Desert Habitat, pt.1

A biome is a type of global habitat characterized by temperature, rainfall, latitude, elevation, and wind pattern.  Plants, which are unable to move about, are used to characterize each type of biome, and are well-adapted to their native habitat.  The desert biome is the driest habitat found on earth.  In a desert, plants are widely spaced and are adapted to life with little or no moisture except for annual periods when brief rains may be heavy, sparse, or not occur at all.  Even within these harsh conditions, plants may be present supporting a rich diversity of species.

_{Clockwise above: Hidden Valley, Joshua Tree solitude by Don Graham, Apr 2013; Sonoran Desert sunset by Jasper Nance, May 2009; Sonoran Desert layers by Art DiTommaso, Apr 2020; Mohave Desert by Melissa Delzio, May 2011}

A common factor of all deserts is the aridity or lack of moisture.  Aridity can be determined by how much water is available in the habitat combined with the rates of evaporation from heat, radiation, and wind exposure.  Another consideration in determining aridity is how much moisture the air can hold.  Neither extremely hot nor extremely cold air easily retains moisture.  Freely running water is only available in the desert for the briefest period of time immediately following a rainfall, a scarce and unpredictable event. 

Deserts form in areas where dry air predominates.  Understanding how weather influences the Earth’s surface helps to identify where to find the world’s desert regions.  At the equator, air is warmed by direct sunlight causing it to rise and absorb large amounts of water from the oceans.  When the warm air mass encounters cooler air at higher altitudes, it spreads out both northward and southward.  Cooler air causes the absorbed moisture to condense into rain in tropical latitudes.  The outward moving air mass, now dryer and cooler, also begins to fall back to the earth’s surface, picking up heat from the sun’s radiation and the hot ground.  Air masses generally descend and move back towards the equator around 30˚ latitude, which is where many deserts occur. 

Deserts can also be formed from rain shadows and temperature extremes.  As prevailing winds encounter mountains, air is forced up the windward side and flows down the leeward side.  As air rises, it cools, once again condensing any moisture into rain which falls on the windward side.  Dry air coming over the mountain forms a rain shadow that leaves little moisture for the habitats on the leeward slopes.  Temperature extremes occur in both hot and cold regions.  Dry, hot air descending from the tropics form warm deserts.  Cold, dry air from polar winds form cold deserts such as those found in  Antarctica and Greenland.

Desert temperatures vary widely, both daily and seasonally.  Dry air and cloudless skies allow a maximum amount of solar energy to reach the surface where it is absorbed and converted to heat, raising daytime temperatures dramatically.  At night, air temperatures plummet as solar radiation ceases and heat is quickly wicked away into the cooler atmosphere.  Temperatures may fluctuate as much as 100˚ in a 24-hour period.  Ultraviolet radiation also presents other challenges to living organisms.  Intense sunlight can damage skin cells, degrade retinal tissue, and destroy chlorophyll molecules in plants. 

Deserts experience seasons, just as those found in other biomes.  In desert areas of Arizona, there are five recognized annual seasons.  Spring, February through April, is characterized by mild temperatures and windy days.  It is a dry season as warm winds wick away all exposed moisture.  Summer, in May and June, is hot and dry, with many species remaining dormant to escape the heat.  Summer monsoons, July through mid-September, signals a change in wind direction bringing moister air northward from the tropics.  Frequent thunderstorms occur most years providing a period of replenishment for shrubs and trees.  Autumn, in October and November, returns to warm and dry conditions.  Winter comes in December and January bringing in mild temperatures and the possibility of a few rain showers.

The climate of the world’s deserts makes survival difficult, but many species have adapted to these conditions in some remarkable ways.  Next week, we will investigate the desert regions of North America and their vast diversity of life.

_{Clockwise above: Blooming hedgehogs by Jasper Nance, Apr 2010; Desert globemallow by Take-A-Hike Arizona, Mar 2009; Desert woollystar, Eriastrum eremicum, Sonoran Desert by SSBiker1, May 2011}