Grazing in Grasslands

Grazing is a form of herbivory in which most of the leaves or other plant parts (small roots and root hairs) are consumed by herbivores. Grazing, both above- and belowground, is an important process in all grasslands. The long association of grazers and grasslands has prompted the hypothesis that grasses and their megaher-bivore grazers are a highly coevolved system, but, as mentioned above, there is some more recent evidence that this might not be the case. However, there is no disagreement that large grazers have been a factor in grassland ecology since their origin. The herbivory actions of many other smaller organisms including small mammals and insects may be equally important. There is no doubt that the impact of native grazers in grasslands can be extensive and work on the East African Serengeti plains estimated that 15% to >90% of the annual above-ground net primary productivity can be consumed by ungulates. However, data from small mammal exclosures suggest that small mammals can also impact grasslands as when small mammals were excluded from plots in Kenya; biomass was 40-50% higher than in adjacent plots where small mammals occurred.

Due to the ability of grasses to cope with high rates of herbivory, many former natural grasslands are now being managed for the production of domestic livestock, primarily cattle in North and South America and Africa, as well as sheep in Europe, New Zealand, and other parts of the world. Grasslands present a vast and readily exploited resource for domestic grazers. However, like many resources, grasslands can be overexploited (discussed in more detail below).

Grazing systems can be roughly divided into two main types - commercial and traditional - with the traditional type often mainly aimed at subsistence. Commercial grazing of natural grasslands is very often at a large scale and commonly involves a single species, usually beef cattle or sheep for wool production. Some of the largest areas of extensive commercial grazing developed in the nineteenth century on land which had not previously been heavily grazed by ruminants; these grazing industries were mainly developed in the Americas and Australia, and to a much less degree in southern and eastern Africa. Traditional livestock production systems vary according to climate and the overall farming systems of the area. They also use a wider range of livestock, including buffaloes, asses, goats, yaks, and camels. In traditional farming systems, livestock are often mainly kept for subsistence and savings, and are frequently multipurpose, providing meat, milk, and manure as fuel.

Grazing aboveground by large herbivores alters grasslands in several ways. Grazers remove fuel and may lessen the frequency and intensity of fires. Most large grazers such as cattle or bison primarily consume the grasses; thus the less abundant forb species (broad-leafed, herbaceous plants) may increase in abundance and new species may invade the space that is made available. Thus, fire reduces heterogeneity in mesic grassland (a few species dominate) while grazers increase heterogeneity regardless of fire frequency. In other words, grazing decouples the impact of fire in productive grasslands (Figure 6). As a result; grazing increases plant species diversity in mesic grasslands. In xeric grasslands, on the other hand, grazing may lower species diversity, particularly by altering the availability of suitable microsites for forb species. These effects are strongly dependent on grazing intensity. Overgrazing may rapidly degrade grasslands to systems dominated by weedy and non-native plant species.

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Figure 6 Aboveground biomass removal by large ungulates modulates plant community responses to fire in mesic grasslands. In ungrazed prairie (top), cover of dominant C4 grasses increased with increasing fire frequency, while cover of forbs decreased, resulting in a loss of diversity. However, in prairie grazed by bison (bottom), the cover of forbs was positively correlated with fire frequency and the cover of grasses was unaffected, resulting in high diversity in spite of frequent fires. From Collins SL, Knapp AK, Briggs JM, Blair JM, and Steinauer EM (1998) Modulation of diversity by grazing and mowing in native tallgrass prairie. Science 280(5364): 745-747.

Grazers may also accelerate the conversion of plant nutrients from forms that are unavailable for plant uptake to forms that can be readily used. Essential plant nutrients, such as nitrogen, are bound for long periods of time in unavailable (organic) forms in plant foliage, stems, and roots. These plant parts are slowly decomposed by microbes and the nutrients they contain are only gradually released in available (inorganic) forms. This decomposition process may take more than a year or two. Grazers consume these plant parts and excrete a portion of the nutrients they contain in plant-available forms. This happens very quickly compared to the slow decomposition process, and nutrients are excreted in high concentrations in small patches. Thus, grazers may increase the availability of potentially limiting nutrients to plants as well as alter the spatial distribution of these resources.

Some grasses and grassland plants can compensate for aboveground tissue lost to grazers by growing faster after grazing has occurred. Thus, even though 50% of the grass foliage may be consumed by bison or wildebeest, when compared to ungrazed plants at the end of the season, the grazed grasses may be only slightly smaller, the same size, or even larger than ungrazed plants. This latter phenomenon, called 'overcompensation' is controversial, yet the ability of grasses to compensate partially or fully for foliage lost to grazers is well established. Compensation occurs for several reasons, including an increase in light available to growing shoots in grazed areas, greater nutrient availability to regrowing plants, and increased soil water availability. The latter occurs after grazing because the large root system of the grasses is able to supply abundant water to a relatively small amount of regrowing leaf tissue.

As with fire, the impact of grazing on grasslands depends upon where in the precipitation gradient the grassland occurs (usually more mesic grasslands can recover more quickly than arid grasslands) as well as the growth form - cespitose (bunch-forming grasses) versus rhizomatous grasses. But another key factor is the evolutionary history of the grassland. In general, grasslands with a long evolutionary history of grazers, as in Africa, are very resilient to grazing whereas grasslands with a short evolutionary history such as desert grasslands in North America can easily be damaged by even light grazing.

Threats to Grasslands and Restoration of Grasslands

Grassland environments are key agricultural areas worldwide. In North America and elsewhere, grasslands are considered to be endangered ecosystems. For example, in US Great Plains up to 99% of native grassland ecosystems in some states have been plowed and converted to agricultural use or lost due to urbanization. Similar but less dramatic losses of mixed and shortgrass prairies have occurred in other areas. While the loss of native grasslands due to agricultural conversion is still occurring in some places, dramatic increases in woody shrub and tree species threatens many remaining tracts of grasslands. Indeed, across the world, the last remaining native grasslands are being threatened by an increase in the abundance of native woody species from expansion of woody plant cover originating from both within the ecosystem and from adjacent ecosystems. Increased cover and abundance of woody species in grasslands and savannas have been observed worldwide with well-known examples from Australia, Africa, and South America. In North America, this phenomenon has been documented in mesic tallgrass prairies of the eastern Great Plains, subtropical grasslands and savannas of Texas, desert grasslands of the Southwest, and the upper Great Basin. Purported drivers of the increase in woody plant abundance are numerous and include changes in climate, atmospheric CO2 concentration, nitrogen deposition, grazing pressure, and changes in disturbance regimes such as the frequency and intensity of fire. Although the drivers vary, the consequences for grassland ecosystems are strikingly consistent. In many areas, the expansion of woody species increases net primary production and carbon storage, but reduces biodiversity. The full impact of shrub encroachment on grassland environments remains to be seen.

Another threat to native grasslands is the increase of non-native grass species. For example, in California, it is estimated that an area of approximately 7 000 000 ha (about 25% of the area of California) has been converted to grassland dominated by non-native annuals primarily of Mediterranean origin. Conversion to non-native annual vegetation was so fast, so extensive, and so complete that the original extent and species composition of native perennial grasslands is unknown. In addition, across the western US, invasive exotic grasses are now dominant in many areas and these species have a significant impact on natural disturbance regimes. For example, the propensity for annual grasses to carry and survive fires is now a major element in the arid and semiarid areas in western North America. In the Mojave and Sonoran deserts of the American Southwest, in particular, fires are now much more common than they were historically, which may reduce the abundance of many native cactus and shrub species in these areas. This annual-grass-fire syndrome is also present in native grasslands of Australia and managers there and in North America are using growing season fire to try to reduce the number of annual plants that set seed and thus reduce the populations of exotics, usually with very mixed results.

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