Grasslands are defined as a composition of at least 25% cover of herb species, dominantly graminoids (grasses) though may include forbs and ferns, and that contain less than 25% cover of trees, and shrubs. Herb cover may be less than 25% where it exceeds tree, shrub, dwarf-shrub, and nonvascular cover, respectively. Grasslands are believed to cover 40.5% of the world's land surface excluding Greenland and Antarctica. This biome is globally used for grazing and has historically been home to many large herds of native ungulate species, such as bison of the North American prairies whose numbers were dramatically reduced in the 1800s. Fire, both natural and human caused, is a major factor in grassland persistence with frequent, low intensity fires reducing encroachment of woody plant species.