Tallgrass prairie is one of the most diverse and productive types of grassland in North America. Historically, tallgrass prairie stretched from Canada to Texas and from Kansas to Ohio. West of the tallgrass prairie lie drier, less productive grasslands, and to the east are deciduous forests. Throughout the presettlement range of this grassland, tallgrass prairie was characterised by three interacting forces: a variable climate that on average provided sufficient precipitation to support forest but was prone to drought; frequent fire that promoted grass growth but suppressed woody species; and grazing by large ungulate herbivores (bison). Today, tallgrass prairie is termed America's most endangered ecosystem because more than 95% of the original prairie has been lost to row crop agriculture, and what remains has been highly fragmented.