Grazing pressure

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Grazing pressure is defined as the number of grazing animals of a specified class (age, species, physiological status like pregnant) per unit weight of herbage (herbage biomass). It is well established in general usage.  [1]

Contents

Definition

Grazing pressure is the demand for feed from herbivores and detritivores within an environment compared to the amount available for consumption. This could come from domestic animals, such as goats and cattle; feral animals, such as rabbits; and wild animals, such as insects, rodents, kangaroos, water buffalo, or moose. Even some microbes are grazers. Total grazing pressure is the ratio of the demand for forage and the supply of forage available. [2] Demand can come from both livestock and native or feral animals. Grassland ecosystems in particular have evolved in the presence of grazing from large herbivores and are well-adapted to it.

Livestock grazing pressure

Goat grazing Goat Grazing.jpg
Goat grazing

Grazing pressure due to livestock can be regulated and controlled more easily compared to that from native and feral animals. [3] Rather than use open fields for grazing among domestic animals, cereal pastures may be used as an alternative. This reduces the grazing pressure on local shrublands. In areas of the Mediterranean, both mature goats and their kids were found to affect local shrubbery. Older goats spent about a third of their time grazing in the shrubs, while their young spent at least half their time. The amount of time spent in each type of grazing are also correlated with the genotype of the goat. While the goats were found in the pastures a majority of the time, their grazing patterns still affected local wildlife and shrub growth. [4] Grazing pressure is not only a problem regarding natural grasslands and shrubbery. In northwestern Europe, the rising goose population has caused an increase in grazing pressure on agricultural lands. [5]

Within the environment

On land

Disturbance of plant life caused by the grazing of large herbivores can be an important determinant of plant community structure. The composition of plant and animal species can be affected by grazing pressure in some environments, in others, the environmental characteristics of the site, including weather and climate, may be more important. Grazing affects plant communities directly through physical removal of plant parts. It can also affect plant communities indirectly by modulation of ecosystem productivity or by changing the pattern of nutrient partitioning of nutrients among different sizes of plants. Thus, grazing can change the population size, diversity and distribution of organisms in an ecosystem. Grazing pressure also influences plant species performance and plant ecological stoichiometry. For instance, plant functional composition of tundra is primary structured by grazing pressure. [6]

Some studies suggest grazing may be beneficial in nutrient-rich conditions and harmful in habitats poor in nutrients. In other cases grazing will not affect ecosystems whatsoever. For instance, in the Mongolian rain forest, Cheng et al. have found that grazing pressure plays a highly, positive important role in species richness in the wetter steppe of the rain forest. The same research has found that in the dryer, desert ecosystem grazing pressure did not affect species richness significantly. [7] Introductions of new types of grazing pressure can change ecosystems if the plants are not adapted to it.

In a study performed by Saccone et al., when experimenting with biodiversity in the Fennoscandia tundra, they found that there was an increase in species richness associated with decrease of shrub cover. [6] It was shown that in the Fennoscandia tundra grazing is an important and efficient biotic filter restricting the spread of dwarf shrubs to mountain tundra snowbeds. In other words, because of grazing and fewer shrubs, there was more biodiversity among the species.

There are mechanisms that plants use to defend themselves against the grazing pressure of herbivores. Carbon-based secondary metabolites inhibit digestion in herbivores. Tough leaves provide physical defense against herbivores. On the other hand, other features of plants may make them more susceptible to herbivores. For example, leaves that are high in nitrogen have a higher dietary value for herbivores. [8] Also, taller plants and forbs are often more sensitive to grazing, while shorter herbs can avoid the grazing by being overlooked by the grazing animals.

In water

In the ocean, grazing pressure tends to increase with decreasing latitude. [9] Many aquatic herbivores graze on phytoplankton: algae that float on the surface of the ocean or are suspended in the water column. With phytoplankton being the main primary producers, grazing on it is the dominant form of grazing in ocean ecosystems. Other form of grazing include grazing on macroalgae anchored at the bottom (kelp) and benthic (bottom dwelling) algae. As a result of the grazing pressure, algae has to divert their energy away from growth and put into repair and the production of chemical defenses against the herbivores. This also means diverting their energy from reproduction. Therefore, the growth of algae in the ocean often depends on herbivores. Furthermore, the eating of algae by animals is not only affecting the algae, but also the ocean community as a whole. As the grazing may limit algae growth and amount of energy transferred to other trophic levels being consumed, the photosynthetic is lost and the ability to fix carbon reduces and diminishes overall growth.

Kelp in coastal water shelters the water column from UV radiation and protects the coasts from waves and storms. Grazing pressure can decrease this protection and shelter. For example, the kelp Macrocytis pyrifera may grow up to 40m. The basal parts of this kelp body are shaded by the surface canopies and are therefore protected, and the undifferentiated cell tissue is exposed to surface levels of UV radiation. Grazing can detach parts of the kelp and the entire reproducing part of the algae in the Macrocytis pyifera becomes suddenly exposed to surface levels of UV radiation.

In a study produced by Rothausler et al., [9] kelps that were not exposed to UV radiation were consumed more by grazers than those kelps that were exposed to UV radiation.

On a microbial level

While grazing pressure is more commonly thought in larger herbivores such as cattle, geese, and goats, it is also found within bacterium and on a more microbial level. Leaf-litter colonizing bacteria are a type of bacterium that have been studied regarding grazing pressure on a smaller scale. While it seems that grazing pressure in such a localized environment would not have any large effects, one study found the opposite. Conducted in 2014, researchers found that when they added nitrogen and phosphorus, as well as increased the temperature, it increased the density of the bacteria within a freshwater environment. While this was a controlled experiment, there are real life consequences that should be considered as global warming becomes a greater issue. If the populations of bacteria increase enough then it is possible that the grazing pressure will reach a detrimental level. Once it reaches this peak, the bacteria will have depleted their food resources and may perhaps cause a collapse within the environmental food chain in that area. [10]

Another negative effect caused by grazing pressure was found in mangrove forest soils. Sulfate, which is used in the oxidation of organic matter, is dependent on the surplus of carbon left by aerobic organisms due to oxygen depletion. When a mix of tidal exposure and camel grazing pressure were placed on the habitat, there was a decrease in primary production, which in turn affected sulfate-cycling. A decrease in sulfate-cycling could cause the trees in the forest to slow down their oxidation process. [11]

Mitigation

The native kangaroo population in Australia is kept under control by hunters. This helps to contain the grazing pressure level while turning a profit for locals. Hunters must abide by a quota set by the Australian federal government. Kangaroo hunting is an effective control method in open areas but are less effective in dense areas where human accessibility is limited. While effective in the native kangaroo population, shooting is not effective within the goat population where herds are much larger. Instead, large herds are mustered together by a plane or trapped. [12] Trapping is illegal within the kangaroo population.

Eastern Grey Kangaroo Grazing Eastern Grey Kangaroo444.jpg
Eastern Grey Kangaroo Grazing

Another method of population control is limiting the water in the area. Australian governments have been known to cap bores or troughs in order to get feral kangaroos or goats to move on. Some farmers have created kangaroo-proof fences with swing or trap gates that allows cattle and sheep in but keeps other animals out. These fences protect water sources and pastures, forcing kangaroos to move on elsewhere for resources. One problem with this method of reducing grazing pressure in one area, is simply pushing the problem on neighboring pastures and croplands.

In other areas of the world, such as Europe, where rising goose levels are becoming a problem for local agricultural pastures, other ideas have been contemplated. It was proposed that by managing nature reserves in the area, this might help decrease the negative effect of grazing pressure on farmland. By increasing the number of geese that can make use of the native flora on the reserves, this will cut costs for the crops lost to geese grazing on farmlands. [13]

Related Research Articles

<span class="mw-page-title-main">Herbivore</span> Organism that eats mostly or exclusively plant material

A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthparts adapted to rasping or grinding. Horses and other herbivores have wide flat teeth that are adapted to grinding grass, tree bark, and other tough plant material.

<span class="mw-page-title-main">Epiphyte</span> Non-parasitic surface organism that grows upon another plant but is not nourished by it

An epiphyte is a plant or plant-like organism that grows on the surface of another plant and derives its moisture and nutrients from the air, rain, water or from debris accumulating around it. The plants on which epiphytes grow are called phorophytes. Epiphytes take part in nutrient cycles and add to both the diversity and biomass of the ecosystem in which they occur, like any other organism. They are an important source of food for many species. Typically, the older parts of a plant will have more epiphytes growing on them. Epiphytes differ from parasites in that they grow on other plants for physical support and do not necessarily affect the host negatively. An organism that grows on another organism that is not a plant may be called an epibiont. Epiphytes are usually found in the temperate zone or in the tropics. Epiphyte species make good houseplants due to their minimal water and soil requirements. Epiphytes provide a rich and diverse habitat for other organisms including animals, fungi, bacteria, and myxomycetes.

<span class="mw-page-title-main">Grassland</span> Area with vegetation dominated by grasses

A grassland is an area where the vegetation is dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur naturally on all continents except Antarctica and are found in most ecoregions of the Earth. Furthermore, grasslands are one of the largest biomes on earth and dominate the landscape worldwide. There are different types of grasslands: natural grasslands, semi-natural grasslands, and agricultural grasslands. They cover 31–69% of the Earth's land area.

<span class="mw-page-title-main">Energy flow (ecology)</span> Flow of energy through food chains in ecological energetics

Energy flow is the flow of energy through living things within an ecosystem. All living organisms can be organized into producers and consumers, and those producers and consumers can further be organized into a food chain. Each of the levels within the food chain is a trophic level. In order to more efficiently show the quantity of organisms at each trophic level, these food chains are then organized into trophic pyramids. The arrows in the food chain show that the energy flow is unidirectional, with the head of an arrow indicating the direction of energy flow; energy is lost as heat at each step along the way.

<span class="mw-page-title-main">Grazing</span> Feeding livestock on forage

In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to roam around and consume wild vegetations in order to convert the otherwise indigestible cellulose within grass and other forages into meat, milk, wool and other animal products, often on land unsuitable for arable farming.

<span class="mw-page-title-main">Kelp forest</span> Underwater areas with a high density of kelp

Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Earth. Although algal kelp forest combined with coral reefs only cover 0.1% of Earth's total surface, they account for 0.9% of global primary productivity. Kelp forests occur worldwide throughout temperate and polar coastal oceans. In 2007, kelp forests were also discovered in tropical waters near Ecuador.

Ecological facilitation or probiosis describes species interactions that benefit at least one of the participants and cause harm to neither. Facilitations can be categorized as mutualisms, in which both species benefit, or commensalisms, in which one species benefits and the other is unaffected. This article addresses both the mechanisms of facilitation and the increasing information available concerning the impacts of facilitation on community ecology.

<span class="mw-page-title-main">Lake ecosystem</span> Type of ecosystem

A lake ecosystem or lacustrine ecosystem includes biotic (living) plants, animals and micro-organisms, as well as abiotic (non-living) physical and chemical interactions. Lake ecosystems are a prime example of lentic ecosystems, which include ponds, lakes and wetlands, and much of this article applies to lentic ecosystems in general. Lentic ecosystems can be compared with lotic ecosystems, which involve flowing terrestrial waters such as rivers and streams. Together, these two ecosystems are examples of freshwater ecosystems.

<span class="mw-page-title-main">Urchin barren</span> Shallow ocean area with destructive grazing of kelp forests

An urchin barren is commonly defined as an urchin-dominated area with little or no kelp. Urchin grazing pressure on kelp is a direct and observable cause of a "barren" area. However, determining which factors contribute to shifting a kelp bed to an urchin barrens is a complex problem and remains a matter of debate among scientists.

Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behavior of their prey, thereby releasing the next lower trophic level from predation.

<span class="mw-page-title-main">Feral donkeys in Australia</span>

Feral donkeys were first brought to Australia as pack animals to replace horses, which had succumbed to native poisonous plants. Now numbering 5 million, they have been declared a pest, owing to their damage to vegetation and erosion of soil. Culling is mainly carried out by marksmen in helicopters, and experiments are being made in fertility control.

<span class="mw-page-title-main">Conservation grazing</span> Use of animals to graze areas like nature reserves to maintain habitats

Conservation grazing or targeted grazing is the use of semi-feral or domesticated grazing livestock to maintain and increase the biodiversity of natural or semi-natural grasslands, heathlands, wood pasture, wetlands and many other habitats. Conservation grazing is generally less intensive than practices such as prescribed burning, but still needs to be managed to ensure that overgrazing does not occur. The practice has proven to be beneficial in moderation in restoring and maintaining grassland and heathland ecosystems. The optimal level of grazing will depend on the goal of conservation, and different levels of grazing, alongside other conservation practices, can be used to induce the desired results.

<span class="mw-page-title-main">Feral goat</span>

The feral goat is the domestic goat when it has become established in the wild. Feral goats occur in many parts of the world.

<span class="mw-page-title-main">Browsing (herbivory)</span> Type of herbivory

Browsing is a type of herbivory in which a herbivore feeds on leaves, soft shoots, or fruits of high-growing, generally woody plants such as shrubs. This is contrasted with grazing, usually associated with animals feeding on grass or other lower vegetations. Alternatively, grazers are animals eating mainly grass, and browsers are animals eating mainly non-grasses, which include both woody and herbaceous dicots. In either case, an example of this dichotomy are goats and sheep.

<span class="mw-page-title-main">Autotroph</span> Organism type

An autotroph is an organism that produces complex organic compounds using carbon from simple substances such as carbon dioxide, generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis). They convert an abiotic source of energy into energy stored in organic compounds, which can be used by other organisms. Autotrophs do not need a living source of carbon or energy and are the producers in a food chain, such as plants on land or algae in water. Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and as stored chemical fuel. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide.

<i>Apophyllum</i> Genus of flowering plants

Apophyllum is a genus of plant native to Australia. It contains only one species, which by definition makes the genus a monotypic genus. This single species is called Apophyllum anomalum, commonly called warrior bush or broom bush.

<span class="mw-page-title-main">Feral goats in Australia</span>

Feral goats are an invasive animal species in Australia. First arriving in the 18th century with European settlers, feral goat populations originated from escaped domestic individuals. Today, feral goats are found across Australia, where they cause economic and environmental damage through overgrazing and competition with livestock and native marsupials.

<span class="mw-page-title-main">Marine habitat</span> Habitat that supports marine life

A marine habitat is a habitat that supports marine life. Marine life depends in some way on the saltwater that is in the sea. A habitat is an ecological or environmental area inhabited by one or more living species. The marine environment supports many kinds of these habitats.

<span class="mw-page-title-main">Marsupial lawn</span>

Marsupial lawns are portions of land where the soil moisture is much higher than in the vegetation surrounding it. These high moisture levels create lawns that attract a large number of grazing by marsupials. Commonly found in Tasmania, the lawns function as habitats for local animals.

<span class="mw-page-title-main">Grazing (behaviour)</span> Method of feeding in herbivores, eating grasses and other plants

Grazing is a method of feeding in which a herbivore feeds on low-growing plants such as grasses or other multicellular organisms, such as algae. Many species of animals can be said to be grazers, from large animals such as hippopotamuses to small aquatic snails. Grazing behaviour is a type of feeding strategy within the ecology of a species. Specific grazing strategies include graminivory ; coprophagy ; pseudoruminant ; and grazing on plants other than grass, such as on marine algae.

References

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  2. Business Queensland, Queensland Govt (July 2013). "Total grazing pressure". Grazing and Pasture Management. Queensland Govt. Retrieved 15 September 2018.{{cite web}}: |last1= has generic name (help)
  3. "Managing Total Grazing Pressure" (PDF).
  4. Baytekin, H; Atasoglu, C; Tolu, C; Yurtman, I. Y.; Atasoglu, C; Savas, T (2012). "Foraging strategies of goats in a pasture of wheat and shrubland". Animal Production Science. 52 (12): 1069–1077. doi:10.1071/AN11251.
  5. Mandema, F.S.; Tinbergen, J.M.; Stahl, J.; Esselink, P.; Bakker., J.P. (2014). "Habitat preference of geese is affected by livestock grazing - seasonal variation in an experimental field evaluation". Wildlife Biology. 20 (2): 67–72. doi: 10.2981/wlb.13046 .
  6. 1 2 Saccone; et al. (2014). "Environmental perturbation, grazing pressure and soil wetness jointly drive mountain tundra toward divergent alternative". Journal of Ecology. 102 (6): 1661–1672. doi: 10.1111/1365-2745.12316 .
  7. Cheng; et al. (2011). "Impact of rainfall variability and grazing pressure on plant diversity in Mongolian grasslands". Journal of Arid Environments. 75 (5): 471–476. Bibcode:2011JArEn..75..471C. doi:10.1016/j.jaridenv.2010.12.019.
  8. Nakamura; et al. (2010). "Differential effects of host plant hybridization on herbivore community structure and grazing pressure on forest canopies". Oikos. 119 (9): 1445–1452. doi:10.1111/j.1600-0706.2010.18255.x.
  9. 1 2 Rothausler; et al. (2011). "UV-radiation versus grazing pressure: long-term floating kelp rafts is facilitated by efficient photoacclimation but undermined by grazing losses". Marine Biology. 158: 127–141. doi:10.1007/s00227-010-1547-9. hdl: 10533/127558 . S2CID   85135042.
  10. Moghadam, F.S.; Zimmer, M. (2014). "Effects of Warming and Nutrient Enrichment on How Grazing Pressure Affects Leaf-Litter Colonizing Bacteria". Journal of Environmental Quality. 43 (3): 851–858. doi:10.2134/jeq2013.12.0503. PMID   25602814.
  11. Balk, M; Keuskamp, J.A.; Laanbroek, H.J. (2015). "Potential Activity, Size, and Structure of Sulfate-Reducing Microbial Communities in an Exposed, Grazed and a Sheltered, Non-Grazed Mangrove Stand at the Red Sea Coast". Frontiers in Microbiology. 6 (33): 1475. doi: 10.3389/fmicb.2015.01478 . PMC   4686736 . PMID   26733999.
  12. "Managing Total Grazing Pressure" (PDF).
  13. Mandema, F.S.; Timbergen, J.M.; Stahl, J.; Esselink, P.; Bakker, J.P. (2014). "Habitat preference geese is affected by livestock grazing - seasonal variation in an experimental field evaluation". Wildlife Biology. 20 (2): 67–72. doi: 10.2981/wlb.13046 .