What makes a desert, of course, is scanty precipitation. And because of the great range in altitude—from 550 meters (1,800 feet) along the river to 2,400 meters (7,800 feet) atop the Chisos—there is a wide variation in available moisture and in temperature throughout the park. This has produced an exceptional diversity in plant and animal habitats. Receiving less than 25 centimeters (10 inches) of rainfall in a year, almost half the park is shrub desert. This plant community begins right next to the river and runs on up to about 1,050 meters (3,500 feet). Another 49 percent of the park is desert grassland, a somewhat less dry environment that you will encounter on mesa tops and foothills to about 1,700 meters (5,500 feet). From there on up, mountain canyons and slopes may sustain typical southwestern woodlands with pinyons, junipers, and oak trees. The Chisos heights receive some 46 centimeters (18 inches) of rain per year and are considerably cooler than the desert. Consequently you will even find 325 hectares (800 acres) of forest in two or three high canyons, where towering Rocky Mountain-type trees persist from cooler, moister times. And that is not all: A lush green jungle grows in a narrow belt along each bank of the Rio Grande and pushes out across the desert along creeks and arroyos. And in the river itself live creatures you wouldn’t expect to find in the middle of the desert! Big Bend National Park is home to more than 70 species of mammals, almost as many species of reptiles and amphibians, a score or more fishes, and a fascinating host of insects and other arthropods. The wide choice of habitats makes Big Bend a birder’s paradise that offers more different resident and migrant birds than any other U.S. national park. Thanks to its location, the park marks the southernmost reach of some U.S. plant and animal species, and the northernmost reach of some Mexican species. Some plants and animals found here occur nowhere else in the world.