The desert regions of North America are largely mountain-barrier deserts, precipitation is prevented from reaching them by the Sierra Nevadas, the Coast Range and the Rockies. There are a large number of cloudless days, and the solar radiation is intense. Hot dry winds blow down out of the mountains causing dust storms which have a definite impact on the life forms there. We find that vegetation, plant size, and plant diversity are all positively correlated to precipitation. Where there is little rainfall all of those characteristics decline. The desert is also a relatively stable habitat which also results in little species variation, the desert plants that have succeeded are where they are due to adaptations to their specific environment. The correlation of species’ diversity with rainfall also exists for animal groups such as ants, grasshoppers, and mammals. Surprisingly one group does not follow this parameter, reptile diversity is at its peak in North American deserts.
North American deserts can be subdivided into three large categories based on seasonal rainfall, and types of successful vegetation. Due to localized variations they can all be further subdivided into semi-arid or true deserts within each class. This more specific breakdown of areas is surely too much for seventh graders but the three larger groups should be of interest to them as they can be seen in so many of the movies that they watch on television. The charts below give some of the details of the three desert areas.
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vegetation types; desert
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altitude
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rainy season
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total rainfall
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and examples
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Sonoran
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0-1200 m
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summer
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100-300 mm
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thorny succulents; saguaro cactus
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Mojave
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800-2000 m
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winter-summer
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100-300 mm
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pygmy woodland; Artemesia, yuccas, Joshua trees
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Chihuahua
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700-1600 m
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summer
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200-400 mm
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open shrub and thicket; cactus, yucca, agave
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Temperatures can vary dramatically, with an extremely low—14.4°C at Kingman, Arizona to an extremely high 50.6°C at Gila Bend. The “hot” desert’s boundaries are roughly approximated by the distribution of the creosote bush (Larrea tridentata).
Desert annuals survive by remaining in the seed stage during the summer and sprouting during the rainy season. They then grow rapidly, flower, and produce seed. The seeds may remain dormant for years until the conditions are right again. Some plants are able to store water that is collected during the rainy season and use it slowly. These plants often maintain a dwarf form and space themselves out over an area. Still other types of plants, such as ocotillos, elephant trees, and jatrophas, are able to control leafing out until rainy weather, and then shed those leaves when it becomes dry again. Mesquite plants have extremely long tap roots, sometimes 30 feet long, which reach deep into the ground for water. And cacti, such as the saguaro and the barrel cacti, have a unique accordion-like shape that allows them to expand into a bloated water container during the rainy season and then draw from that source during the dry times. Some leaf adaptations include the ability to curl, or twist the narrow edge up reducing the surface area exposed to the sun. Hairy leaves or stems catch moisture from the air and shiny or waxy leaves trap and hold the moisture they have. And in some desert plants the leaves are absent and the stems take over leaf function.
Some physiological adaptations of desert plants include the ability to survive with very little water, the ability to control water-using life functions such as transpiration and photosynthesis, a tolerance for high concentrations of salts in the body tissues and special salt excreting organs.
The animals of the desert share many of the same adaptive behaviors and physiological traits. Some of the most common are nocturnal lifestyle, burrowing, obtaining water from prey, ability to derive sufficient fluid from seeds or other vegetative material or low metabolic needs. Some animals have special salt-excreting glands, or produce semi-solid urine. Birds function at a higher body temperature, and are able to fly at high (cooler) altitudes. Their feathers sometimes act as insulators from excessive sun or radiant heat. They can of course travel great distances to obtain water and some can “carry” water in their breast feathers and down. Like mammals of the desert, birds are most likely to be active early in the morning or at dusk when it is cooler. Mammals are likely to be nocturnal and live in burrows. Small mammals such as the rodents are able to get their water needs filled by their seed diet, but larger mammals must have liquid water. Some of the larger carnivores get substantial amounts of their water requirement from their prey. Most mammals have relatively low metabolic rates (compared to mammals in other biomes) and their food requirement is low, their urine output is low, and they can go a long time without water. Animals that burrow to avoid the sun in the summer also must be able to survive relatively cold winters. Many of them hibernate through the cold season. An adaptation of desert animals to the blowing winds would be their small ear openings with hair or scales, long eyelashes, and thickened eyelids. Some even have nostrils with closure valves. Many desert animals tend to be pale in color, but scientists disagree as to whether this is an adaptation for camouflage or for heat radiation.
An example of the types of animals to be found in a typical square kilometer of North American hot desert would include about ten small mammal species including varieties of ground squirrels, mice, hares, rabbits, and gophers. There would be four to nine bat species, and at least one representative of the following; fox, coyote, skunk, and badger. The birds would be represented by five to twenty-five breeding species; including quail, dove, roadrunner, owl, night hawk, vulture, woodpecker, raven, wren, and flycatcher species. The reptile group would be well represented by the spadefoot toad, tortoise, gecko, iguanids, lizards, rattlesnakes, racers, longnose, king, and bullsnakes. This would not be the end of the species list as there would also be some eight hundred to twelve hundred arthropod species. Invertebrates would include worms, scorpions, spiders, beetles, bees, termites and ants.
The desert is a very complex environment despite its lack of rain. Desert species have a high rate of predation and it appears that the number of organisms is limited more by the amount of resources and the very high degree of competition than by the degree of harshness of the physical environment.
Many examples of the types of plant adaptations to the desert biome can be found in plants easily available in local nurseries and stores. A good activity to do with the children is to set up a desert terrarium with a variety of cactus and succulent plants. Directions for a terrarium are given in Activity 1.