prairie survives, ranging from less than 1% mostly in "scattered remnants found in pioneer cemeteries, restoration projects, along highways and railroad rights-of-way, and on steep bluffs high above rivers" to 4%. Ecosystem Tallgrass prairie is capable of supporting significant biodiversity. Parts of the ecoregion are among the "top ten ecoregions for reptiles, birds, butterflies, and tree species. Tallgrass species are found in the understory layer." Oak (blackjack oak (Quercus marilandica) and post oak (Q. stellata) ) and hickory tree species occur in some areas, but generally in moderate densities. Bison (Bison bison) were a dominant species. The tallgrass prairie biome depends on prairie fires, a form of wildfire, for its survival and renewal. Tree seedlings and intrusive alien species without fire tolerance are eliminated by periodic fires. Such fires may either be set by humans (for example, Native Americans used fires to drive bison and improve hunting, travel, and visibility) or started naturally by lightning. White-tailed deer fecal matter has nutrients for plant biodiversity in Tallgrass prairie area. Boundaries As its name suggests, the most obvious features of the tallgrass prairie are tall grasses, such as indiangrass (Sorghastrum nutans), big bluestem (Andropogon gerardi), little bluestem (Schizachyrium scoparium), and switchgrass (Panicum virgatum), which average between tall, with occasional stalks as high as . Prairies also include a large percentage of forbs, such as lead plant (Amorpha spp.), prairie rosinweed (Silphium spp.), gayfeathers (Liatris spp.), sunflowers (Helianthus spp.), asters (Aster and Symphyotrichum spp.), coneflowers (Echinacea spp., and Rudbeckia spp.), and many other species. Technically, prairies have less than 5–11% tree cover. A grass-dominated plant community with 10–49% tree cover is a savanna. After the steel plow was invented by John Deere in 1833, this fertile soil became one of America's most important resources. Over 95% of the original tallgrass prairie is now farmland. Remnants The tallgrass prairie survives in areas unsuited to plowing: the rocky hill country of the Flint Hills, which runs north to south through east-central Kansas; the eastern fringe of the Red River Valley (Tallgrass Aspen Parkland) in Manitoba and Minnesota; the Coteau des Prairies, which extends from South Dakota through Minnesota and into Iowa; and the far north portion of Oklahoma. In Oklahoma, the tallgrass prairie has been maintained by ranchers, who saw the hat-high grass as prime grazing area for cattle. The Tallgrass Prairie Preserve in Osage County, Oklahoma, and the somewhat smaller Tallgrass Prairie National Preserve