native warm season 

graminoids] 

September—Prairie dogs present 

Native forb 14 99 -52–100 14 0 -151–15 14 82 18–100

Non-native forb 3 94 89–100 3 13 0–25 3 82 65–100

Non-native forb (H) 18 44 -72–72 18 2 -70–50 18 42 19–60

NCSG 11 62 32–84 11 17 0–55 11 20 -86–70

NNCSG 7 19 -84–100 7 0 -180–42 7 80 1–100

NWSG 19 57 -97–91 19 0 -189–45 19 50 -13–91

Total 27 48 30–70 27 9 3–14 27 40 21–60
17

Bison Habitat Selection 

The NorthStar GPS collar failed approximately 1 month after it was successfully launched and 

secured to a bison. Successful launch was verified prior to deployment. While troubleshooting the 

problem with the NorthStar technical representative, we learned that the failed collar was one of a 

cohort of early prototypes having a design flaw that allowed moisture into the transmitter housing, 

resulting in high failure rates. The Habit Research GPS collar stopped transmitting data when the animal 

fitted with the collar was fatally struck by lightning. We were unable to recover data from this unit. 

Available locations from the animal fitted with the NorthStar collar were plotted (fig. 4). These 

locations were recorded from March 3, 2008, through April 10, 2008. Although we conducted no formal 

analysis of habitat selection, visual analysis of the location data indicated that this cow used the whole 

pasture with the majority of the locations in areas initially identified as restored shortgrass prairie, 

though this type predominates on the 2007 vegetation map. 

Figure 4. Location data gathered from one cow bison fitted with a GPS radio-collar at Rocky Mountain Arsenal 

National Wildlife Refuge, Colorado, March 3, 2008, through April 10, 2008.
18

Discussion

Our study found few significant differences in either productivity or utilization among native 

vegetation types of the RMA bison pasture in 2007 or 2010 but did find higher productivity in areas 

identified as crested wheatgrass, an introduced plant type, in 2007. The 2007 data showed low overall 

offtake rates. The 2010 data indicated that real offtake was occurring and that in the presence of two 

grazers (prairie dogs and bison) total offtake increased significantly by late in the growing season

compared to areas where only bison grazed. We also showed that high levels of prairie dog grazing on 

native forbs appears to reduce bison offtake of this functional group. Lack of greater observed 

differences in vegetation offtake in areas containing bison and prairie dogs compared to areas with only 

bison may have been because bison were not yet influencing pasture vegetation in a measurable manner

by 2010. Fahnestock and Detling (2002) found similar results in mixed-grass prairie and attributed it to 

low stocking rates of bison. However, by 2010 the pasture was at the carrying capacity that RMA NWR 

personnel deemed appropriate for bison (Tom Ronning, U.S. Fish and Wildlife Service, oral commun., 

March 15, 2009), and the lack of demonstrated utilization effects by either bison or prairie dogs was 

most likely due to a loss of statistical precision resulting from our inability to stratify samples by actual 

vegetation cover types present in the pasture. 

Several factors contributed to this study being inconclusive. An accurate vegetation map is 

critical to collection of accurate, precise, and representative data describing vegetation cover, 

productivity, and utilization and to interpretation of that data. For a variety of reasons that are beyond 

the scope of this report, we were unable to complete an accurate and error-free vegetation classification 

map. This prohibited us from executing a data collection and analysis plan that would have organized 

data on the vegetation classes present in the pasture. An accurate map of pasture vegetation would have 

allowed us to constrain our data analysis of any particular plant group to only those areas actually 

containing that group thus reducing the variation among biomass values for all of the plant group 

estimates as well as reducing the size of the confidence interval estimates. In many cases the reduction 

would have been large (based on a review of the number of extremely low values and zeros associated 

with each class in the data) and would have resulted in more precise estimates of productivity and actual 

offtake. Bison may have consumed significant amounts of biomass of particular vegetation types, but 

we were not able to demonstrate this because of the large degree of noise in the data.

Secondarily, additional and incompatible vegetation management actions (herbicide 

applications, burning, disking, and so forth) were conducted in the pasture while this study was 

underway. Several rounds of widespread herbicide applications went uncommunicated to USGS staff 

prior to vegetation sampling in 2010. These activities also contributed to larger confidence intervals 

about vegetation biomass estimates and hampered our ability to determine causal relations between 

bison or prairie dog grazing and observed vegetation responses. 

In the future, if RMA NWR managers desire to fully understand the effects of bison grazing on 

pasture vegetation, then other, confounding sources of vegetation change need to be eliminated. Two of 

the original project goals were to evaluate bison selection among the dominant vegetation types in the 

pasture and to evaluate bison impacts on pasture vegetation through herbivory. Before either objective 

can be met, an accurate pasture vegetation map that has known levels of error needs to be completed. 

During the two rounds of error checking that occurred in section 34, vegetation data were collected at 

the species level (A. Maes, U.S. Fish and Wildlife Service, oral commun., March 4, 2008). These data 

and the vegetation data collected during the 2010 ground-truthing effort associated with the USGS pilot 

vegetation map could help dramatically improve the existing map. 

It is also important to ensure that sample sizes (for both transects and exclosures) are adequate to 

minimize the effects of heterogeneity observed in the 2007 dataset. In both sampling years, a large 
19

number of sites had greater recorded vegetation productivity outside of the grazing cages than inside. 

Although the 16 sites sampled in 2007 was undoubtedly too small a sample, even sampling 80 sites—as 

was done in 2010—was not enough to overcome our inability to stratify by vegetation classes. An 

alternate method of locating or sampling cage pairs might also be considered to reduce variability and 

the number of locations with higher production outside of cages. At a randomly selected site, individual 

pairs (or trios) of sample plots could be selected based on observer determination of similarity and then 

randomly assigned to a treatment (caged, uncaged). Another method might be to sample several 

randomly selected uncaged plots near each caged plot to reduce variation of grazed-plot estimates. 

To fully determine the effects of various herbivores on vegetation, then sample size and 

allocation of samples within vegetation types should include consideration of prairie dogs. Prairie dog 

towns are distinctly different from other cover types, and bison have been found to use areas around 

young prairie dog towns preferentially because of the predominance of perennial grasses, enhanced 

nitrogen content of vegetation, and lower ratios of dead to live plant material near prairie dog towns 

(Schwartz and Ellis, 1981; Coppock and others 1983a,b; Krueger, 1986; Cid and others, 1991). 

Therefore, if management goals include discriminating between the effects of prairie dogs and bison, or 

the effects of prairie dog presence on bison use of the landscape, then prairie dog towns will need to be 

considered as a separate vegetation cover type. Our limited data showed differences in bison grazing 

intensity (offtake) on forbs when prairie dogs were present and differences in biomass of native forbs in 

areas where prairie dogs were present. The increased grazing pressure in areas with bison and prairie 

dogs should be considered as well in the management of existing and proposed pasture areas. 

Allocating samples proportionally among vegetation types will allow within-type sample sizes to 

represent the proportional distributions of each vegetation type in the pasture. Alternatively, focusing 

sampling efforts within a restricted number of target vegetation types would provide useful information 

on those types and is an effective sampling strategy for studies having limited resources. Assigning 

random sample locations in the project GIS before the field season begins—such as was done in 2010—

is the most efficient and error-free way to place sampling locations. Uploading transect coordinates into 

handheld GPS units is the most precise way to locate them in the field, after which they may be 

permanently marked on-site. Also, if RMA NWR managers desire to monitor the effects of bison on 

shrub composition, density, abundance, and structure, then a focused shrub monitoring plan will also 

need to be developed. 

An early objective of this study was to quantify breeding-songbird-community changes 

associated with bison reintroduction. The refuge collects data on these and other grassland species, but 

only data collected before the reintroduction of bison to the refuge was available. Continued collection 

of breeding-bird data both on and off the bison pasture would provide information to help determine the 

impacts of bison on key avian species. Another objective identified in the early stages of this research 

was bison habitat selection. Due to failure of one GPS collar and loss of the other, we were not able to 

meet this objective. This objective may be achievable, but a strong commitment to maintaining active 

collars on bison will be necessary so that data collection can continue without interruption. An accurate 

vegetation map will also be necessary before habitat availability and selection estimates can be 

accurately made. 

Ultimately, the success of monitoring programs designed to measure herbivory effects is 

dependent on following study design and protocols closely and in a timely manner. We encourage RMA 

bison program managers to carefully consider each element described above as they continue to develop 

their bison management program to ensure a scientifically sound basis for determining future 

management actions. 
20

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21

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