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Appendix D-33 

APPENDIX D PNW Causal Paper Ungulates 

The Influence of Ungulates on Non-native Plant Invasions in Forests and 

Rangelands: A Review 

____________________________________________________________ 

Catherine G. Parks, Michael J. Wisdom, and John G. Kie 

U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, 

La Grande, OR. 

Abstract

Herbivory by wild and domestic ungulates can strongly influence vegetation composition and 

productivity in forest and range ecosystems. However, the role of ungulates as contributors to 

the establishment and spread of non-native invasive plants is not well known. Ungulates 

spread seeds through endozoochory (passing through an animal’s digestive tract) or 

epizoochory (attached to an animal’s body); hence, animal-mediated spread of invasive plants 

is a common phenomenon. Manipulative experiments of ungulate grazing effects on nonnative plant introduction, establishment, and spread are limited. Herbivory can alter 

successional patterns and rates when selective foraging favors survival, growth, and 

reproduction of plants with low palatability, although the impact can differ greatly among 

ecosystems. Descriptive studies in various habitats have shown that non-native species 

invade sites with or without livestock grazing, and other studies have tested the utility of 

prescribed grazing to reduce biomass and occurrence of invasive plants. Understanding the 

cause-effect relations between ungulates and invasive plant dynamics is a critical management 

need that deserves high priority for research. We summarize existing knowledge and identify 

gaps and research needs. 

Additional Keywords. 

Exotic plants, invasive plants, cattle grazing, herbivory. 
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Appendix D-34 

The Influence of Ungulates on Non-native Plant Invasions in Forests and 

Rangelands: A Review 

________________________________________________________________________ 

Introduction 

Herbivory by wild and domestic ungulates has profound effects on vegetation development 

and productivity in forest and range ecosystems (Hobbs 1996). Livestock grazing has long 

been recognized as an agent of change affecting the composition, structure, and development 

of rangeland plant communities (Fleischner 1994, Milchunas et al. 1998). More recently, 

herbivory by wild ungulates also has been recognized as an ecological force in forest 

ecosystems (Augustine and McNaughton 1998, Riggs et al. 2000, Kie and Lehmkuhl 2001). 

Substantial populations of domestic and wild ungulates, particularly cattle, elk (Cervus 

elaphus), and mule deer (Odocoileus hemionus), are present throughout expansive areas of 

western North America. An estimated population of 200,000 cattle and 430,000 mule deer 

and elk graze public lands in Oregon for at least part of each year (fig. 1). For example, 

ungulate densities on the Wallowa-Whitman National Forest in northeast Oregon typify the 

densities present on public lands in much of the intermountain West. Total ungulate numbers 

on the Wallowa-Whitman exceed 80,000 animals. Densities of elk and mule deer on winter 

range average 3 elk per square kilometer and 5 deer per square kilometer. In addition, cattle 

density averages about 3 cattle per square kilometer (table 1.) 

Empirical studies have provided evidence that ungulate herbivory contributes to long-term 

changes in ecosystems by changing nutrient turnover; rates, patterns, and perhaps trajectories 

of succession; and competition among plant species (Molvar et al. 1993, Pastor et al. 1993, 

Hobbs 1996, Pastor and Cohen 1997, Augustine and McNaughton 1998). Ungulate grazing 

and browsing can function as a chronic disturbance, exerting a continuous influence over long 

periods, as opposed to episodic disturbances such as fire, epidemic insect defoliation, and 

timber harvest that change forest structure and composition dramatically at a given point in 

time (Klein 1968, Kie et al. 2003). 

One aspect of ungulate herbivory is its potential as a contributor to the establishment, spread, 

and persistence of non-native invasive plant species. The establishment and spread of these 

plants is an increasingly important issue for land managers in western North America, who 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-35 

are faced with expansive invasions of non-native plants that are displacing native plant 

communities at an alarming rate. Non-native plants are spreading and invading 

approximately 700,000 hectares of U.S. wildlife habitat per year (Babbitt 1998, Pimental et al. 

2000). At least 17 million acres of western federal lands were reportedly infested in 1996, 

more than quadrupling the range affected between 1985 and 1995 (Westbrooks 1998). 

Invasive plants are having profound ecological impacts on Western landscapes, where they 

can form new metastable states that deflect or halt the natural dynamics of native ecosystems 

(Hobbs and Mooney 1993). 

It has been asserted that livestock grazing is a major cause of non-native plant invasions 

(Belsky and Gelbard 2000). However, no manipulative studies, with appropriate treatments 

and controls, have rigorously tested this hypothesis. Likewise, to our knowledge, 

manipulative experiments of grazing effects of wild ungulates on non-native plant species 

dynamics have yet to be conducted. Although scientific support is growing for the hypothesis 

that large herbivores facilitate the invasion and establishment of non-native plants, substantial 

controversy exists about the specific processes in time and space and the associated 

predictions of effects. We summarize the current scientific understanding of the role of 

ungulates in the introduction, establishment, and spread of non-native plants in forest and 

rangeland ecosystems, and discuss associated knowledge gaps and research needs. We focus 

our summary on forest and rangeland ecosystems in western North America, where herbivory 

by a variety of wild and domestic ungulates is common, and where non-native plant species 

are supplanting native plant species at rapid rates. 

Definitions 

Invasive species are plants with the ability to spread into native ecosystems, expand over large 

areas, and often are considered “undesirable” by society owing to their ecologic or economic 

impacts. The majority of invasive species that are considered most deleterious to Western 

forests and rangelands were introduced to North America during the last century from other 

continents. For consistency, we use the term “invasive” to define a non-native plant species 

whose introduction does, or is likely to, cause economic loss or detrimental impact to native 

plant communities and ecological processes. Non-native means, with respect to a particular 

ecosystem, any species that is not indigenous to that ecosystem. 
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Appendix D-36 

Ungulates as Contributors to Plant Invasions 

Both theoretical and empirical evidence suggest that domestic and wild ungulates may 

contribute to plant invasions by (1) selective foraging on and resultant reduction or 

elimination of native plant species, thus favoring an increase in invasive plants; (2) transport 

of invasive plant seeds into uninfested sites through endozoochory, which occurs when 

animals consume seeds in one area and later regurgitate or defecate them in another area; (3) 

epizoochory, which is the transport of seeds attached to animal skin, fur, or hooves, into 

another location where the seeds detach; and (4) disturbance or alteration of soils or 

trampling of vegetation, any of which confers advantage to invasive plant seeds that often are 

adapted for establishment in subsequent openings and bared soil. 

Selective Foraging 

Few studies have examined the effects of ungulate herbivory on ecosystem processes, but 

many have examined ungulate effects on the species composition of plant communities 

(Hobbs 1996, Augustine and McNaughton 1998, Vesk and Westoby 2001). Herbivores 

influence recruitment, growth, and mortality rates of plants, which are correlated with 

community traits and with interspecific competion (Huntly 1991). Herbivory is highly 

variable in space and time, and the spatial and temporal patterns of herbivory can modify the 

composition, structure, and dynamics of plant communities. Herbivory alters the profile of a 

plant community through differential use of palatable versus unpalatable plants, and thus can 

alter successional trajectories when selective foraging favors survival and reproduction of 

plants with low palatability. Palatable, native perennial plant species may produce fewer seeds 

and seedlings in heavily grazed or browsed situations (Crawley 1983, Parks et al. 1998) and 

thereby place unpalatable invasive species at a competitive advantage. 

Secondary compounds (e.g., phenolics and tannins) and structural compounds (e.g., lignins 

and cellulose) in plants reduce their palatability to ungulates, thus conferring defense against 

herbivory and an advantage to plants that contain these compounds (Augustine and Mc 

Naughton 1998). Many invasive plants have such defenses, and thus are unpalatable to 

herbivores, or only palatable during short phenologic periods (Kelsy and Locken 1987, Sheley 

and Petroff 1999). As a consequence, less intensive grazing of the invasive plant provides 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-37 

competitive advantage to these species over more palatable native plants, allowing the former 

to effectively invade, establish, and spread. 

Despite the competitive advantages that ungulate herbivory may confer to invasive plants, the 

impact of grazing animals can differ greatly among ecosystems. Specific effects may depend 

largely on the response variables chosen to evaluate herbivory (Milchunas and Lauenroth 

1993). For example, Stohlgren et al. (1999) evaluated 20 long-term livestock grazing 

exclosures in Colorado, Wyoming, Montana, and South Dakota, and concluded there was 

little evidence to suggest that grazing has an effect on native species richness or on the 

accelerated spread of most invasive plant species at landscape scales. This study, as with 

most other grazing studies, did not exclude native ungulates from ungrazed treatments. Other 

studies in various habitats have shown that non-native species can invade sites with or without 

livestock grazing (Lacey et al. 1990, Frank et al. 1995, Rambo and Faeth 1999). Moreover, 

Milchunas et al. (1998) concluded that the shortgrass steppe of North America is insensitive 

to cattle grazing because plants have evolved and adapted to long evolutionary history of 

grazing by native bison (Bison bison). Moreover, Milchunas et al. (1998) found ungrazed 

shortgrass steppe to be more susceptible to invasion by “weed” species than were heavily 

grazed communities. Mack and Thompson (1982) also found this true of steppe communities 

east of the Rocky Mountains. By contrast, however, Mack and Thompson (1982) argued that 

steppe communities west of the Rocky Mountains historically had few large herbivorous 

mammals and therefore lack herbivore-adapted traits and resiliency in response to current 

anthropogenic increases of both domestic and wild herbivores. 

Despite research that questions the role of grazing as a major contributor to plant invasions, 

findings from many observational studies (as summarized by Belksy and Gelbard 2000) 

support this premise. Conducting research in a spotted knapweed (Centaurea maculosa

Lam.) infested area that had been repeatedly grazed, Olson and Wallander (1997) found root 

and shoot biomass of Idaho fescue (Festuca idahoensis Elmer.) was significantly reduced 

from ungrazed controls, whereas the spotted knapweed biomass was unchanged with grazing. 

These results suggest that repeated grazing may reduce the ability of Idaho fescue to compete 

with spotted knapweed. Moreover, Sheley et al. (1997) found that clipping to simulate 

intensive livestock grazing enhanced the establishment of diffuse knapweed (Centaurea 
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Appendix D-38 

diffusa Lam), but moderate defoliation had no effect. Such variability in results reflect the 

need for manipulative studies where confounding and extraneous factors are properly 

controlled or accounted for, and where effects of specific treatments of interest are evaluated 

in an experimental manner. 

Numerous studies have tested the utility of “prescribed livestock grazing,” particularly with 

sheep and goats, to reduce biomass and composition of invasive plants. Olson et al. (1997) 

grazed sheep intermittently for 3 consecutive years on range infested by spotted knapweed; 

they found that knapweed density and seed viability were lower on grazed treatments versus 

ungrazed treatments. Grazing treatments also produced more Kentucky bluegrass (Poa 

pratensis L., a non- native grass) cover and increased bare soil. Other studies report some 

level of invasive plant reduction with controlled levels of grazing, selective timing of grazing, 

or multiple species of herbivores (Lacey and Sheley 1996, Sheley and Petroff 1999). Cattle 

appear to be less effective at reducing invasive plants than sheep or goats (Vallentine and 

Stevens 1994, Lawrence et al. 1995), or alternatively, can increase the cover of invasive 

plants in grazed treatments (Maxwell et al. 1992). 

Transport of Invasive Plant Seeds by Endozoochory and Epizoochory 

Several intentional and unintentional introductions of invasive plants have been associated 

with livestock management, and some introductions have resulted in widespread invasions 

(Baker 1978, Mack 1991, Sheley and Petroff 1999). Landscape-scale spread of invasive 

plants can occur when seeds are moved along transportation corridors from infested sites or 

from infested ungulate forage, attached to or held within infested animals, or attached to 

vehicles used to transport infested forage or animals. Domestic and wild ungulates spread 

seeds through endozoochory or epizoochory (Janzen 1984). Animal-mediated spread of 

invasive plants is a common, world-wide phenomenon (Schiffman 1997). Although invasive 

plant foliage and other structures generally harbor unpalatable plant compounds and often are 

avoided, they can be consumed by large herbivores, even when not intentionally selected or 

preferred. Fruits and seeds are readily eaten by large herbivores intentionally or inadvertently 

in the course of general grazing, and may form a substantial part of their diet (Gill and 

Beardall 2001). By moving seeds away from parent plants, which are usually competitively 

superior, animal dispersers are likely to increase the probability of seedling survivorship 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-39 

(Howe and Smallwood 1982, Herrera 1995). Because wild ungulates and livestock have 

predictable patterns of habitat selection (Johnson et al. 2000, Coe et al. 2001), animaldispersed seeds are likely to be spread among environmentally similar sites. Consequently, 

invasive plants are often deposited in conditions similar to sites where such plants have 

already become established (Howe and Smallwood 1982). This role of ungulates to deposit 

seeds in environments where establishment can be successful can be particularly important for 

colonization of invasive plants in fragmented landscapes (Schiffman 1997). 

Invasive species with thick seed coats dispersed by endozoochory, in addition to benefiting 

from transport, may have enhanced germination owing to gut scarification (Schiffman 1997). 

Invasive plant seeds have been recovered from feces and found viable in numerous studies. 

Seeds of leafy spurge (Euphorbia esula L.) are viable for 4 days following sheep ingestion 

(Olson and Wallander 2002). Thill et al. (1986) fed achenes of common crupina (Crupina 

vulgaris Cass.) to several large herbivores and found none in sheep feces; but horses, mule 

deer, and cows excreted 5, 3, and 25% respectively. The percentage of achenes consumed 

and excreted that are viable ranged from 29% in cows to 81% in deer. Seeds of spotted 

knapweed fed to mule deer and sheep were collectable and viable for more than 2 days 

(Wallander et al. 1995). 

It is not certain how important endozoochory is in initiating or increasing infestations in 

natural systems (Gill and Beardall 2001). Malo et al. (2000), however, quantified seeds in 

dung deposited by domestic and wild herbivores in Mediterranean plant communities; they 

found that cows dispersed the most seeds (68%), followed by red deer (Cervus elaphus) 

(20%), rabbits (7%), and fallow deer (Dama dama) (5%). Malo et al. (2000) concluded that 

the effect of seed input to the seed bank from ungulate transport may be low at large and 

medium-sized spatial scales, but very important at small scales and for colonization processes. 

Epizoic transport may also be an important mode of introduction and spread of invasive plants 

at several landscape scales. De Clerck-Floate (1997) suggests that cattle are effective 

dispersers of hound’s-tongue (Cynoglossum officinale L.) on rangelands of British Columbia. 

Likewise, sheep are known to spread leafy spurge seeds (Olson et al. 1997). The fleece of a 
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Appendix D-40 

single sheep grazing on calcareous grasslands of Germany was found to carry as many as 

8,500 seeds of 85 vascular plant species (Fischer et al. 1996). 

Effects of Site Disturbance or Alteration 

Direct physical or chemical effects such as plant trampling, disturbance of soil crusts, creation 

of bare soil, and inputs of nitrogen to the soil through dung and urine, can influence the 

success of native species (Augustine and McNaughton 1998) and thereby the “invasibility” of 

a plant community by non-native species (Lonsdale 1999). 

By consuming aboveground biomass of plants, domestic ungulates also reduce the amount of 

biomass available for conversion to litter. Substantial litter reduction can cause a subsequent 

increase in bare ground (Schulz and Leininger 1990), erosion, and soil compaction, all of 

which favor germination and establishment of invasive plants (Sheley and Petroff 1999). 

Many studies have shown that increased nitrogen availability (e.g. through animal dung and 

urine deposition) encourages community dominance by invasive plant species (Harris 1967, 

Hobbs 1989). A recent greenhouse study, however, evaluated five levels of nitrogen 

availability on the comparative growth of native and invasive plant species and found the 

invasive plants did not display a more positive response (Lowe et al. 2002). 

Direct trampling by ungulates can alter the important symbiotic relations of microbiotic crusts 

and mycorrhizae with native plants, thus indirectly increasing the invasibility of the 

community by non-native plants that are enhanced by disturbed site conditions (Eckert et al. 

1986, Rosentreter 1994). Microbiotic crusts are living mats of lichens, mosses, algae, and 

cyanobacteria that cover rangelands world-wide. Microbiotic crusts play a key role in 

nutrient cycling, increase soil stability and soil moisture, interact with vascular plants, and 

perform other important functions (Eldridge and Green 1994). The need for scientific 

understanding regarding the ecological role of these crusts is growing (Quigley and Arbelbide 

1997), along with growing concern about management practices that adversely impact the 

crust layer. In general, trampling and grazing are deleterious to survival of microbiotic crusts 

(Brotherson et al. 1983, Johansen 1986, Jeffries and Klopatec 1987, Beymer and Klopatek 

1992). 
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Appendix D-41 

The influence of grazing on mycorrhizae also is a concern because mycorrhizal fungi form 

symbiotic relations with the roots of most vascular plants native to North America (Molina et 

al. 1992). Allen et al. (1989), however, reported no detectible changes in vesicular-arbuscular 

mychorrhizal fungi in two species of semiarid tussock grasses after clipping. Conversely, 

Bethlenfalvay and Dakessian (1984) found that colonization of forage grasses by vesiculararbuscular mychorrhizal fungi was significantly reduced by sheep grazing, but found no 

differences in colonization of grazed and ungrazed broadleaf plants. These authors 

documented that grazing causes a shift in the composition of the native vegetation and 

accompanying mycoflora. 

Interactions Between Grazing and Other Disturbances 

Most invasive species are extremely efficient at exploiting the initial decreases in native plant 

competition that can follow a disturbance. If native plant species vigor, abundance, or 

diversity are reduced following disturbance, invaders may establish vigorous populations that 

ultimately exclude the recovery of native species (Pimm 1984, Drake 1990). The interaction 

of multiple disturbances such as fire, logging, grazing, and road building have been noted to 

allow non-native species to invade or colonize native ecosystems (Vitousek et al. 1996). 

Predicting specific responses, however, is exceedingly complex. Grazing by ungulates 

reduces the standing biomass of plants available for burning, hence grazing can influence the 

frequency, extent, and intensity of wild and prescribed fires (Belsky and Blumenthal 1997). 

Fire, directly or indirectly, can influence the establishment and spread of many invasive plant 

species (D’Antonio and Vitousek 1992, Milberg and Lamont 1995, D’Antonio 2000, 

Goodwin and Sheley 2001, Harrod and Reichard 2001). Once established, the invasive 

plants themselves may modify fire frequency and intensity, as is the case with cheat grass 

(Bromus tectorum L.) which in some environments is a persistent species because of its 

synergistic relationship with fire (Mack and D’Antonio 1998). 

Effects of Disturbance-Mediated Ungulate Distribution 

Many factors beyond food availability regulate the distribution of ungulates (Coughenour 

1991, Reimoser and Gossow 1996). Interference factors, such as postfire rehabilitation, 

logging, fuels reduction practices, and increased road traffic, can redistribute ungulates across 

landscapes (Wisdom 1998, Weisberg et al. 2002). As a result of such interference, herbivores 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-42 

may concentrate their use, and hence their impacts, in a smaller portion of their habitats. 

Overall, grazing by livestock tends to be more intense and potentially deleterious to native 

plant communities than grazing by wild ungulates simply because livestock densities can be 

higher and spatially constant, owing to fencing, water improvements, mineral 

supplementation, and other management (Hobbs 1996). However, migratory wild ungulates 

and their effects are also highly influenced by management inputs. Setting ungulate density 

through hunting seasons is a traditional tool of wildlife managers, but managing game animal 

numbers alone may not alter the relative grazing pressure on plant communities. Historical 

patterns of wild ungulate grazing were based on migration that tracked the young, rapidly 

growing stage of development in plant communities along phenological gradients covering 

entire landscapes. Anthropogenic habitat fragmentation and the elimination of migration 

corridors, however, may have led to a homogenization of grazing intensities because these 

landscapes no longer receive the variable grazing pressure of nomadic ungulates (Knopf 

1996). Extended or continual grazing may have major indirect effects on the functional 

composition of isolated reserves (Augustine and McNaughton 1998), and thereby could 

increase their risk of non-native plant invasion. 

Knowledge Gaps and Research Needs

Current knowledge of ungulate herbivory as a contributor to the invasion, establishment, and 

spread of non-native plants is severely limited by a lack of manipulative experiments. Most 

studies to date have been observational, unreplicated, and lacking the experimental controls 

needed to eliminate or account for confounding sources of variation. For example, most 

studies have examined ungulate effects on composition or succession of plant communities by 

comparing only extant herbivory with total ungulate exclusion or evaluate the effects of only 

one ungulate species even though two or more species were typically present in the areas 

under study. Only a few studies evaluate multiple densities of ungulates. Inferring causeeffect relations from such studies is questionable. Importantly, we are unaware of any 

herbivory studies that were designed to make inferences to large landscapes typically of 

interest to resource managers. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-43 

Given the rapid rate of establishment and spread of invasive plants in Western landscapes, we 

feel that new research on the influence of ungulates should consider the following points in 

study design: 

(1) The influence of all ungulate species present in the system under study, including both 

wild and domestic species. 

(2) The effects of multiple densities of each ungulate species, including the evaluation of 

potential thresholds, below which the densities of ungulates may or may not contribute 

to spread and establishment of non- native plants. 

(3) The interactions of ungulate herbivory with episodic disturbances as potential joint 

contributors to non-native plant spread and establishment; 

(4) The specific mechanisms by which each species of invasive plant may be influenced 

by ungulate herbivory, such as through trampling of vegetation, soil disturbance, 

endozoochory or epizoochory, or interspecific competetion. 

(5) Manipulative studies, with particular emphasis on experiments designed to evaluate 

specific effects of ungulates under appropriate treatments and controls needed to 

understand cause-effect relations and mechanisms. 

(6) Research that allows inferences to be made at large landscape scales of interest to 

managers such as watersheds that are typically managed for a variety of resources in 

an integrated manner (Hann et al. 1997). 

(7) Interdisciplinary research in various vegetation types to establish a multiple-scale 

framework for restoration of native communities on invasive-species-infested areas 

that have been historically overgrazed by domestic ungulates. 

Addressing these needs as part of future research on ungulate herbivory is critical to the 

design of effective management strategies to mitigate the continued spread and 

establishment of invasive plants, and to determine the best practices to reestablish native 

communities. Understanding the cause-effect relations between ungulates and native and 

invasive plant dynamics is a critical management need that deserves high priority for 

research. 
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-44 

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Appendix D-51 

Figure 1. Numbers of ungulates that spend some time each year on public lands in Oregon. 

Based on 2001 data from Oregon Department of Fish and Wildlife reports and U.S. Forest 

Service national grazing statistics. 

CATTLE SHEEP WILD HORSES ELK MULE DEER

NUMBERS

0

50000

100000

150000

200000

250000

300000

350000
Preventing and Managing Invasive Plants Final Environmental Impact Statement April 2005 

Appendix D-52 

Ungulate 

Type 

Number of 

Animals 

______Area( km2

_)______

 Total range Winter range 

______Density _______

Total range Winter range 

Mule Deera 36,680 18,076 7,543 2.03/km2 4.86/km2 

Elkb 19,740 13,497 5,869 1.46/km2 3.36/km2 

Cattlec 24,150 7,725 ______ 3.13/km2 ______ 

Total 80,570 39,298 13,412 6.63/km2 8.22/km2 

Table 1. Density of grazing ungulates on the Wallowa Whitman National Forest in 

northeast Oregon 

a

1998 population census (personal communication, Tim Schommer, Wallowa Whitman 

National Forest) 

b 2000 population census (personal communication, Tim Schommer, Wallowa Whitman 

National Forest) 

c 2001 grazing statistics, Wallowa Whitman National Forest