The Niobrara River valley has unusually diverse plant groups and ecosystems. The area is noted in scientific literature for the many plants that exist here or beyond their normal geographic limits. Plants of eastern, western, and northern forest ecosystems and three Great Plains prairie ecosystems converge here. Approximately 160 plant species are at the edge of their natural range in the river valley. Several factors cause this unusual biological diversity. The river valley provides an unbroken east-west riparain corridor connecting the dryer western landscape with the more humid midwestern prairie and eastern forest. Plants typical of each condition intermingle in the transition zone. The river valley also provides a variety of habitats due to differing slope, moisture, and soil conditions. Also, as climate conditions changed over geologic time, plants typical of past colder conditions survived due to the cool, wet, north-facing branch canyons.