North American deserts are caused by the rain shadow effect. They are situated on the east side of mountains that cause atmospheric moisture to condense and precipitate on the west side of the mountains.
The Sierra Nevada Mountains. create the rain shadow that stops water vapor from moving into the No. American deserts.
No. American Deserts’ climate is more variable than one might expect. Most think of the desert as hot and dry. They are dry but the heat is seasonal. Average annual precipitation is 10 inches or less. Temperatures range from 20 o F to 110 o F. The limited rainfall limits the diversity of plants and as a consequence the animals. Soils have very limited organic matter in them. The consist mostly only of parent materials (sand and gravel). Without organic material, the soil have little capacity to hold what little precipitation there is.
Huh? That’s right! It gets plenty cold in North American Deserts. Arizona gets its share of snowfall, especially at higher elevations. These are Saguaro Cacti. They are the icon of the Sonoran Desert.
Saguaro Cacti under the desert sun in the Vekol Valley north of Tucson. These are about 50 or 60 feet tall and probably 300 to 400 years old. I remember checking the thermometer on my GPS on this day. It was 114 o F and my skin was bone dry. As fast as I’d sweat, the water evaporated off my skin.
Notice how evenly, and widely dispersed these plant in this picture are. The taller plants are Joshua Trees and the small shrubs are Tumbleweeds. Desert plants’ roots are shallow and they spread over a great distance. This allows them to absorb water as soon as it falls and before it evaporates or percolates down through the soil.
These are creosote bushes. Like the other plants, they too are evenly and widely dispersed. Creosote bushes are loaded with creosote, a very distasteful chemical that deters insects. Because desert plant growth is slow, chemical deterrents are a good way to prevent the loss of growth.
Desert cacti are plants have fleshy stems that store water. Their shallow, widespread roots absorb rainfall quickly and their stems may triple in diameter. Leaves are reduced to spines. This adaptation serves two purposes. First the lamina, the flat part of the leaf is gone. This all but eliminates transpiration. Secondly, the spines are sharp. Browsers are unable to eat the plants. A couple of feet of stem could represent tens of years of growth. Without sharp spines a browser could eat it in a few bites. Such an investment must be protected.
The desert sun can be so intense that cacti tissues can cook the plant. Many have white fibers the cloak the stem and reflect sunlight so the plant doesn’t overheat. Enough light gets through to drive photosynthesis. This species is called Old Man’s Beard Cactus. The upper surface of the Golden Barrel Cactus is covered by a thick growth of yellow fibers. They reflect sunlight when it is directly overhead and at its most intense.
This is Palo Verde (Green Stick). It has no leaves. This prevents loss of water to transpiration. It’s a woody plant whose epidermis is rich in chlorophyll. Its bark is paper thin to allow light to reach the epidermis for photosynthesis.
Thick leaves store water for long periods of time. They’re a common adaptation of leafy plants in the desert. This is a Jade Plant. Plants with fleshy, water storing leaves and stems are called succulents . Plants that are adapted to the dry conditions of the desert are called xerophytes . Xerophytes can be succulent or like the Palo Verde have no leaves, or they can have the adaptions of cacti.
Dry, scale-covered skin locks in moisture making many reptile species well adapted to desert life. Lizards and snakes are actually abundant and a couple of turtles are desert dwellers as well. I found this Three-toed Box Turtle in the Sonoran Desert near Tucson, Arizona. Same turtle is pictured in his habitat in the next slide.
The Desert Tortoise is a North American species that feeds largely on cactus fruits and stems. It manages to avoid the spines.
To survive the heat, Desert Tortoises return to their burrow each morning before the sun becomes extremely hot. Animals that are adapted to desert heat by being active at dawn and dusk are referred to as crepuscular .
The Gila Monster is one of only two venomous lizard species in the world. It is a No. American desert native that can survive up to 40% loss of body water . It will appear skinny and shriveled and near death but when rare rainfall occurs, it drinks its fill and rehydrates. One or two meals per year can sustain it.
Horned Lizards are well camouflaged. Canyon Spiny Lizard (left) and a Collared Lizard (right)
Coachwhip Snake. Long, skinny and aggressive, but not venomous!
Thorn-scrub Hooknose Snake. My friend and I caught two of these in one night. They were the 12 th and 13 th documented specimens of this species caught in the U.S. We were right on the Mexican border. We were stopped by the Border Patrol four times that night (by the 4 th stop they knew us on a first name basis). Not venomous and only about 10 or 12 inches long.
There are many species of rattlesnakes in the desert. This is an adult Blacktail Rattlesnake. Its diet is comprised almost exclusively of Kangaroo Rats.
The famous Sidewinder . This rattlesnake uses a unique means of locomotion called sidewinding. It touches two points of its body to the ground and sends a wave along its length. The position of the two points touching the ground move along its body propelling it forward. This allows it to crawl on hot desert sand. By having two ever changing, small parts of its body on the ground at any one time it reduces the risk of burning its skin on the hot sand.
Sidewinders are characterized by two tiny “horns”; one above each eye.
Sidewinders leave characteristic J-shaped tracks in the sand as they move.
My trip to Arizona three years ago was to see rare desert amphibians. Most amphibians require moist habitats to keep their skin moist and permeable to oxygen. Often the Vekol Valley gets all of its rain in one event, usually in early August. We planned our trip to be there during that time. We got lucky. There was an unbelievable storm that dumped about 8 inches of rain in an hour. We drove out into the desert at night and found a low lying area that held water. It was easy to find in the dark. We just had to follow the sound of chorusing frogs. Thousands of frogs, about 6 species, seemed to come from nowhere. It was 114oF that day and the ground was as hard as cement. But when the flooding rains came, there were frogs everywhere!!! Desert frogs burrow into the sand to reduce evaporative water loss and they only surface occasionally to feed or when it rains, to breed.
Sonoran Green Toad – the inflated bubble is the male’s vocal sac. It amplifies the toad’s voice so females can hear him.
The Sonoran Toad is the largest of the frogs in the desert. This one was about 8 inches long. The glands behind its eyes are called parotoid glands. They secrete a toxin that causes rapid heart beat. Because of its size, this toad produces copious quantities of the toxin (called bufotenin). Drug users were selling these toads to people who licked the parotoid glands to get high. Their heart raced, delivered a blast of oxygen to their brain causing euphoria. Then many of them died. It is treated like a narcotic and it’s illegal to own them in Arizona and California.
These are mating Desert Treefrogs. There were no trees. This species spends its life underground, surfacing like the other frogs, only when it rains.
The electrical storms were violent the night we were out looking for frogs.
I’m a bird watcher. I’ll travel to other continents to see birds. Their species diversity in the desert was surprising. Many species will fly 40 to 50 miles each day to load up on water before returning to the part of the desert. Montezuma Quail
Phenopepla - looks like a black Cardinal with a skinny beak. This was on my wish list to see!
There are several species of woodpeckers in the desert, but there are no trees. They adapt by creating holes in Saguaro Cacti. Gila Woodpecker
Abandoned woodpecker holes are taken over by owls. Without the woodpeckers, there would be no owls in the desert. Western Screech Owl
This is a Road Runner. They are fast!!! I watched them chase down lots of lizards. Roadrunners specialize on lizards, snakes and insects. This was number 1 on my wish list to see!
Black-tailed Jack Rabbits (not rabbits but Hares – see Grasslands Power Point) range into the desert. Their giant ears are cooled by the wind and as their blood circulates through them, it is cooled. When their blood flows into their body, the hare is cooled.
There were ground squirrels in the desert, just as they are in the Grasslands. This is a Black-tailed Prairie Dog.
Another ground squirrel. I don’t recall what species.
This is a highly adaptable species called the Collared Peccary. I see this species in Central American Rainforests. In the desert they’re called Javalinas, but they’re the same species. They’re aggressive pack animals.
Kangaroo Rats hop on their back legs, like kangaroos. They are extremely common and they’re the most important prey of large snakes in the desert. They are adapted to the desert’s lack of water by being able to make their own water. They eat a diet rich in lipids. Most mammals can’t digest lipids. But Kangaroo Rats have enzymes that do so. A byproduct of the breakdown of lipids is water. As long as these little guys can find fatty, oily, or waxy foods, they’ll be able to manufacture all the water they need.