Grasslands

Grasslands are a major floralistic association of the Earth. They are characterized by grasses (Poaceae) and forbs (non-woody plants) of various heights and densities. On some grasslands, trees are interspersed, especially along watercourses. The climatic cause for widespread grasslands is a greater potential evapotranspiration than precipitation for the year. In such cases, the landscape cannot be dominated by trees because of the moisture stress. Grass species become shorter and sparser as potential evapotranspiration becomes progressively larger than precipitation. Ultimately, the grasslands grade into desert where the potential evapotranspiration is more than twice precipitation. There are also non-climatic reasons by which grasses can dominate the landscape, such as places in which the soils contain high concentrations of minerals such as nickel that are toxic to many tree species.

Geographers usually differentiate grasslands into two biomes dominated by grasses. The first is the tropical savanna and the second is the midlatitude grasslands. Their characteristics were given in the biomes article earlier in this Handbook. There are major grasslands on all the continents save Antarctica. The regional names are familiar to geographers: Examples include the Great Plains (North America), pampas (South America), steppe (Russia), veldt (Africa), and rangelands (Australia). Each continent has its own “flavor” of grassland floral and faunal species, yet the grassland formations are distinct from those in forests and other floralistic associations.

Within the last few thousand years, probably 40 percent of continental surfaces were grasslands but the percentage is declining as a result of the warming of the planet as it has emerged from the Pleistocene (ice age) and the unintentional destruction of the ecosystem by overgrazing (see Desertification). A billion people live in the grasslands of the world, so these areas are quite susceptible to human modifications. Overall, grasslands have declined in their ability to sustain animals and people. For instance, the tallgrass prairie of the eastern Great Plains has all but disappeared. Agriculture is prolific in the more precipitation-rich grasslands with all the major grain crops having originated in grasslands and being variants of grasses. These crops include millet, sorghum, corn, rice, and rye. When considering the human influence, it is apparent that up to a third of the species of grasses in regions such as the Great Plains are not native to the region.

In general, there is lesser diversity of plant and animal species in grasslands compared to adjacent forests. However, this cannot be taken to mean a paucity of life. All sorts of bacteria, fungi, insects, and earthworms are at home in grassland soils. Moreover, plentiful large herbivores are one signature of native grassland conditions. In North America, the bison once populated the Great Plains in untold millions. In today’s African savanna, elephants, zebra, and wildebeests are still largely present but in declining numbers. The herbivores may, in fact, help keep grasses dominant in some places as herbivores are attracted to competing plant types such as young trees and bushes.