Impacts of Grazing
The ecological forces—herbivory, physical impact, and deposition—of grazing ungulates have shaped natural grazing ecosystems around the world. Grazing ecosystems evolved with and depend upon herbivory, heavy hoof action, nitrogen deposits, and decomposing carcasses of large migratory ungulates. When introduced into ecosystems that did not evolve with frequent grazing, these forces can alter biological communities and ecosystem function.
Herbivores consume plant leaves, stems, flowers, seeds, and sometimes roots. Patterns of herbivory largely determine plant community composition, structure, and productivity.
Through hoof action, pawing, and wallowing, grazing animals trample plants, break up soil surfaces, incorporate seed into the soil, and compact soils.
Grazing animals contribute to nutrient cycling by depositing nitrogen-rich urine and dung, and their carcasses can provide an important contribution to the food web.
In human-controlled grazing systems, the detrimental or beneficial effects of grazing are largely determined by how and where grazing is used. The negative impacts of livestock grazing are often the result of misuse. The benefits of domestic livestock grazing rarely come by accident, and are likely the result of careful program design, regular monitoring, and flexibility in modifying treatments. The ecological impacts of grazing depend on the type of ecosystem, plant community, and conditions of a particular site.
Grazing Selects For or Against Plant Species
Grazing affects the species composition of a plant community through herbivores selecting or avoiding specific plants, and through differential tolerance of plants to grazing (Szaro 1989). Continued selective grazing can reduce the competitive vigor of grazed plants and release ungrazed species from competition. Trampling can also indiscriminately injure plants, and may reduce their competitive and reproductive capacities within the plant community.
Grazing Changes Vegetative Structure
The physical structure of plant communities is often changed by grazing (Huntly 1991). Fleischner (1994) cites a number of examples where defoliation by grazing herbivores altered plant height and canopy cover, and changed species composition to include structurally different types of plants. Trampling may also change the structure of plant communities by breaking and beating down vegetation.
Grazing Increases or Decreases Biomass Production
Ungulates play a major role in regulating primary production (energy produced by photosynthesis) in grazing ecosystems (Huntly 1991). Defoliation can promote shoot growth and enhance light levels, soil moisture, and nutrient availability (Frank et al. 1998). Overgrazing, however, can significantly reduce biomass production.
Grazing Increases or Decreases Seed Production, Dispersal, and Germination
Grazing animals can decrease flower and seed production directly by consuming reproductive structures, or indirectly by stressing the plant and reducing energy available to develop seeds. Grazing animals can also disperse seeds by transporting seed in their coats (fur, fleece, or hair), feet, or digestive tracts (Wallander et al. 1995, Lacey et al. 1992). For some plant species, grazing ungulates may facilitate seed germination by trampling seed into the soil.
The response of wildlife to domestic grazing varies by habitat. While wildlife habitat may be degraded by mismanaged livestock grazing, proper management often improves wildlife habitat. Livestock grazing can have direct and indirect impacts on wildlife. Direct impacts include the removal and/or trampling of vegetation that would otherwise be used for food and cover, and livestock-wildlife interactions that may result in wildlife displacement or disease transmission. Indirect impacts result from changes in plant community composition, structure, and productivity which together largely determine wildlife habitat suitability.
Grazing Disturbs Soil Surfaces
Trampling, pawing, and wallowing by ungulates disturb the soil and in some cases completely destroy soil crusts (Fleischner 1994, Belsky and Gelbard 2000). Microbiotic soil crusts play an important role in regulating nutrient cycling, biomass production, soil stability, and water infiltration (Belnap 1994). In ecosystems that evolved with frequent grazing disturbance, soil crust disruption maintains natural ecosystem processes and biological communities. However, some authors argue that in arid and semi-arid ecosystems, loss of microbiotic crusts can have detrimental long-term effects (Belsky and Gelbard 2000, Fleischner 1994).
Grazing Increases or Decreases Erosion
Reduced vegetative cover and disturbed soil surfaces may result in increased wind and water erosion (Belnap and Gillette 1998). However, organic components of feces and urine from grazing animals can build soil organic matter reserves, resulting in soils having increased water-holding capacity, increased water-infiltration rates, and improved structural stability. These changes can decrease soil loss by wind and water erosion (Hubbard et al. 2004).
Grazing Compacts Soils
The most severe effect of trampling may be compaction of soils, which damages plant roots (Watkins and Clements 1978) and causes roots to become concentrated near the soil surface (Dormaar and Willms 1998). These changes may prevent plants from acquiring sufficient resources for vigorous growth (Belsky and Gelbard 2000).
Grazing Incorporates Organic Matter
The hoof-action of large grazing ungulates can incorporate plant material into soils and increase organic matter.
Grazing Contributes to Nutrient Cycling and the Food Web
Grazers enhance mineral availability by increasing nutrient cycling within patches of their waste and increasing nitrogen availability to plants (Holland et al. 1992). In natural grazing systems, the decomposing carcasses of wild ungulates provide feasts for decomposers and scavengers, constituting a central node in the food web (Dunne et al. 2002). However, in grazing systems managed by humans, livestock carcasses are often removed from the environment.
Grazing Can Alter Fire Regimes
Fire frequency, intensity, and behavior are dictated largely by type, condition, and quantity of vegetation (DiTomaso and Johnson 2006). Grazing alters fuel-load characteristics by changing plant community composition, structure, and biomass. Altered fuels can change fire-return cycles, fire intensity, and spread patterns. This can result in further changes in plant community composition as fire-intolerant populations give way to fire-tolerant ones. Grazing can be used to intentionally manipulate fire fuel-loads in prescribed burning programs (DiTomaso and Johnson 2006).
The forces and influences discussed above make grazing a valuable vegetation management tool. While the misuse of domestic livestock grazing can increase populations of invasive plants, proper grazing management can promote desirable vegetation and reduce invasive plant populations.
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