In the trophic relationship between producers and different types of consumers, the number and mass of the organisms at each step is limited by the amount of energy available since same amount of energy is always lost at every step of the reaction as per the laws of thermodynamics. Progressively at each step the bio mass (number of organisms) becomes smaller as it reaches the top (i.e. reaches the level of carnivores).
This relationship which can be graphically represented with a wide base and a narrow top is called an ecological pyramid. In an ecological pyramid producers are at the base. Generally ecological pyramids are of three types. These are – pyramid of numbers, pyramid of biomass and pyramid of energy.
Pyramid of numbers:
Pyramid of numbers denotes the relationship between producers, herbivores and carnivores (different levels of consumers) at successive trophic levels in terms of their number. The concept of pyramid of numbers was proposed by Charles Elton (1927) who pointed out the difference in the numbers of organisms at different levels.
The producers (autotrophic plants) will occupy the base and they are the largest in number. They support the herbivores whose number should be less than their own (plants) number. Still higher in the pyramid secondary level consumers are lesser in number than herbivores. Finally only a very small number or carnivores occupy the top of the pyramid.
A situation similar to the one mentioned above exists in a pond also. Here successively starting from the phytoplanktons – zooplanktons – Crabs, larvae – small fish – large fish, the number gets reduced.
In the above two situations the pyramid is said to be upright. If we apply the concept of pyramid of numbers to a forest ecosystem the situation will be slightly different. Here for instance the producers will be large sized trees which are generally less in number from the base of the pyramid.
Fruit eating birds, Insects etc. which depend on the tree are more in number, but then the carnivores which depend on the birds and insects are smaller in number and the pyramid once again becomes upright. In parasitic food chain however the pyramids are always inverted. This is because a single plant may support many herbivores and each herbivores in turn supports many parasites.
A specific comment seems to be necessary in the case of inverted pyramids. Pyramids of numbers do not – reflect the actual functional relationship between different trophic levels. For instance in a parasitic chain even there is only a single plant at the base and a number of parasites at the top in reality the total biomass of a single plant is more than the accumulated bio mass of all the parasites.
Pyramid of biomass:
These are more accurate in the representation of trophic relationship between the organisms. They show a quantitative relationship between different trophic levels. The pyramids of bio mass are always upright as there is a gradual decrease in the quantity of the biomass.
Pyramid of Energy:
Among the three types of ecological pyramids the pyramid of energy perhaps gives the best picture of the overall functioning of the ecosystem. At the entry point i.e. producers, energy is always mere and at every level quantum of energy transferred gets reduced and it is reflected in terms of biomass.
In contrast to the other two pyramids, this pyramid (of energy) represents the rate of flow of energy (food) through the food chain. The shape of the pyramid of energy is always upright as the energy gets tapered as it passes on from one level to another.