Grasslands are environments in which herbaceous species, especially grasses, make up the dominant vegetation. Natural grasslands, commonly called prairie, pampas, shrub steppe, palouse, and many other regional names, occur in regions where rainfall is sufficient for grasses and forbs but too sparse or too seasonal to support tree growth. Such conditions occur at both temperate and tropical latitudes around the world. In addition, thousands of years of human activity—clearing pastures and fields, burning, or harvesting trees for materials or fuel—have extended and maintained large expanses of the world's grasslands beyond the natural limits dictated by climate. Precipitation in temperate grasslands (those lying between about 25 and 65 degrees latitude) usually ranges from approximately 10–30 in (25–75 cm) per year. At tropical and subtropical latitudes, annual grassland precipitation is generally between 24 and 59 in (60 and 150 cm). Besides its relatively low volume, precipitation on natural grasslands is usually seasonal and often unreliable. Grasslands...