Energy Flow, Trophic Level & Food Chain | Biology Class 12 - NEET PDF Download

Introduction

• The chemical energy of food is the main source of energy required by all living organisms. 
• The transfer of energy from one trophic level to another trophic level is called energy flow.
• The flow of energy in an ecosystem is unidirectional.
• That is, it flows from the producer level to the consumer level and never in the reverse direction.
• Hence, energy can be used only once in the ecosystem.
• But the minerals circulate and recirculate many times in the ecosystem. 

This energy flow is based on two different laws of thermodynamics:

• First law of thermodynamics, that states that energy can neither be created nor destroyed, it can only change from one form to another.
• Second law of thermodynamics, that states that as energy is transferred more and more of it is wasted.

Trophic level

The producers and consumers in the ecosystem can be arranged into different feeding groups and are known as trophic level or the feeding level.

1. The producers (plants) represent the first trophic level.
2. Herbivores (primary consumers) present the second trophic level.
3. Primary carnivores (secondary consumers) represent the third trophic level
4. Top carnivores (tertiary consumers) represent the last level.

Energy Flow in Ecosystem

• A large amount of energy is lost at each trophic level. It is estimated that 90% of the energy is lost when it is transferred from one trophic level to another. 
• Hence, the amount of energy available decreases from step to step. 
• Only about 10% of the biomass is transferred from one trophic level to the next one in a food chain. 
• And only about 10% chemical energy is retained at each trophic level. 
• This is called 10% law of Lindeman (1942).  
• When the food chain is short, the final consumers may get a large amount of energy. But when the food chain is long, the final consumer may get a lesser amount of energy.

Food Chain

• At each step in a food chain, a portion of the energy captured by producer is lost as heat given off during the chemical breakdown of food by respiration (metabolic heat).
• Thus, energy flows  through the ecosystem in a single direction and is not recycled. In contrast, nutrients cycle between organisms and the physical environment.
• The transfer of food energy from one to another organism leads to loss of energy as heat due to metabolic activity.
• The amount of solar radiation reaching the surface of the earth is 2 cals/sq.cm/min. It is more or less constant and is called solar constant or solar flux. About 95 to 99% of the energy is lost by reflection. Plants utilize only 0.02% of the energy reaching earth.
• The energy trapped by the producers (primary production) is utilized by the consumers. The producers are directly consumed by herbivores that are eaten by the primary carnivores that in turn are consumed by the secondary carnivores.
• The consumers store some amount of energy in their tissues. This energy, stored by the consumers, is called secondary production. Only about 10 to 20% of the primary production is converted into secondary production. The remaining 80 to 90% is lost by the consumers in the form of faeces.