Due to its over 5,000-foot elevation gradient and the highly dissected nature of its valleys and mesas topography, the HLR is home to a diverse set of ecosystem types and the many animals that rely on their habitats for survival. The plant communities that form the foundations of these ecosystems have evolved over many millennia based on factors such as temperature, moisture, aspect, elevation, competition among the plants and interactions with the animals inhabiting the landscape. The diversity of habitat types includes: high desert, valley grasslands and agricultural fields, steam and pond ecosystems, riparian ecosystems, sagebrush shrublands, pinon-juniper woodlands, Gambel oak woodlands, aspen groves, mixed-conifer and Douglas-fir forests, subalpine spruce-fir forests, and, subalpine meadows.