Pasture Lands

Pasture land is an anthropogenic biome represented by lands where predominant plants are grasses, grasslike plants, forbs, or shrubs. These lands are managed as grazing systems designed for the production of domestic animals for consumption, including production for meat, milk, and other major animal products. Since historical times these sites have been predominantly located in natural grasslands, savannas, steppes, wetlands, open and dense shrublands, tropical evergreen forests/woodlands, boreal evergreen forests/woodlands, boreal deciduous forests/woodlands, temperate needle-leaf evergreen forests/woodlands, some deserts, and tundra. For millennia, the widespread presence of human populations and their activities altered these ecosystems around the world, both intentionally and unintentionally. These effects started with low-impact practices like hunting and gathering, but through the centuries humans developed and controlled many tools that allowed them the permanent use of lands for agriculture, forestry, livestock, and settlements. These activities changed many natural processes and with them the composition and structure of flora and fauna, as well as the hydrologic and biochemical cycles at local, regional, and global scales.

Development of Pasture Lands

Historically, people managed livestock to produce food, directly as meat and dairy products and indirectly as animal power that increased cropland production. In the 18th century livestock were managed at local scales because technology and transport facilities were poor, and because livestock depended on the availability of food resources in areas where disease constraints allowed these activities. For this reason, pasture lands were kept geographically close to human settlements and barely distributed around the world. However, during the following centuries there was a considerable increase in human population around the world, as well as a growth in technology development. Due to these factors, the demand for livestock products expanded pasture distribution in almost every corner of the planet in order to produce enough commodities to sustain the growing population. As a result, the location and extension of pasture lands were undergoing important shifts. The increase of this biome exerted an enormous pressure in all wild lands, principally in the drier biomes of the Americas, Australia, central Asia, and southern Africa, but also in the moister wooded biomes of sub-Saharan Africa, Central America, and Eurasia. As a result of the increase of human use of natural resources and appropriation of the net primary production, the global extension of pasture lands increased considerably since the Industrial Revolution, growing from 3 percent of the ice-free land cover before 1700 to 26 percent in 2000.

Because of these developments, pasture lands have now become one of the most extensive anthropogenic biomes, occupying more than 26 percent of the ice-free land in the world in an area around 13,513,576 square miles (35 million square kilometers), including large areas where there previously was little or no livestock grazing in North America, South America, and Australia. In many areas, pasture lands have expanded to occupy virtually all the land that can be grazed and for which there is no other activity. Except in bare areas located in dry or cold deserts, or in dense forests, pasture lands are present to some extent in all regions around the world. In North America a large extension of the continent has been considered as a pasture, except in areas such as the northern extreme in Canada and Greenland, where the extreme temperatures impedes this land use. Also the rough mountain systems of the Rocky Mountains in western North America, both the Sierra Madre Oriental and Occidental in Mexico, the Andes Range and Patagonia along the southwestern coast of South America, and the Brazilian highlands have been difficult places to sustain livestock production. The Amazon rainforest is also considered one of the few places in the world that has not been transformed to a pasture land. In Africa, the scenario is similar; most of the continent has been grazed except the Sahara Desert in the north and the tropical forest in central Africa. In Eurasia, the extreme weather conditions and complexity of the mountain system such as the Himalaya Range, the Gobi Desert, the Scandinavian and Siberian regions, as well as the tropical rainforests at the southeastern extreme of the Asiatic continent, have disabled these areas to use the land as pasture, while the rest of the world has virtually been transformed to pasture lands.

Pasture land productivity differs in different sites on the planet and is mainly defined by climatic and soil conditions. Managed grazing is preferentially employed in areas that are much drier and stabler than the biome mean, while regions with large interannual precipitation and temperature variation strongly affect vegetal production, grazing capacity, and human living conditions. Savannas, grasslands, shrublands, and deserts support the largest extent of managed pastoral systems; these dry-land biomes cover about 51 percent of the total land area of the Earth (around 25,888,845 square miles or 67 million square kilometers), and they support 78 percent of the global pasture lands of the Earth. Other biomes also support managed grazing systems but in less proportion. For example, temperate deciduous and temperate evergreen broadleaf forests and woodlands support grazing systems, and about 656,374 square miles (1.7 million square kilometers) of tropical evergreen broadleaf forests (corresponding to 10 percent of all tropical forests on the planet) have been cleared for managed grazing. Nowadays pasture land area is expanding annually in wild regions such as the Amazon Basin, Congo, and southeast Asia rainforest.

Besides savanna, shrubland and desert are the most-used biomes for grazing systems, although they are limited in their soil type and the fertility associated with it. For instance, many types of grassland are supported in soils composed of aridisols, entisols, and alfisols that are not fertile enough to sustain a grazing system for a long period of time. In the colder boreal biomes, grazing takes place preferentially on alfisols and spodosols but not on frozen gelisols, which are the most common soil order found in these regions, the reason why these lands are still under low grazing pressure. In humid tropical regions ultisols represent the best place to maintain a grazing system, while oxisols that are also well represented in the Amazon Basin, Congo, and southeastern Asia are nutrient-poor soils for managed grazing systems.

The five countries with the most pasture lands are Australia (1,698,850 square miles or 4.4 million square kilometers), China (1,544,409 square miles or 4 million square kilometers), United States (926,645 square miles or 2.4 million square kilometers), Brazil (656,374 square miles or 1.7 million square kilometers ), and Argentina (540,543 square miles or 1.4 million square kilometers ). However, based on the proportion of total land area that a country uses for grazing, Mongolia, Botswana, and Uruguay are the most-affected countries, with 80 percent, 76 percent, and 76 percent of pasture land coverage, respectively. Countries with the highest stocking rates are Malaysia with 320 animal units per square kilometer, India with 272 animal units per square kilometer, North Korea with 213 per square kilometer, and Vietnam with 184 per square kilometer.

Consequences of Pasture Land Expansion and Management

Although managed grazing systems have been developed throughout the world for thousands of years, the spatial extension and intensity of grazing systems have considerably increased in the last three centuries. Pasture land development is influenced not only by a complex range of environmental conditions but also by many social and economic factors that have a strong influence on extension and pressure. Throughout human history, pastures have been managed under different conditions leaving the land in a broad number of states. Pasture lands have been managed in different ways according to property and access rights. Historically, three main types of land tenure have been recognized: private (owned by an individual or a company), communal, and public property. The interaction of this management generally is noxious for the ecosystem as a result of a complex and inadequate set of rules imposed by state and local officials who pretend to control the use of natural resources. This complexity generally leads to conflicts among stakeholders who claim access to pasture lands.

However, not all grazing systems or practices lead to these negative impacts. Domestic cattle (cows, horses, and goats) may have positive, neutral, or negative impacts on vegetation structure and composition, depending on the grazing intensity and the site. In areas where grazing intensity is light to moderate, the habitat of wild ungulates, the effect can be positive in stimulating the growth of some species and increasing diversity in the landscape. For the most part, however, where livestock is raised, pasture land is aimed toward the goal of producing the greatest possible number of animals. At these sites, generally bounded by fencing, cattle cause plants to recover. In these cases the rate of extinction of species tolerant of grazing increases dramatically, but there are also increases in the rate of colonization of grazing-tolerant species or those that are not consumed by livestock. Gradually the original vegetation (before introducing livestock) is replaced, and there comes a point where even when grazing decreases, the vegetation generally does not return quickly to its original state and can take many decades to return, or even not return to its original state. The impacts on soil structure are also notable. Much of the nutrients are either recycled naturally in the ecosystem, or removed and not redeposited in the place of origin, and the extensive areas without plant cover created by overgrazing promote soil erosion. After many years of inadequate, intense, chronic, and uncontrolled management, many pasture lands are now exposed to three main processes that have negative effects on ecosystems: desertification, woody encroachment, and deforestation.

The bioclimatic and edaphic conditions under which managed grazing occur have, to some extent, contributed to the development of these three processes. Desertification has occurred in arid regions of the southwestern United States, Australia, South Africa, and Argentina as a result of chronic grazing and pronounced climatic variability with special influence of drought periods. Woody encroachment has occurred in semi-arid to mesic environments as a result of large-scale grazing, fire suppression, and climatic variability. Deforestation continues to expand principally in the humid tropics in South America, Africa, and Asia in part because of grazing development on infertile soils that often cannot maintain large-scale livestock production.

As the human population grows and land scarcity increases, intensification and improvements in traditional animal production should be implemented to satisfy human demand for livestock products. Unfortunately, most pasture productivity is declining in many arid regions, and many grazing systems are disappearing in tropical forests. These are being replaced by more concentrated grazing systems that can lead to greater degradation of pasture land. In the northeastern United States, northwestern Europe, and densely populated areas of Asia, animal production has become mechanized and dependent on external fertilizer and feed inputs. Industrial meat production is growing rapidly, and in general in these systems animal numbers exceed the carrying capacity of the land, and waste is saturating the surrounding environment causing habitat fragmentation and eutrophication of freshwater and marine ecosystems.

Intensification of animal production and grazing systems is likely to continue, and it will require the expansion of pasture lands and improve livestock production through a supplemental feeding. In both cases, however, expansive management will cause greater degradation processes and will intensify environmental impacts over the land surface. Even more, the emergence of large economies such as China, India, Brazil, Russia, Mexico, South Korea, Indonesia, and Turkey as new centers of demand and production will potentially promote the expansion of pasture lands globally. Humid tropical ecosystems represent the only viable way to expand global grazing systems beyond their current geographic extent, but this expansion into marginal areas has already more or less reached the limits imposed by climate and soil factors, not to mention the degradation of some of the most biodiversity-rich areas in the world.

The success of pasture expansion into forest will depend mainly on local and regional policies in concerned areas. But since the prospect of expansion on pasture lands is limited, the intensification of pasture production on the most suitable land, and the loss of marginal pastures, is likely to continue. Climate change is also likely to alter pasture land systems. The impact on pastures will be greater than in other anthropogenic biomes. While the cropland biome can be more easily manipulated by irrigation or use of fertilizers, pasture lands depend largely on weather conditions. On dry pasture lands the impact is projected to be dramatic, reducing forage yields by as much as 25 percent by 2030, but on the other hand, pastures located in cold areas are expected to benefit from rising temperatures.

The management and long-term conservation of pasture lands requires a broad understanding of the patterns and processes that have determined their structure and composition, as well as the impact that humans have had on these lands for centuries. Today, we live at a time in which it is necessary to reconcile conservation strategies with livestock management strategies, and this will only be possible by integrating the knowledge of many disciplines. For this integration to occur it is essential to promote social and governmental participation in the design, dissemination, and implementation of best-management practices involving the conservation of ecological processes in the ecosystems in order to maintain their goods and services. It is necessary that management policies be based on sound scientific data to ensure better management of ecosystems, and that these policies integrate environmental and social needs focused on restoring and preserving the health of ecosystems in the long term.