Ecological Pyramid
Ecosystem of Class 12
An Ecological pyramid is a graphic representation of an ecological parameter, like number of individuals present in various trophic levels of a food chain with producers forming the base and top carnivores the tip.
Trophic structure, i.e., the interaction of food chain and the size metabolism relationship between the linearly arranged various biotic components of an ecosystem is characteristic of each type of ecosystem. The trophic structure and function at successive trophic level, i.e., produces herbivores, carnivores, may be shown graphically by means of ecological pyramids where the first or problem level constitutes the base of the pyramid and the successive levels, the tiers reaching the apex. The ecological pyramids are also called as Eltonian pyramids as its concept was proposed by Elton (1927).
TYPES OF ECOLOGICAL PYRAMIDS
Ecological pyramids are of three general types :
- Pyramid of numbers
- Pyramid of biomass
- Pyramid of energy
The pyramid of numbers on biomass may be upright or inverted depending upon the nature of the food chain in the particular ecosystem, whereas pyramids of energy are always upright.
PYRAMID OF NUMBERS
They show the relationship between producers, herbivores and carnivores at successive trophic levels in terms of their number.
Grassland EcosystemIn a grassland, the producers which are mainly grasses, are always maximum in number. This number then shows a decrease towards apex, as the primary consumers (herbivores) like rabbits, mice etc., are lesser in number than the grasses; the secondary consumers such as snakes and lizards are lesser than the rabbits and mice. Finally,the top (tertiary) consumers hawks or other birds, are least in number. Thus, the pyramid becomes upright.
Fig. : Upright pyramid of numbers showing decreasing number at successive stages(A) Grassland ecosystem (B) Crop ecosystem
Pond EcosystemSimilarly, in a pond ecosystem the pyramid is upright. Here the producers, which are mainly the phytoplanktons as algae, cyanobacteria etc. are maximum in number, the herbivores which are smaller fishes; rotifers etc., are lesser in number than the producers; and the secondary consumer (carnivores) such as small fish eating each other, water beetles etc., are lesser in number than the herbivores. Finally, the top (tertiary) consumers, the bigger fish are least in number.
Forest EcosystemIn a forest ecosystem, however, the pyramid of numbers is somewhat of different shape. The producers, which are mainly large sized trees, are lesser in number, and form the base of the pyramid. The herbivores, which are fruit eating birds, elephants, deers etc. are more in number than the producers. Then there is a gradual decrease in the number of successive carnivores, thus making the pyramid again upright.
Parasitic food chain
In a parasitic food chain the pyramids are always inverted. This is due to the fact that a single plant may support the growth of many herbivores and each herbivore in turn may provide nutrition to several parasites, which support many hyperparastites. Thus, from the producer towards consumers, there is a reverse position, i.e., the numberof organisms gradually shows an increase, making the pyramid inverted in shape. Actually the pyramids of numbers do not give a true picture of the food chain as they are not very functional. They do not indicate the relative effects of the geometric, food chain and size factors of the organisms. They generally vary with different communities with different types of food chain in the same environment. It becomes sometimes very difficult to represent the whole community on the same numerical scale (as in forests).
PYRAMID OF BIOMASS
They are comparatively more fundamental as they, instead of geometric factor, show the quantitative relationship of the standing crops. In grasslands and forests, there is generally a gradual decrease in biomass of organisms at successive levels from the producers to the top carnivores. Thus, pyramids are upright. However, in a pond as the producers are small organisms, their biomass is least, and this value gradually shows an increase towards the apex of the pyramid, thus making the pyramid inverted in shape.
Fig. : Upright pyramid of biomass : Showing decrease in biomass in successive trophic levels (A) Grassland ecosystem (B) Tree ecosystem
PYRAMID OF ENERGY
Of the three types of ecological pyramids, the energy pyramids give the best picture of overall nature of ecosystem. Here, number and weight of organisms at any level depends not on the amount of fixed energy present at any one time in the level just below but rather on the rate at which food is being produced. In shape it is always upright, as in most of the cases there is always a gradual decrease in the energy content at successive trophic levels from the producers to various consumer.
Fig. : Upright pyramid of energy (A) Grassland ecosystem (B) Aquatic ecosystem
ECOLOGICAL EFFICIENCY
Ecological efficiencies are the percentage ratios between energy flow at different trophic levels of a food chain. It was first studied by Lindeman (1942) who called it as Progressive efficiency.
Ecological efficiencies/Trophic level efficiency = 
These include a number of ratios which indicate the ratios between energy transfers e.g. Trophic level energy intake efficiency. It is most important kind of ecological efficiency. It is the ratio between the amounts of energy assimilated at two successive trophic levels. It is also called Lindeman’s efficiency and varies from 1-5 percent for producers (first trophic level) and 10-20 percent for higher trophic levels.
Assimilation efficiency
It is the percentage ratio between the net production and the energy assimilation at a particular trophic level. It varies from 10 to 50 per cent.
Photosynthetic efficiency
It is percentage ratio between total photosynthesis (or Gross primary productivity) and incident total solar radiation. On an average, it varies from 1 to 5 percent.
Net production efficiency
It is percentage ratio between apparent photosynthesis (Net primary productivity) and gross primary productivity. It is about 50 percent.
