ENERGY FLOW FROM PRODUCERS TO CONSUMERS IN AN ECOSYSTEM
Energy continuously flows through the biological world in one direction with new energy from the sun constantly entering the system to replace the energy that is dissipated as heat. Green plants, the primary producers of a terrestrial ecosystem, generally capture about 1% to 5% of the energy that falls on their leaves converting it to food energy. This percentage may be a little higher in especially productive ecosystems.
When these plants are consumed by herbivores or primary consumers, only a portion of the plants accumulated energy is actually converted into the bodies of the organisms that consume them. The same applies to secondary consumers, carnivores, which feed on the herbivores. Only some of the potential energy stored in the herbivore tissues is converted into the body of the carnivore. This reducing trend is sustained from one trophic level to the next.
Efficiency of energy transfer
The amount of energy ingested and retained at each trophic level goes toward heat production. A great deal of the energy is used for digestion and work. Usually 40% or less of energy goes toward growth and reproduction.
Raven and Johnson (1996) observed that an invertebrate typically uses about a quarter of the 40% ie 10% of the food it eats to its own body and thus into potential food for its predators. The comparable figure varies from about 5% in carnivores to nearly 20% for herbivores however, 10% is a good average value for the amount of organic matter that reaches the next trophic level. Because the energy lost at each trophic level is so great, food chains generally consist of only three or four steps. Very little energy remains in the system as usable energy after it has been incorporated successfully into the bodies at four trophic levels.
In other words, only about 10% of the energy fixed in the food is fixed in the body of the animal that eats that food. The trophic efficiency is in general related to the formula- exploitation efficiency x assimilation efficiency x production efficiency (Chapman and Reiss, 1995).
Pyramid of Energy
A pyramid of energy shows the flow of energy from one trophic level of a community to the next and the rate at which energy flows up a food web. The units of the pyramids of energy are energy/area/time e.g. kcal/M2/yr i.e. (Kilocalories/Square meter/year) and are usually measured over a period of time. Following the law of conservation of energy which states that energy can neither be created nor destroyed but is always conserved, pyramids of energy can never be inverted as can be found with pyramids of numbers or of biomass. The productivity of herbivores cannot exceed the net primary productivity and that of the first level carnivores must be less than the productivity of the herbivores. The productivity of the second-level carnivores must be less than the productivity of the first-level carnivores and so on.
The ecosystem acquires its energy from the producers (photosynthetic plants) which produce energy from the reception of sunlight. Through the food chain or food web, the energy becomes available to all organisms within the ecosystem.
The energy acquired by the producers is transferred from one trophic level to the next along the food chain.
The length of food chain is limited to 3 or rarely 4 levels because too much energy is lost at each transfer point.
Following the law of conservation of energy, the pyramid of energy cannot be inverted.