- In an ecosystem, green plants act as producers and the food prepared by green plants is consumed by a series of consumers.
- In a food chain, each source of food is called a trophic level.
- It can also be defined as the level representing organisms in a food chain at which the transfer of food and energy takes place.
- In a food chain, green plants (producers) represent the first trophic level, herbivores (primary consumers) represent the second trophic level, carnivores (secondary consumers) represent the third trophic level and the top carnivores (tertiary consumers) represent the fourth trophic level.
- There is a continuous transfer or loss of energy at each successive trophic level of a food chain.
- According to the ten percent law, only 10% of the total energy entering a particular trophic level is available for transfer to the next trophic level and the rest of the energy is utilized and wasted by the organisms for their metabolic activities at each trophic level.
- In an ecosystem, a definite relationship exists among the number, biomass and energy of producers, primary consumers, secondary consumers and tertiary consumers.
- When this relationship is expressed in a graph, a pyramid is obtained which is called ecological pyramid.
- Ecological pyramids were developed by Charles Elton in 1927 AD.
- In an ecological pyramid, various levels of food chains are represented sequence wise with producers at the base and top carnivores at the apex.
- Ecological pyramids are prepared on the basis of number of organisms, their biomass and energy in each trophic level.
- Ecological pyramids are mainly of three types:
- Pyramid of number
- Pyramid of biomass
- Pyramid of energy
1. Pyramid of number:
- It is the graphic representation of the number of organisms present in unit area of various trophic levels of a food chain with producers at the base and top consumers at the apex.
- The base of the pyramid of number is broad whereas its apex is tapering. It is because of the progressive decrease in number of organisms from the second trophic level to the final trophic level.
- This type of pyramid is always upright or erect in shape.
2. Pyramid of biomass:
- It is the graphic representation of biomass of organisms present in unit area of various trophic levels of a food chain with the biomass of producers at the base and that of top carnivores at the apex.
- The pyramid of biomass of terrestrial ecosystem is upright or erect as there is gradual decrease in the biomass of organisms from the second trophic level to the final trophic level.
- However, the pyramid of biomass of aquatic ecosystems (pond, lake, river etc.) is inverted, i.e. broad apex and tapering base which is because of the progressive increase in the biomass of the organisms from the second trophic level to the final trophic level.
- In an aquatic ecosystem, most number of producers (algae) is microscopic and unicellular. They are more in number but their biomass is less than that of primary consumers.
- The biomass of secondary consumers is more than that of primary consumers and the biomass of the final trophic level is the largest of all the trophic levels. Therefore, the pyramid of biomass of an aquatic ecosystem is always inverted.
3. Pyramid of energy:
- Pyramid of energy is defined as the graphic representation of total energy available in each of the trophic level of a food chain.
- At each trophic level of a food chain, there is a loss of energy and only 10% of the total energy entering a particular trophic level is available for transfer to the next trophic level.
- Therefore, the pyramid of energy is always upright with a broad base and a tapering apex.