Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
Ecosystems Earth on Edge
Earth on Edge
Ecosystems  
Get Involved
Science MattersThe ProgramBuy the Book and Video  

Grasslands Ecosystems Profile

Intro | Profile | Ecosystem Assessment

Depleted grasslands
Depleted grasslands in Mongolia.
In Mongolia, livestock products, including live animals, milk, meat, wool and hides, account for one-third of that country's gross domestic product. In Africa much of the livestock is used to provide daily nourishment for many rural populations who depend on their animals for sustenance. Those of us whose livelihoods depend on grasslands are vitally aware of their value, but many of us who only use the products these ecosystems provide are not. We know that grasslands contain scenic landscapes and unique mixes of species, but they are also home to nearly 800 million people. Grasslands encompass the savannas of Africa, the grasslands of Australia, the cerrado and campo of South America, the prairies of North America, and the steppes of Central Asia.

Highly dynamic ecosystems, grasslands support flora, fauna, and human populations worldwide. Grasslands provide food, energy, biodiversity conservation, carbon storage, tourism, and habitats for endemic bird and wild herbivore species. They also recycle water and build and maintain soil stabilization mechanisms. However, in taking advantage of some of these goods and services, human activities have had a devastating effect on grasslands.

How much grassland is left?

Estimates of grasslands' global extent range from 41 to 56 million square kilometers (km2), covering from 31% to 43% of Earth's land surface. Why the broad estimates? Some researchers include more (or less) tundra or shrubland in their assessments. Broadly defined, grasslands are "areas dominated by grassy vegetation and maintained by fire, grazing, and drought or freezing temperatures." If it weren't for these maintenance factors, trees and shrubs in many grasslands would increase and the grassland could eventually become forest.

Map
The Global Extent of Grasslands
(click on image to enlarge)
In central North America, tall-grass prairie has decreased by nearly 97%, converted to urban and agricultural areas. Conversion, particularly to croplands and other agroecosystems, has been the primary reason for the diminishing extent of grasslands globally. In selected grassland ecoregions, conversion to croplands has been as high as 76% in South America and from 20% to 40% in other regions.

What are some of the most important goods and services agroecosystems provide?

Goods

Services

Food (meat) — Domestic and wild animals sustained by grassland forage provide vital sources of meat, milk and other products.

Biodiversity — Grasslands comprise 35 of 136 terrestrial ecoregions identified as outstanding examples of diverse ecosystems.

Food (cereals) —Wheat, rice, rye, and other major crop grains originated in grasslands, a prime source of crop genetic resources.

Carbon storage — Grasslands store about 34% or the terrestrial global stock of carbon, most of it in the soil rather than in vegetation.

Energy — Fuelwood and windfarms are important sources of energy originating in grasslands.

Tourism — Developing countries with vast grasslands reap substantial economic gains from grassland recreation and tourism.

What are the primary pressures threatening grassland ecosystems?

The primary pressure on grassland ecosystems is outright conversion to other uses. Conversion to agroecosystems and urban areas especially brings pressures from the infrastructure needed to support these new systems. Road building causes fragmentation, which can degrade habitat and adversely impact biodiversity. Furthermore, fragmentation caused by the spread of nonnative species can not only disrupt patterns of growth of native species but also degrade freshwater systems within grasslands, as we saw in South Africa.

Grassland Blocks in the Great Plains (click on image to enlarge)

The most severe modification of grassland ecosystems, however, may be desertification. Scant rains in arid grassland ecosystems make these ecosystems particularly susceptible to damage from human management and slower to recover from degradation. Desertification results when land is severely degraded in arid, semiarid, and dry sub-humid areas. It's caused by factors that include both climate variations and human activities.

Another pressure factor, which originates from both natural and human causes, is fire. A natural cause of fires is lightening strikes. Fire is useful in grassland areas because it prevents bushes from encroaching, removes dry vegetation, and recycles nutrients. Farmers use it to clear land for conversion to agriculture and to control pests. But too many fires and extensive burning can degrade grasslands. About 500 million hectares of tropical and subtropical savannas, woodlands, and open forest are burned each year.

Overgrazing is also an important degrader of grassland condition, especially when livestock numbers are high, animals are confined to small grazing plots without rotation, vegetation is sparse, and soils are easily eroded. And the amount of forage, or livestock food, a grassland can produce depends in part on soil conditions. Indicators show that more than half of all remaining grasslands have some degree of soil degradation. In many regions overgrazing and drought pose a serious threat to the future of grasslands.

«previous | next »

Source: This profile is adapted from the companion book, World Resources 2000-2001.

For comprehensive data about the world's ecosystems, visit EarthTrends at http://www.earthtrends.wri.org/.

Agricultural | Forests | Coastal | Grasslands | Freshwater | Urban
The Value of Ecosystems


 
 
Discussion Guide | Buy the Book and Video | Moyers Mailing List | Site Map
Bulletin Boards | Classroom Materials | Resources | Glossary | Site Credits