five life zones represented are the Lower Sonoran, Upper Sonoran, Transition, Canadian, and Hudsonian. This is equivalent to traveling from Mexico to Canada. Differences in elevation and the resulting variations in climate are the major factors that form the various life zones and communities in and around the canyon. Grand Canyon National Park contains 129 vegetation communities, and the composition and distribution of plant species are influenced by climate, geomorphology and geology. Lower Sonoran The Lower Sonoran life zone spans from the Colorado River up to . Along the Colorado River and its perennial tributaries, a riparian community exists. Coyote willow, arrowweed, seep-willow, western honey mesquite, catclaw acacia, and exotic tamarisk (saltcedar) are the predominant species. Hanging gardens, seeps and springs often contain rare plants such as the white-flowering western redbud, stream orchid, and Flaveria mcdougallii. Endangered fish in the river include the humpback chub and the razorback sucker. The three most common amphibians in these riparian communities are the canyon tree frog, red-spotted toad, and Woodhouse's Rocky Mountain toad. Leopard frogs are very rare in the Colorado River corridor; they have undergone major declines and have not been seen in the Canyon in several years. There are 33 crustacean species found in the Colorado River and its tributaries within Grand Canyon National Park. Of these 33, 16 are considered true zooplankton organisms. Only 48 bird species regularly nest along the river, while others use the river as a migration corridor or as an overwintering habitat. The bald eagle is one species that uses the river corridor as a winter habitat. River otters may have disappeared from the park in the late 20th century, and muskrats are extremely rare. Beavers cut willows, cottonwoods, and shrubs for food, and can significantly affect the riparian vegetation. Other rodents, such as antelope squirrels and pocket mice, are mostly omnivorous, using many different vegetation types. Grand Canyon bats typically roost in desert uplands but forage on the abundance of insects along the river and its tributaries. In addition to bats, coyotes, ringtails, and spotted skunks are the most numerous riparian predators and prey on invertebrates, rodents, and reptiles. Raccoons, weasels, bobcats, gray foxes, and mountain lions are also present but are much rarer. Ungulate species such as mule deer and desert bighorn sheep frequent the river corridor. Since the removal of 500 feral burros in the early 1980s, bighorn sheep numbers have rebounded. Mule