An ecosystem includes a habitat with all its living species which interact with the physical and chemical environment. The size of an ecosystems can vary from a few 100 m² to a regional scale to global scale

• Physical environment of an ecosystem: Atmosphere
  
  • the atmosphere is the gas space extending from the Earth surface into space.

    Troposphere : layer closest to Earth surface (0- 20km) and contains most of the mass of the Earth atmosphere. The major components are oxygen (21%), nitrogen (78%), argon (0.93%), O₃ (ppm) ,CO₂ (ppb) . All weather phenomena occur within the troposphere. The air density, temperature and water vapor concentration decrease with increasing altitude . Water vapor concentration greatest in the tropics, but decreases toward the poles.

    Stratosphere : 10 - 50 km above the planet's surface. The air temperature in the lower stratosphere are relatively constant and reach - 60 to -70^oC at 25 km. The stratosphere contain the ozone layer (20 -30 km). O3 loss in Antarctica , ozonehole

    Mesosphere : (50 - 80 km). Temperatures, which reach -80 ^oC km. Residual gases begin to stratify according to molecular mass.

    Thermosphere : 100 -300km. Aurora forms in the thermosphere

  • Solar radiation
    The amount of solar energy input reaching the Earth surface varies with altitude. High altitude (near poles) receive less sun radiation, whereas equatorial region receive more sun energy.
    Mixing in the atmosphere and oceans (circulation) redistribute solar energy

  • Atmospheric circulation
    • Equatorial Cells : rising / descending air
      Solar energy warms up the air. Warmer air is less dense ( hot air balloon) and begins to rise.
      Solar radiation is strongest near the equator , causing massive upward airflows close to 0 ^o latitude . As these warmer air masses rise and expand they have to move aside the colder more denser air, a process which consumes energy. Air expansion requires energy, concequently the rising air masses cool down.
      The rising equatorial air masses have a high moisture content. As the rising air cools down its ability to hold water vapor decreases. Moisture begins to condensate resulting in precipitation. Thus, tropical regions (near the equator) receive abundant rainfall.
      At higher altitudes the upward motion of the air masses slows down and eventually stops. However, the underlying air is still pushing upwards, forcing the top air masses to move aside. The aloft air which has lost its moisture moves horizontally and eventually drops down . This is the beginning of a circulation in each hemisphere between the equator (ascending) and 20- 30^o North or 20-30^o South. (descending) . World deserts (Atacama) are located along these latitudes.

    • Polar Cells
      At the poles cold, dense air is moving downward. This pushes the bottom air to the side moving away from the poles. The polar circulation extends to 60 ^o North and South

    • Mid latitude cells
      Between polar and equatorial cells are the mid latitude cells (30 - 60 ^o North and South. They connect polar and equatorial cells like a wheel running between two gears. circulation cells

  • Coriolis force ( image)
    Physics of the rotating sphere dictate that the rotation of the Earth imparts a force on objects moving over the Earth surface. Globular circulation Without the Coriolis force the 30 ^o Overall the descending air masses at 30^o North and 30^o South will generate winds that should spread in North and South direction. Because of the Coriolis forces, these winds are deflected such, that they have an East to West component. These winds will move from 30 ^o North toward the equator in a NE to SW direction (the NE trade winds) and from 30 ^o South toward the equator in a SE to NW direction (the SE trade winds) . The westerlies in the northern hemisphere are generated by the air flow moving from 30^o North in NE direction. In the southern hemisphere the westerlies move from 30^o South in SE direction.
    Fig. 56.2, hurricane , clockwise/cc

  • oceanic circulation: Fig. 56.3
    When wind blows over a water surface, a directional force is acting on the water. One can show that the resulting shear forces cause the water to move in a direction which is 90^o to the right of the wind direction in the northern hemisphere . In the southern hemisphere water will move 90 ^o to the left of the wind direction. The dominant winds produce prevailing flow patterns in the world oceans.

    Warm water currents:
    Gulf Stream : Waters from the tropics flow into the North Atlantic.
    North Atlantic Drift : northern extension of the Gulf( to Iceland, Norway, and Spitsbergen)
    Kuroshio :Pacific's equivalent Gulf Stream, tropical waters to Japan.
    Agulhas Current: strongest NS current in southern hemisphere stays close to Africa coast
    South Equatorial Current : westward current is diverted northward by the corner of Brazil feeding the sources of the Gulf Stream.
    Equatorial Countercurrent : The Trade Winds build up water on the western side of the oceans; the Equatorial Counter-current flows eastward, restoring the balance.

    Cold water currents:
    East Greenland : Cold Arctic waters make their way southward into the Atlantic.
    Oyashio : The Pacific twin of the East Greenland Current is heavy with the cold waters moving south from the Bearing Sea.
    California Current : A wide, cold and sluggish current follows the west coast of the U.S. as far as Baja California.
    Labrador Current: Cold waters and icebergs are brought down from Baffin Bay and meet the warm Gulf Stream
    Benguela current : drives water northward and westward into midocean. This produces a strong upwelling of water and nutrients from the ocean bed, supporting enormous numbers of fish.
    Humboldt Current: brings cold water to the west coast of south America .

  • There are divergent flows (away from each other ) and convergent flows (toward each other). Divergent flows produce upwelling, while convergent flows result in downwelling. Upwelling at California coast, coast off Peru, increases nutrients in water. Middle ocean , from 30 ^o North to equator mainly upwelling . Nutrient rich waters at equator have a high biological productivity.

  • El Nino "Normal conditions" : Average winter temperature The cool Humboldt current and easterly trade winds causing upwelling bring cool waters to the coast of Peru.
    Average cloud distribution : Wester pacific, high evaporation rate, abundant rain. Polar jet stream moves from W to E, marks boundary between tropical and cold polar air. Polar jet moves across Pacific, sometimes picking up additional moisture from the subtropical jet stream and makes landfall in northern California, Oregan, Alaska (high rainfall).
    Periodically (3 - 10 years) the warm, eastward-flowing Equatorial Countercurrent strengthens and the ocean of the South American coast warms up. The Humboldt Current is disrupted. El Nino temperature distribution . Atmospheric response to the warmer ocean temperatures: El Nino cloud distribution , Effect Coastal areas of Peru and Ecuador (usually dry) torrential rain and flooding, Collapse of the monsoons in Asia, which can bring severe drought to Indonesia and northern Australia. Droughts in areas of Africa and central North America

• Flow of energy and materials in ecosystems
  
  • Productivity: is the conversion of energy into biomass. Biomass production of an organism can be measured by its growth and reproduction.
    Autotrophic organisms are the primary source of energy and biomass. Secondary production of organic matter by using existing organic matter as energy source (heterotrophic organisms)

  • Five trophic levels : Trophic level is the position in a food chain, determined by the number of energy-transfer steps to that level
    • Primary producers:(support all other trophic levels directly or indirectly )
      Photosynthetic autotrophs synthesize sugars and other organic molecules using light energy. Examples : terrestrial plants, aquatic photosynthetic protists, and cyanobacteria. Non-photosynthetic primary producers are chemosynthetic bacteria that oxidize inorganic componds as source of energy and synthesize organic componds primarily from CO2.
    • Primary consumers
      Herbivores which are consuming primary producers. Examples: terrestrial insects, snails, grazing mammals, seed-eating birds, aquatic zooplankton, and some fish.
    • Secondary consumers
      Carnivores that eat herbivores. Examples: Insct eaters (spiders, frogs, birds), mammals , fish, sea-stars.
    • Tertiary consumers
      Carnivores that eat other carnivores.
    • Detritivores
      Feed on organic wastes and dead organisms. Examples: bacteria , fungi, savengers (roaches)

  • Oceanic circulation regulates primary production of biomass
    East US coast North Atlantic , North Pacific

  • Distribution of biomass :
    Gross primary production (GPP): total biomass produced by primary producers
    Net primary production (NPP): NPP = GPP - respiration
    oxygen light dark bottle method Fig. 56.4 , Fig. 56.5 , Fig. 56.1 ,

  • Food web
    Biomass is moved through an ecosystem through the trophic levels
    Soil food web , Marine food web , Fig. 56.6

  • Ecological Efficiency :
    The efficiency at which consumers incorporate organic material (the food they eat) into new biomass depends on the trophic position of the organism. For example herbivors have a low ecological efficiency since the conversion of ingested food into own biomass is not very efficient. In other words, they have to eat large quantities of food to sustain themselves. Carnivores have a higher ecological efficiency because their food is chemically similar to their own tissue and can therefore easily be converted into their own biomass.

    Ecological Efficiency (E) can be defined as
    E = (Production by next higher tropic level / Production available at lower trophic level) x100
    The value of E varies : e.g. herbivores E = 0.2 to 0.4 , carnivores : E = 0.3 - 0.5
    Fig. 56.7

  • Energy and Biomass Pyramids:
    The productivity of each higher trophic level declines, because only a small portion of the energy of each trophic level is transferred to the next trophic level. Fig. 56.8