Dr Syed Zulfiqar Andrabi
Ecological Pyramids (Eltonian Pyramid):
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 pyramid. The trophic level of an ecosystem can be expressed in a diagrammatic way in the form of ecological pyramids. The concept of ecological pyramid was developed by Charles Elton in 1927 and after his name these pyramids are also called as Eltonian pyramids.
The ecological pyramid basically consists of three parts the base body and the apex. The producers form the base the body consists of successive trophic levels and the top carnivores form the apex. Based on the parameters selected to depict the trophic relationship, an ecological pyramid may be of the following types.
- Pyramid of number: This pyramid shows the relationships between producers, herbivores and carnivores at successive trophic levels in terms of their numbers. When plotted the relationships among the number of producers, primary consumers, secondary consumers, tertiary consumers and so on in any ecosystem it forms a pyramid structure called the pyramid of number.
There are three types of pyramid of number.
(a)Upright pyramid of number: In an aquatic and a grassland ecosystem numerous small autotrophs support lesser number of herbivores which support further smaller number of carnivores and hence the pyramid structure is upright.
Pyramid of number in a Grassland ecosystem
(b)Partly upright /Irregular Pyramid of number: In forest ecosystem lesser number of producers support greater number of herbivores who in turn support a fewer number of carnivores.
Pyramid of number in a forest ecosystem
(c)Inverted pyramid of number: In parasite food chain one primary producer support numerous parasites which support still more hyperparasites.
Pyramid of number in a parasitic food chain
- Pyramid of Biomass:
The pyramid of biomass is comparatively more realistic than pyramid of number. If the numbers of consumers at each trophic level are multiplied by their weight then what we obtain is the pyramid of biomass. When we plot the biomass (net dry weight) of producers, herbivores, carnivores and so on we have a pyramid of biomass. Two types of pyramid of biomass are found.
(a)Upright pyramid of Biomass: When larger weight of producers supports a smaller weight of consumers an upright pyramid results e.g. forest ecosystem.
Pyramid of biomass in a forest ecosystem
(b)Inverted Pyramid of Biomass: When smaller weight of producers supports larger weight of consumers an inverted pyramid of biomass is formed e.g. aquatic ecosystem (pond ecosystem).
Pyramid of biomass in an aquatic ecosystem
- Pyramid of energy: This pyramid gives the best picture of the overall nature of an ecosystem. The pyramid of energy is based on the total energy content of energy trophic level. As the quantity of energy available for utilization in successive trophic levels is always less because there is loss of energy in each transfer thus energy pyramid is always upright.
Pyramid of energy
The limitations of pyramid of numbers and pyramid of Biomass which provide information only about the quantity of organic matter available at a particular time but not of productivity and the pyramid of energy eliminates these drawbacks.