ECOLOGICAL PYRAMIDS

An ecological pyramid is the graphic representation of, trophic levels of a food chain.

Ecological pyramids were developed by Charles Elton (1927) and are, therefore, also called Eltonian pyramids.

Three types of ecological pyramids are recognized, viz., (i) pyramid of number, (ii) pyramid of biomass, and (iii) pyramid of energy, giving graphic representation to three important parameters at different trophic levels in food chain respectively, number of individuals, amount of biomass and amount of energy.

(i) Pyramid of number:

In most ecosystems, the number of producers is maximum.

During transfer of food at any trophic level, only 10% of the food present in one trophic level becomes part of the next trophic level.

90% of the food is either lost in wastage or broken down during cellular respiration for providing energy for various life activities.

Producers, thus can support fewer herbivores and herbivores can support still fewer carnivores and so on.

Thus the number of top carnivores is too small to support any other trophic level and don't act as prey to any other organism.

Fig. : Pyramid of numbers in a grassland ecosystem.

(For example, only three top-carnivores are supported in an

ecosystem based on production of nearly 6 millions plants)

Pyramid of numbers, though upright in most cases, like a pond or a grassland but may not be so always.

In some cases it may be inverted, i.e., the number of the organisms at each successive trophic level is higher than that in preceeding one and the size decreases gradually at each successive level; e.g., a large-sized tree (producer) may support and provide nourishment to several birds (herbivores).

The number of ectoparasites like mites, ticks, lices, -bugs etc., dependent upon birds for nourishment is much larger than birds.

The number thus increases at each successive level.

The pyramid of number may become spindle-shaped for a tree as herbivorous birds are usually eaten by eagle or falcon.

The number of eagles is much less than that of birds feeding upon tree.

The number of organisms thus increases at lower trophic levels and finally decreases at higher trophic levels.

(ii) Pyramid of biomass :

Biomass is the amount of living matter (expressed as weight) at any particular trophic level at a given time.

Pyramid of biomass in terrestrial ecosystems is usually upright.

For upright pyramid, total biomass of plants (producers) in a specific area is more than that of herbivores (primary consumers) and it gradually decreases at each successive trophic level.

It is least in top carnivores. It is upright for tree and grassland ecosystems.

Fig. : Pyramid of biomass. Showing a sharp decrease in biomass at higher trophic levels

In aquatic ecosystem, the pyramid of biomass may be inverted, e.g., biomass of zooplanktons is higher than that of phytoplanktons as life span of former is longer and the latter multiply much faster though having shorter life span.

A number of generations of phytoplanktons may thus be consumed by single generation of zooplanktons.

Biomass of fish may still be larger as fish are larger in size with longer life span and a number of generations of zooplanktons can be consumed by fishes.

However during transfer, only 10% of the biomass of one generation is passed on to next trophic level.

Fig. : Inverted pyramid of biomass in Pond Ecosystem

(iii) Pyramid of energy:

The pyramid ofenergy is always upright because the flow of energy is unidirectional from producer to consumer level.

The energy content is maximum in producers.

The energy decreases at each trophic level of food chain, as part of the energy is lost as heat and major part of energy is liberated during respiration for use in various activities.

Only 10% of the energy of previous trophic level is received by next trophic level, as proposed by 10 per cent law of Lindeman (1942).

Just to illustrate 1,000 calories of solar energy is needed to produce 10 calories of energy stored in a plant (if plants trap 1 % solar energy).

Herbivores, feeding upon plant, will retain 1 cal of available stored energy and carnivores feeding upon them will gain only 0.1 cal of usable energy.

Fig. : An ideal pyramid of energy. Observe that primary producers convert
only 1 % of the energy in the sunlight available to them into NPP

Limitations of ecological pyramid

(i) It does not take into account the same species belonging to 2 or more trophic levels, e.g., insectivorous plants.

(ii) It assumes a simple food chain and does not accommodate a food web.

(iii) Saprophytes, decomposers, microbes and detrivores are not given any place in ecological pyramids.