Grassland extent and change

Definitions for grasslands vary. Some studies classify grasslands by vegetation while others characterize them by climate, soils, and human use of the ecosystem.

In this study, we define grasslands as terrestrial ecosystems dominated by herbaceous and shrub vegetation and maintained by fire, grazing, drought and/or freezing temperatures. According to this definition, grasslands encompass not only non-woody grasslands but also savannas, woodlands, shrublands, and tundra. This broad definition has allowed PAGE analysts to highlight many of the important goods and services provided by this ecosystem: livestock production as well as grassland biodiversity, carbon storage, and tourism and recreation.

The following summarizes key findings of the PAGE study regarding the condition of grassland ecosystems, as well as the quality and availability of data.

Conditions and trends

• Grasslands cover some 40 percent of the earth’s surface (excluding Greenland and Antarctica).

• Grasslands are found in every region of the world; Sub-Saharan Africa and Asia have the largest total area in grassland, 14.5 and 8.9 million square kilometers respectively.

• Of 136 terrestrial ecoregions identified as outstanding examples of the world’s diverse ecosystems, 35 are grasslands, supporting some of the most important grassland biodiversity in the world today.

• The five countries with the largest grassland area are Australia, the Russian Federation, China, the United States, and Canada.

• The five countries with the highest percentage of grassland area, all in Sub-Saharan Africa, are Benin, Central African Republic, Botswana, Togo, and Somalia.

• Regional data for African herbivores show generally steady long-term population trends within the Serengeti ecosystem. Areas outside the protected area boundaries and with fewer law enforcement activities experienced decreases in densities of already-low wildlife populations. .Of nearly 600 key areas for threatened bird species in the Neotropics, 42 are grasslands; 12 percent of the threatened birds are specific to grasslands.

• Twenty-five of the 145 major watersheds of the world are made up of at least 50 percent grassland. Sub-Saharan Africa has the most extensive grassland watersheds; Europe, the least.

• Grasslands are found most commonly in semi-arid zones (28 percent of the world’s grasslands), followed by humid (23 percent), cold (20 percent), and arid zones (19 percent).

• Human populations are highest in the dry grasslands (arid, semi-arid, and dry sub-humid) of Sub-Saharan Africa followed by Asia. Human populations are lowest in the dry grasslands of Oceania.

• Temperate grasslands, savannas, and shrublands have experienced heavy conversion to agriculture, more so than other grassland types including tropical and subtropical grasslands, savannas, and woodlands.

Information status and needs

• Global estimates of grasslands are complicated by diverse definitions of grassland, and variability in the designation of boundaries between land cover types.

• Higher-resolution satellite data, available now and expected to become more accessible within the next few years, could improve the information base. These data, however, will most likely remain expensive to obtain, especially for extensive areas.

• Expansion of our knowledge of grassland condition is hindered by disagreement on the characteristics of a healthy grassland ecosystem and the difficulty of identifying the best methods to determine ecosystem health.

• Various satellite sources primarily from the U.S. and Europe are being perfected to better detect, monitor and analyze fires over time. NASA’s website presents current (1999-2000) fire counts and additional fire information at 4km resolution in monthly intervals but these data are not yet available for general analysis. Studies using these data are required to analyze the long-term effects of frequent fires on grassland systems.

Quality and availability of data

PAGE measures and indicators	Data sources and comments
Extent of current grasslands	Land cover characterization developed by International Geosphere/Biosphere Program (IGBP) using global satellite data at l-km resolution (GLCCD 1998), modified by WRI using Olson (1994a and b); WRI global, electronic dataset of watersheds of the world (Revenga et al. 1998).
Extent of dry grasslands	Aridity zones of the world mapped by United Nations Environment Programme according to the ratio of mean annual precipitation to mean annual potential evapotranspiration (UNEP 1992, 1997).
Extent of woody vegetation	Land cover characterization developed by University of Maryland Geography Department identifying percent woody and herbaceous cover across the world’s terrestrial surface (DeFries et al. 2000).
Extent of historical grassland	Major habitat types of the world representing geographic areas of similar environmental conditions before major modification by humans (WWF-U.S. 1999).
Trends in grassland conversion	Regional data reported by United States Geological Survey (USGS) and the Nature Conservancy (TNC) for North America; IUCN - The World Conservation Union for Europe; State of the Environment Advisory Council for Australia; United States Agency for International Development (USAID) for Kenya.
Modification of grasslands
Agriculture	GLCCD (1998) land cover characterization as modified by PAGE; methodology may over-represent grassland modification in some parts of the world, such as southern Africa.
Urbanization/human settlements	Population data from inventory of national censuses (CIESIN 2000); see also see below for road fragmentation using Digital Chart of the World road’s database (ESRI 1993).
Desertification	Use of aridity zones and human population data to describe effects of land degradation in dry areas as presented in the World Atlas of Desertification (UNEP 1992, 1997)
Fire	Satellite data from European Space Agency (ESA) for fires in Africa, Latin America, SE Asia, and Oceania detected during 1993 (Arino and Melinotte 1997).
Domestic livestock	Various studies in scientific literature; datasets from FAO and ILRI described in chapter on food, forage and livestock.
Fragmentation	Fragmentation index developed by the World Wildlife Fund (Dinerstein et al. 1995; Ricketts et al 1997); spatial, electronic database of road networks worldwide from Digital Chart of the World (DCW) (ESRI 1993) presented in chapter on biodiversity.
Non-Native Species	Dataset for North America compiled by WWF-US (Ricketts et al. 1997), described in chapter on biodiversity.