There is some sort of relationship between the numbers, biomass and energy contents of the producers and consumers of different orders in any ecosystem.
These relationships, when represented in diagrammatic ways, are referred to as ecological pyramids, which are of the following three types:
1. Pyramid of Numbers:
The first attempt to provide a quantitative law concerning the trophic levels was given by Elton (1927) who coined the term ‘pyramid of numbers’. The pyramid of numbers deals with the relationship between the numbers of primary producers and consumers of different orders. In all cases, the base of such a pyramid always represents the numbers of primary producers and the subsequent structures on this base are represented by the number of consumers of successive levels, the top representing the number of top carnivores in that ecosystem. In the ecological pyramid so formed, the higher the step in the pyramid, the lower the number of individuals and the larger their size.
The shape of the pyramid of numbers may be upright or inverted (Fig. 5.3a). In a cropland ecosystem, the numbers of crop plants shown at the base of the pyramid are very large; the numbers of grasshoppers are usually less than the numbers of green plants; numbers of frogs is lesser than the number of grasshoppers and finally the number of top carnivores is the least in the series of organisms forming a food chain.
But in an inverted pyramid of numbers (Fig. 5.3b), the number of primary consumers (herbivorous birds feeding upon the tree fruits) is always greater than the number of primary producers (a tree), and the number of parasites living and feeding upon the bird’s body is still greater.
In order to explain the inverted nature of pyramid of numbers, ecologists proposed the idea of pyramid of biomass where the weight of primary producers forms the base. As the term suggests, a pyramid of biomass takes into account, for a given unit area, the biomass of the producers, the biomass of the herbivores, the biomass of the first-level carnivores, and so on.
In (Fig. 5.4 (a)), two types of ecosystems are shown where the pyramid of biomass is upright. The biomass of a single tree is naturally very high than the biomass of a number of birds feeding upon the tree. Similarly, the biomass of even a very large number of bird parasites is lesser than that of the birds. Therefore, in upright pyramid the biomass decreases with successive trophic levels.
The pyramid of biomass becomes upright in a case where it was inverted if the numbers were considered. However, in some instances the inverted pyramids of biomass are also met with. In a pond or lake ecosystem, (Fig. 5.4 (b)) the biomass of diatoms and other phytoplankton is quite negligible as compared with that of the crustaceans and small herbivorous fish that feed on these producers. The biomass of large carnivorous fish living on small fishes is still greater. In fact, this is the condition in most aquatic ecosystems, including the sea.
3. Pyramid of Energy:
A pyramid of energy shows the flow of energy from one trophic level of a community to the next. The shape of pyramid of energy is always upright or triangular because in this the time factor is always taken into account. The pyramid of energy represents the total amount of energy utilized by different trophic level organisms of an ecosystem in limit area over a set period of time (usually per square metre per year).
Fig. 5.5 shows that the quantity of energy trapped by green plants is highest among organisms of different trophic levels and, therefore, the base of pyramid is broad. Similarly, in an aquatic ecosystem the large numbers of phytoplankton quickly complete their life cycles and sets of new populations or crops of phytoplankton are formed every few hours or days.
Thus, the cumulative energy contents that these generation after generation of phytoplankton trap in course of a year is certainly much more than trapped by only a few generations of herbivorous fishes in the corresponding time and space. The energy content trapped by the carnivores living on the herbivorous fishes is the least. Therefore, the pyramid of energy can never be inverted.
This follows directly from the first law of thermodynamics, the law of conservation of energy. If primary producers, or organisms of any trophic level are destroyed, the organisms of the next higher trophic level will automatically perish for want of food or source of energy and ultimately the upright pyramidal shape is maintained.
In Florida silver-springs, the energy contents of different trophic level organisms, as collected by H.T. Odum (1957), over a period of one year make an upright pyramid. The energy contents in kcal/m/yr from 208010 in primary producers decrease to 3368 to 383 to 21 in consumers of first, second and third orders respectively.
Thus, a top carnivore for the organic production of only 21 kcal needs a very broad base of green plants equivalent to 208010 kcal of energy. However, Farlow (1976) has used the principle of pyramid of energy to answer the question of whether or not dinosaurs were poikilothermic. The ratio of energy intake and energy of the produced biomass i.e., of input and output is called the ecological efficiency. This can be studied at any trophic level.