Ecology

• Deserts are one of several types of biomes
• Biomes


  Major divisions of the terrestrial environment, distinguished primarily by predominant plants
• Natural history


  The study of how organisms in a particular area are influenced by factors like climate, soils, predators, competitors, etc.
• Climate is the most important factor in understanding distribution of biomes
• Regardless of continent, a desert has plants with certain functional traits that arise through selective pressure in the desert environment

  
• Biomes


  • Generally defined by the average temperature and precipitation of a given location

  
• Uneven heating of earth's surface by the sun and tilt of earth

  1. Produce predictable variations in climate

  2. Latitudinal and seasonal variations

  3. Temperatures less seasonal near equator, rainfall may be markedly seasonal

  4. Deserts receive little precipitation

  
• Coriolis effect


  Causes apparent deflection of winds clockwise in the N hemisphere and counterclockwise in the S hemisphere
• When terrestrial biome distribution is strongly associated with latitude, biomes are also influenced by microclimate and soil type

  
• Soil


  A complex mixture of living and non-living material, classified based on vertical layering (soil horizons)
• Soil horizons


  • O horizon
  • A horizon
  • B horizon
  • C horizon
• Environmental conditions shape each biome's characteristic biology
• Biomes


  Biological communities defined primarily by their predominant plants and associated with particular climates
• Tropical rainforests


  • Most occur within 10o latitude of equator
  • Little temperature variation between months
  • Annual rainfall of 2,000 - 4,000 mm relatively evenly distributed
  • Quickly leaches soil nutrients
  • Organisms add vertical dimension

  
• Predominant plants in tropical rainforests

  • Tall trees
  • Epiphytes (e.g. orchids, ferns)
  • Mycorrhizae (plant & fungus symbiotic association)

  
• More than ½ of the world's species of animals and plants are found in tropical rainforests
• More than 20% of world's O2 produced in Amazon rainforest
• Mycorrhizae


  Symbiotic association between fungi and roots of plants (especially trees), helping trees obtain nutrients from the soil
• Tropical dry forests


  • Usually located between 10o - 25o latitude
  • Climate more seasonal than tropical rainforest: rainy season (5-6 months) followed by a dry season (6-7 months)
  • Soils generally richer in nutrients, but vulnerable to erosion
  • Height of forest influenced by precipitation
  • Many animals make seasonal migrations
  • Shares many animal and plant species with tropical rainforests
  • Heavily settled by humans with extensive clearing for agriculture
• Tropical savannas


  • Most occur north and south of tropical dry forests within 10o - 20o of the equator
  • Climate alternates between wet / dry seasons
  • Seasonal lightning often leads to wildfires, especially at beginning of wet season
  • Soil layers have low water permeability, saturated surface soils keep trees out
  • Many animals move with rainfall and food
  • Livestock ranching is main livelihood
• Predominant plants in tropical savannas

  • Grass
  • Shrubs
  • Isolated trees

  
• Animals found in tropical savannas

  • Zebras, gazelles, wildebeest, lions, elephants, giraffes (Africa)
  • Kangaroos, emus (Australia)
• Water plays a central role in living organisms
• Body water content is 50 to 90%
• Life evolved in water
• Life has biochemistry within aquatic medium
• Life must maintain appropriate internal water concentrations
• Particularly critical for desert organisms, which have been focus of much study
• Concentration gradients


  Influence the movement of water between an organism and its environment
• Water tends to move down concentration gradients
• Magnitude of gradients determines whether an organism tends to lose or gain water from environment
• Must consider an organism's microclimate in order to understand its water relations
• Evaporation


  Accounts for how much water lost by terrestrial organisms
• As water vapor in the air increases


  The water concentration gradient from organisms to air is reduced
• Evaporative loss decreased


  As water vapor in the air increases
• Evaporative coolers work best in dry climates
• Relative humidity


  Common measure of water content in air, calculated as 100 x (actual water vapor density / saturation water vapor density)
• Water vapor density


  Mass of water vapor per unit volume of air (g/m3 or mg/L)
• Saturation water vapor density

  Maximum quantity of water vapor air can potentially hold at a certain temperature
• Saturation water vapor density increases with temperature