Deserts

 

Deserts are land where evaporation
exceeds precipitation.

 

  • The arid regions of the world occupy 25 to 35 percent of the Earth's landmass
     
    • It is the lack of precipitation that is the distinctive characteristic of all deserts
       
  • Deserts lie between 15 and 30 latitude in response to the effect of the subtropical high pressure systems
     
  • Temperate deserts lie in the rain shadow of mountain barriers or are located far inland
     

 

 

 

  • Most of the arid environments are found in the Northern Hemisphere
     
    • The Sahara in North Africa is the world's largest desert (9 million km2)
    • Temperate deserts of Central Asia (e.g., Gobi Desert)
    • Temperate deserts of North America

 

 

  • Most of the deserts in the Southern Hemisphere lie within the subtropical high-pressure belt
     
    • The deserts of southern Africa include three regions:
      • Namib Desert,
      • Karoo
      • Kalahari Desert
    • 40 percent of Australia's land is classified as desert

       
  • The vegetation cover, dominant plants, and groups of associated species differ in response to differences in:
     
    • Moisture
    • Temperature
    • Soil drainage
    • Topography
    • Alkalinity
    • Salinitity

 

 

  • Cold deserts and high elevation hot deserts (e.g., Great Basin of North America, Gobi)
     
    • Can be considered shrub steppes or desert scrub
    • Dominated by genus Artemisia (sagebrush) and chenopod shrubs
       

Desert scrub in Wyoming is dominated by sagebrush (Artemisia)

 

 

 

 

Saltbrush scrubland in Victoria, Australia is dominated by Atriplex and is an ecological equivalent of the Great Basin shrublands of North America

 

 

 

 

 

  • Hot deserts range from those lacking vegetation to ones with some combination of chenopods, dwarf shrubs, and succulents
     
  • Southwestern North America (Mojave, Sonoran, Chihuahuan)
     
    • Bur sage and creosote bush
    • Acacia, saguaro, palo verde, ocotillo, and so forth
       

 

 

Hot desert: Chihuahuan desert in northern Mexico. The soil consists of sand-sized particles of gypsum

 

 

 

 

Dunes in the Saudi Arabian desert near Riyadh

 

 

 

 

  • Plants and animals adapt to the scarcity of water via
    • Drought evasion
    • Drought resistance

 

 

Drought Evasion
 

  • Drought-evading plants flower only when moisture is present and persist as seeds during drought periods
     
  • Drought-evading animals adopt an annual cycle or go into a dormant state (e.g., estivation*)
    • Spadefoot toad (Scaphiopus)
       

* dormancy of an animal during a hot or dry period.

A spadefoot toad emerges from its desert burrow to breed when rains come.

 

 

 

Drought Resistance

 

  • Desert plants may be deep-rooted woody shrubs (e.g., mesquite) whose taproots reach the water table:
    • mesquite
    • tamarisk
       
  • Other plants may have shallow roots that extend no more than a few centimeters below the surface (e.g., cacti), enabling a quick uptake of water when it is available

     
  • Precipitation is highly periodic.
  • It comes in pulses as clusters of rainy days 3-15 times a year.
  • Only 1-6 may be large enough to stimulate biologic activity.
  • Desert ecosystems experience periods of inactive steady-states broken by periods of production and reproduction.
  • Both processes are stimulated by rain and continue through short periods of adequate moisture.

 

  • Net primary production (NPP) is low in deserts.

 

  • Desert ecosystems support a diversity of animal life
    • Insects: beetles, ants, locusts
    • Reptiles: lizards, snakes
    • Birds
    • Mammals: grazing and seed-eating herbivores, generalist carnivores