The ecosystem is characterized by the energy flow and the circulation of materials through its members. Different organisms of an ecosystem are linked together by their nutritional requirements. Individuals related in this manner constitute a food chain.
Each food chain contains many steps comprising of producers, herbivores, primary carnivores and so on. Each step of the food chain is called trophic level.
An ecological pyramid is a graphical representation in the shape of a pyramid to show the feeding relationship of groups of organisms and the flow of energy or biomass through the different trophic levels in a given ecosystem. It is a standard practice to place producers at the bottom of the pyramid with primary consumers, secondary consumers, tertiary consumers, etc. above them.
The concept of ecological pyramids was introduced by Charles Elton (1927) and are therefore also known as Eltonian Pyramids.
Upright :The pyramid has larger base that gradually tapers towards the tip.
Inverted : The pyramid has narrow base that gradually broadens towards the tip.
Spindle shaped :The pyramid is narrow both at base and tip with a broader part in the middle.
TYPES OF ECOLOGICAL PYRAMIDS
Ecological pyramids are of three main types – pyramid of numbers, pyramid of biomass and pyramid of energy.
Pyramid of numbers
A bar diagram that indicates the number of individuals per unit area at each trophic level in a food chain represents the pyramid of numbers. The length of each bar gives the measure of the relative number. Pyramid begins with the producers which are usually greater in number and are placed at the bottom. Pyramid of numbers shows the relationship between producers, herbivores and carnivores at successive trophic levels in terms of their numbers.
The shape of pyramid of numbers may be upright, inverted or spindle shaped depending upon whether producer individuals are greater or lesser in number.
In a grassland ecosystem, producers are always maximum in number (grasses) while top carnivores (hawks) are least in number. So, in grassland ecosystem, the pyramid of numbers is upright i.e., decreasing towards the top as shown in figure.
In a forest ecosystem however, the pyramid of numbers is somewhat different. The producers which are 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. Thus it shows a spindle shaped structure as shown in the following figure.
However, 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 hyperparasites. Thus, from the producer towards consumers, the number of organisms gradually shows an increase, making the pyramid inverted in shape.
The pyramid of numbers is actually the result of three phenomena which usually operate simultaneously.
i) First one is the geometrical fact that even if the weight of large organisms were equal to the weight of the smaller ones, the number of smaller organisms would be vastly greater than that of the larger ones.
Because of the geometry, therefore, the existence of a valid pyramid of numbers in a natural group of organisms does not necessarily mean that there are fewer of the larger organisms on a weight basis.
ii) The second phenomenon contributing to the pattern of many small organisms and few large ones is the food chain. Useful energy is always lost (in the form of heat) during the transfer through each step in the food chain. Consequently, much less energy is available at the higher trophic levels.
iii) The third factor involved in the pyramid of numbers is the inverse size-metabolic rate pattern. The smaller the organism, the greater is its metabolism per gram of biomass. Consequently, smaller the organism, smaller the biomass which can be supported at a particular trophic level in the ecosystem.
The pyramid of numbers is not very fundamental as an illustrative device since the relative effects of the “geometric,” “food chain,” and “size” factors are not indicated. Its form will vary widely with different communities, depending on whether producing individuals are small (phytoplankton, grass) or large (oak trees). Likewise, numbers vary so widely that it is difficult to show the whole community on the same numerical scale.
Pyramid of biomass
An ecological pyramid of biomass shows the relationship between biomass (total amount of living or organic matter in an ecosystem at any time) and trophic level by quantifying the amount of biomass present at each trophic level of an ecological community at a particular moment of time.
Pyramid of biomass may be straight or inverted. In grassland and forest ecosystem, 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 ecosystem 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.
The pyramid of biomass is of more fundamental interest since the “geometric” factor is eliminated, and the quantitative relations of the “standing crop” (amount of living biomass or number of living organisms present in unit area of an ecosystem) are well shown. In general, the biomass pyramid gives a rough picture of the overall effect of the food chain relationships for the ecological group as a whole.