ECOLOGICAL PYRAMIDS

            Graphic representation of trophic structure and function of an ecosystem, starting with producers at the base and successive trophic levels forming the apex is known as ecological pyramids.

            Ecological pyramid may be of three types – pyramid of numbers, pyramid of biomass and pyramid of energy.

            1. Pyramid of Numbers:- It represents the number of individual organisms at each trophic level. We may have upright or inverted pyramid of numbers depending upon the type of ecosystem and food chain.

            As for example – Grassland and a pond ecosystem show an upright pyramid of numbers. The producers in grassland are grasses which are very large in numbers. So the producers form a broad base. The herbivores in the grassland are insects while tertiary carnivores are hawks and other birds which are gradually less and less in number and hence the apex of the pyramid becomes gradually narrower.

            Pyramid of numbers may be inverted (e.g., parasitic food chain). The producers like a few big trees harbor fruit eating birds acting like herbivores which are larger in number. A much higher number of lice, bugs, etc., grow as parasites on these birds while a still greater number of hyperparasites like bugs, fleas and microbes feed upon them, thus making an inverted pyramid.

            2. Pyramid of Biomass:- It represents the total biomass (dry matter) at each trophic level in the food chain. The pyramid of biomass can also be upright or inverted.

            As for example – The pyramid of biomass in forest ecosystem is upright. This is because the producers (trees) accumulate a huge biomass while the biomass of consumers feeding on them declines as it moves to higher trophic levels, resulting in broad base and narrowing tip.

            The pyramid of biomass may be inverted (e.g., pond ecosystem). Here the total biomass of producers (phytoplanktons) is much less as compared to herbivores (zooplanktons, insects), carnivores (small fish) and tertiary carnivores (big fish). Thus, the pyramid takes an inverted shape with narrow base and broad apex.

            3. Pyramid of Energy:- It represents the total amount of energy present at each trophic level. Pyramid of energy gives the best representation of the trophic relationships and it is always upright.

            At every successive trophic level, there is a huge loss of energy (about 90%) in the form of heat, respiration, etc. Thus, at each next higher level only 10% of energy passes on. Hence, there is a sharp decline in every level of each successive trophic level as it moves from producers to top carnivores. Therefore, the pyramid of energy is always upright.