Direct and indirect effects of livestock grazing intensity on a grassland food web

Abstract

Background/Question/Methods

Livestock graze the majority of North American grasslands and livestock production is an important contributor to many rural economies. Developing sustainable grazing practices for public and private grasslands depends on a comprehensive understanding of how livestock grazing intensity affects biodiversity and ecological function of grasslands. However, sustainable livestock production on the country’s rangelands is difficult to identify because few studies have examined the economic and ecological impacts of livestock grazing together. Also most of the ecological literature is focused on the effects of livestock on soils and plants with little information on native fauna. We combined landscape-scale experimental manipulations of cattle stocking rates and structural equation modeling to partition direct and indirect effects of grazing intensity on grassland soils, flora and fauna (ground-nesting songbirds and invertebrates) and on livestock performance. The study area is The Nature Conservancy’s Zumwalt Prairie Preserve in northeastern OR which is located in one of the last large remnants of the Pacific Northwest Bunchgrass prairie.

Results/Conclusions

Increasing cattle stocking rate resulted in direct increases to soil compaction and cattle performance, but decreased vegetation structure. Invertebrate taxa showed differential responses to grazing intensity. The abundance of several invertebrate taxa identified as important to nesting passerines declined with increased stocking rate, but these effects were indirect and were mediated through changes in vegetation structure. Results were also suggestive of a direct negative effect of increasing stocking rate on native pollinators, but patterns were not statistically significant. Although univariate analyses suggested high stocking rates negatively impacted avian abundance of some species, model selection procedures suggested that grassland passerines should not be included in the final model, implying effects of stocking rate may not be a primary factor structuring grassland passerine abundance or productivity. The lack of differences in plant species composition among stocking rates suggest short-term effects of grazing intensity are influenced by altered physical properties like vegetation and soil structure, but that those affects may be largely restricted to basal trophic levels when evaluated in the context of the grassland food web. These results, combined with previous results yielded from the same experiment, suggest low-to-moderate stocking rates as applied in this experiment may be a more suitable grazing practice for sustaining grassland biodiversity and function than high stocking rates.

Full text

America’s Grasslands: Status, Threats, and Opportunities - Proceedings of the 1st Biennial Conference on the Conservation of America’s Grasslands
i
America’s Grasslands:
Status, Threats, and
Opportunities
August 15-17, 2011
Sioux Falls, SD
Suggested Citation
Glaser, A., ed. 2012. America’s Grasslands Conference: Status, Threats, and Opportunities.
Proceedings of the 1st Biennial Conference on the Conservation of America’s Grasslands. August 15-
17, 2011, Sioux Falls, SD. Washington, DC and Brookings, SD: National Wildlife Federation and South
Dakota State University.
Edited by
Aviva Glaser, National Wildlife Federation
Published
June 2012 by National Wildlife Federation and South Dakota State University
Cover image: Windblown arms of Indian grass and big bluestem, Spring Creek Prairie, Nebraska.
Credit: ©Michael Forsberg, www.michaelforsberg.com
Proceedings of the First Biennial
Conference on the Conservation
of America’s Grasslands

America’s Grasslands: Status, Threats, and Opportunities - Proceedings of the 1st Biennial Conference on the Conservation of America’s Grasslands 17
al. 1999, Jones 2000). There is less quantitative data on
invertebrate and wildlife responses to different livestock
grazing management scenarios. To fully understand
effects of livestock grazing on ecosystems, effects on
other rangeland occupants cannot be ignored. We need
to expand our livestock grazing investigations beyond the
obvious changes in plants, and pay attention to potential
bottom-up effects (both direct and indirect) on other
herbivores and consumers. Experimental studies are needed
that evaluate livestock grazing effects on terrestrial food
webs with multiple trophic levels and taxonomically diverse
consumers (invertebrates and vertebrates) (Polis et al. 2004).
Approach
Study Ecosystem
We conducted this experiment at The Nature Conservancy’s
(TNC) Zumwalt Prairie Preserve (ZPP) in northeastern
Oregon (lat 117° 3’ N, long 45° 31’W); ZPP is located on
the largest relict (approx 65,000 ha) of the Pacic Northwest
Bunchgrass Prairie in North America, an ecosystem type
which once covered approximately 800,000 ha in the
northwestern United States and Canada. Compared to
other prairies in North America, relatively little is known
about these semi-arid temperate grasslands as the majority
disappeared quickly after Euro-American settlement
(Bartusevige et al. in press). Because the Zumwalt Prairie is
slightly higher, drier, colder and more geographically isolated
than other bunchgrass prairies in western North America it
has remained relatively intact. Little of the prairie has been
farmed, and spring/summer cattle grazing is the primary
land use.
Most previous investigations of livestock grazing have been
conducted in areas that have evolved in the presence of
extensive herds of large native herbivores [e.g., bison (Bison
bison)]. Grasslands and their associated fauna, which
evolved in the presence of large herbivores, are expected
to be relatively insensitive to, or even dependent upon,
grazing by domestic livestock compared to grasslands that
supported few large herbivores in the Holocene (Milchunas
and Lauenroth 1993, Stohlgren et al. 1999, Pykala 2000).
In addition the study site (like approximately 75% of the
western US) is characterized by high elevation rangelands (>
1000 m), short growing seasons (< 150 days), and relatively
low annual precipitation (< 50 cm). Thus, arid, mountainous
grasslands such as those in the study area are more likely
to show a different treatment effect to livestock stocking
Responses of a Pacic Northwest
Bunchgrass Food Web to
Experimental Manipulations of
Stocking Rate
Presenter: Patricia L. Kennedy, Oregon State
University (pat.kennedy@oregonstate.edu)
Other Authors: Timothy DelCurto, Sandra J. Debano, Tracey
N. Johnson, Samuel Wyffels, Chiho Kimoto, and Ryan
Limb, Oregon State University; Robert V. Taylor, The Nature
Conservancy; Heidi Schmalz, University of Idaho
Introduction
Livestock graze the majority of rangelands globally,
contributing $74 billion in business to many rural economies
and forming a major component of US agricultural
production (USDA-ERS 2010). However, sustainable
livestock production on the country’s rangelands is currently
limited by our incomplete knowledge of the impact of
grazing on ecosystem services associated with rangelands,
including those provided by native fauna. Vegetative
changes resulting from grazing are used to explain varying
patterns of biodiversity in grazed and ungrazed areas.
However, the effects of livestock grazing on animals
(both vertebrates and invertebrates) is poorly understood
because the majority of livestock grazing studies only involve
plants, abiotic environments and livestock (see reviews by
Milchunas et al. 1998, Olff and Ritchie 1998, Stohlgren et
Figure 5. Sericea lespedeza seed viability in response to
heat treatments of 1 and 2 minute durations. Points are the
percent of viable seeds of 3 100-seed replicates at each
temperature treatment.

America’s Grasslands: Status, Threats, and Opportunities - Proceedings of the 1st Biennial Conference on the Conservation of America’s Grasslands
18
• Plants: Overall paddock-level visual obstruction (our
index of vegetation structure) decreased and structural
heterogeneity increased with increasing stocking rates,
and those effects carried over one year after grazing had
ceased (Johnson et al. in review a). There were no signicant
effects of stocking rates on plant composition (Limb et al.,
unpublished data).
• Invertebrates: Invertebrate taxa varied in their resistance
to livestock grazing intensity. Several common taxa (e.g.,
grasshoppers and leafhoppers) were highly resistant –
showing no response to grazing treatment. Other taxa,
including native bees, spiders, and Lepidoptera showed
decreases in abundance and diversity and changes in
community composition with increased grazing intensity,
with some responses evident even at fairly low levels of
grazing intensity (~20% utilization) (Kimoto 2011).
• Birds: The high stocking rate had a negative effect on
bird and nest abundance of several species and avian
community composition differed between control and
heavily-grazed paddocks. Although stocking rate inuenced
vegetation structure, the only nest failures related to
stocking rate were from trampling. Trampling rates were
higher in paddocks with more cattle, but also depended
on number of days cattle were present (Johnson et al. in
review a, b).
Discussion
The soil and vegetation results suggest signicant and
potentially long-term changes caused by livestock grazing
can occur. High stocking rates had signicant effects on
all food web components. However, the changes in soil
properties and vegetative structure observed in the low to
moderate stocking rates did not have signicant effects on
higher vertebrate trophic levels. The persistence of these
changes is not known because we only applied treatments
for two years. In addition, we do not know how the rate
of removal inuences the development of thresholds and
responses of faunal communities in this system. This will be
examined in future research.
References
Adler, P. B., Milchunas, D.G., Lauenroth, W.K., Sala, O.E.,
Burke, I.C. (2004) Functional traits of graminoids in semi-
arid steppes: a test of grazing histories. Journal of Applied
Ecology 41, 653-663.
rate experiments than grasslands that have a long history
of native ungulate herbivory, more precipitation and longer
growing seasons (Adler et al. 2004).
Our study organisms for this experiment were selected
members of the grassland food web which included
vascular plants, terrestrial invertebrates, and breeding
songbirds and their predators. Members of both animal
taxa (invertebrates and birds) are providers of important
ecosystem services. Invertebrates are involved with a wide
variety of supporting services, including providing pollination
for native and agriculturally important plants, pest control
through the actions of natural predators, and as food
resources for other organisms, including birds. Breeding
grassland birds also provide a number of ecosystem
services, including cultural services because society values
their existence for aesthetic reasons, and regulating services
because of their important roles as both predators and prey.
Both groups are also known to be sensitive to changes in
soil and vegetation characteristics that are impacted by
grazing. North American grassland bird populations have
shown dramatic declines in recent years and appear to be
declining more markedly than all other avian guilds on this
continent (Sauer and Link 2011).
Experimental Design
We used a randomized complete block design with one
factor (livestock grazing) and four grazing treatment levels
(stocking rate) to evaluate our predictions. Details on the
experimental design are presented in Table 2 and Johnson
et al. in press.
Summary of Results
The results of this experiment are summarized for each
trophic level below.
• Soils: Penetration resistance (i.e., compaction) increased
at medium and high stocking rates relative to paddocks
with low stocking rates or no cattle. While herbaceous litter
increased across the study area, increases were greater
in ungrazed and lightly grazed paddocks (22% and 18%,
respectively) than in areas experiencing moderate to high
grazing (mean increase of 9.6% and 9.5%, respectively.
Aggregate soil stability was reduced in the soil sub-surface
when exposed to moderate or high stocking rates relative
to lightly grazed or areas excluded from livestock grazing
(Schmaltz 2011).

America’s Grasslands: Status, Threats, and Opportunities - Proceedings of the 1st Biennial Conference on the Conservation of America’s Grasslands 19
Polis, G. A., M. E. Power, and G. R. Huxel, Editors. (2004)
Food webs at the landscape level. University of Chicago
Press, Chicago, Illinois, USA.
Pykala, J. (2000) Mitigating human effects on European
biodiversity through traditional animal husbandry.
Conservation Biology 14, 705-712.
Sauer, J.R. and W. A. Link. (2011) Analysis of the North
American Breeding Bird Survey using hierarchical models.
Auk 128, 87-98.
Schmalz, H. (2011) Soil spatial heterogeneity and measured
soil responses: factors in an ecological grazing experiment
on a bunchgrass prairie. M. Sc. Thesis. University of Idaho,
Moscow, Idaho, USA.
Stohlgren, T.J., Schell, L.D., Heuvel, B.V. (1999) How grazing
and soil quality affect native and exotic plant diversity in
Rocky Mountain grasslands. Ecological Applications 9,
45-64.
(USDA-ERS) United States Department of Agriculture
Economic Research Service. (2010) U.S. beef and cattle
industry: Background statistics and information. www.ers.
usda.gov; last accessed May 26, 2011.
Wyffels, S. (2009) Inuence of stocking density on grazing
beef cattle performance, diet composition, foraging
efciency, and diet quality on a late-spring early-summer
native bunchgrass prairie. M.Sc. Thesis, Oregon State
University, Corvallis, Oregon, USA.
Bartuszevige, A. B, P. L. Kennedy and R. V. Taylor. In press.
Sixty-seven years of landscape changes in the last, large
remnant of the Pacic Northwest Bunchgrass Prairie.
Natural Areas Journal.
Johnson, T. N., P. L. Kennedy, T. DelCurto and R. V. Taylor.
In press. Bird community responses to cattle stocking
rates in a Pacic Northwest bunchgrass prairie. Agriculture,
Ecosystems & Environment.
Johnson, T. N., P. L. Kennedy, and M. A. Etterson. In review.
Estimating risk of cause-specic nest-failure for grassland
passerines in experimentally-grazed paddocks. Journal of
Wildlife Management.
Jones, A. (2000). Effects of cattle grazing on North American
arid ecosystems: A quantitative review. Western North
American Naturalist 60, 155-164.
Kimoto, C. (2011) Effect of livestock grazing on native bees
in a Pacic Northwest Bunchgrass prairie. M.S. Thesis,
Oregon State University, Corvallis, Oregon, USA.
Milchunas, D.G. and W. K. Lauenroth. (1993) Quantitative
effects of grazing on vegetation and soils over a global range
of environments. Ecological Monographs 63, 327-366.
Milchunas, D. G., W. K. Lauenroth, and I. C. Burke.
(1998) Livestock grazing: animal and plant biodiversity
of shortgrass steppe and the relationship to ecosystem
function. Oikos 83, 65-74.
Olff, H., and M. E. Ritchie. (1998) Effects of herbivores on
grassland plant diversity. Trends in Ecology and Evolution
13, 261-265.
Table 2. Phase I grazing treatments randomly assigned to each 40-ha paddock within each block (n =4) on the Zumwalt Prairie Preserve,
northeastern Oregon, USA.
Treatment Animal unit monthsbMean Percent Utilizationc (SD)
Controla0.00 9.52 (3.05)
Low 14.4 20.18 (4.08)
Moderate 28.8 31.66 (5.72)
High 43.2 46.09 (11.68)
a Control treatments represented no use by domestic livestock. However, native herbivores (e.g., ungulates, insects) were present in control
paddocks. We present mean utilization for control paddocks to account for background levels of native herbivory at the study site.
b One animal unit is dened as a mature cow and calf. We assume each animal unit consumes 20 kg/day and a grazing period of 42 days.
c Utilization was averaged over the two treatment years (2007-08). Methods for determining utilization are presented in Wyffels (2009).