Energy Flow in Ecosystems

• Energy enters food chains in the form of light energy.
• Three things can happen to light when it reaches an object: it can be reflected, transmitted or absorbed.
• Only light energy that is absorbed by chlorophyll molecules in producers can be converted into chemical energy in glucose and so enter the food chain.
•  Light energy that is not absorbed by chlorophyll will eventually be absorbed by other objects on the ground,such as water, rocks or animals, and will be converted to heat.
• A lot of this energy is used to evaporate water and so drive the water cycle.
• More energy is lost as heat during photosynthesis and respiration.
• Consumers take in concentrated chemical energy in the form of the organic molecules that make up the biomass of the plants oranimals they eat.
• A lot of biomass is not absorbed by consumers (plant fibre, wood, bone, fur, etc.) and the energy in this biomass is passed on to decomposers, who can use it.
• And much of the energy that is absorbed is lost as heat through the various metabolic reactions, especially respiration and friction due to movement.
• The heat energy  is given out to the surroundings by radiation, convection and conduction, and cannot be regained by living organisms.
• These losses are particularly big in warm-blooded and very active animals.

I Pyramids of Numbers

• These show the numbers of organisms at each trophic level in a food chain.
• The widths of the bars represent the numbers using a linear or logarithmic scale, or the bars may be purely qualitative.
• In general as you go up a food chain the size of the individuals increases and the number of individuals decreases.

II Pyramids of Biomass

• Consider the total mass of living organisms (i.e. the biomass) at each trophic level.
• The biomass should be dry mass(since water stores no energy) and is measured in
  kg m-2.
• The biomass may be found by drying and weighing the organisms at each trophic level, or by counting them and multiplying by an average individual mass.
• Typically, only around 10% of the biomass in each level is passed on to the next level.

Mass is lost at each stage of a food chain for two reasons:

•  Some of the biomass absorbed by a consumer is used  in respiration and is converted to carbon dioxide and water, which are excreted (when organisms respire they lose mass!).

•  Some of the biomass is simply not eaten by the consumers in the next trophic level, or is ingested but then egested again without being absorbed. This unused biomass can include plant cellulose cell walls, wood, bones, teeth, skin and hair. Many consumers are surprisingly fussy about what they eat. This biomass becomes detritus and is used by decomposers.

III Pyramids of Energy

• These pyramids represent the flow of energy into each tropic level, so describe a period of time (usually a year).
• The units are usually something like kJ m-2y-1.
•  Pyramids of energy are always pyramidal (energy can be lost but cannot be created), and are always  very shallow, since the transfer of energy from one trophic level to the next is very inefficient .