ABSTRACT
The dramatic vegetation gradients along the border are summarized in Biotic Communities of the American Southwest-United States and Mexico, the accompanying vegetation map, Webster, and various regional floras. In the Puerto Blanco Mountains of Organ Pipe Cactus National Monument (9 miles north of the Sonora border), saguaro and brittlebush returned to Arizona soon after the beginning of the Holocene about 11,000 years ago but were living in xeric woodlands. Although the late Holocene desertscrub communities likely resembled the original late Miocene Sonoran Desert, relatively modern desertscrub communities were developed for about 5%-10% of the 2.4 million years of the Pleistocene. As for many large mammals, the modern distribution may not accurately reflect their physiological range limits because of human predation in the last 11,000 years. The Sonoran Desert toad is a regional endemic while the slider and boa constrictor occur today in Sonora in wetter, more tropical areas to the southeast.
