Deserts
In the United States, three “hot deserts” receive precipitation in the summer months (Mojave, Sonoran, Chihuahuan) and one “cold desert” receives precipitation in the winter (Great Basin).
These ecosystems receive less than 10 in (250 mm) of annual precipitation. Far from desolate, the deserts support high levels of biodiversity including iconic species such as Joshua trees, Mexican free-tailed bats, desert pupfishes, cutthroat trout, pronghorn antelope, desert tortoises, Gila monsters, sage grouse, bighorn sheep, desert iguanas, bristlecone pines, cuckoos, ocotillo, desert poppies, saguaro cactus, kangaroo rats and pigmy rabbits.
Desert biodiversity is the result of evolutionary divergence driven by landscape heterogeneity such as the remote valleys, mountain tops, dry lake beds, sand dunes, and caves across the West. By 12,000 BCE, when the Pleistocene ended, lakes that filled many valleys receded, leaving fishes and other aquatic animals isolated among highly divergent aquatic habitats including lakes with salinities that vary from fresh water to three times the salinity of ocean water. The isolation of populations in terrestrial refugia and desert oases provided the conditions for evolutionary divergence that led to a rich biodiversity.
However, deserts are also impacted by anthropomorphic stressors such as water withdrawal, mining, roads, pollution, over-grazing, energy development, invasive and feral species, and urban sprawl.
