The food producer forms the base of the pyramid and the top carnivore forms the tip. The ecological pyramids are the three categories.

I. Pyramid of numbers,

II. Pyramid of biomass, and

III. Pyramid of energy or productivity.

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Pyramid of Numbers

This deals with the relationship between the numbers of primary producers and consumers of different orders. But it is very difficult to count all the organisms; so, a pyramid of numbers does not completely define the tropic structure for an ecosystem.

A pyramid of numbers does not take into account the fact that the size of organisms being counted in each tropic level can vary. A count in a forest would have a small number of large producers, the big trees, which support a large number of herbivores and which in turn support a number of carnivores. Thus, depending upon the size and biomass, the pyramid of numbers may not always be upright; it may even be completely inverted.

Pyramid of Biomass

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Another approach is to weigh individuals in each tropic level instead of counting them. This would give us a pyramid of biomass, i.e., the total weight of all organisms at a given level. For most ecosystems on land, the pyramid of biomass has a large base of primary producers with a smaller tropic level perched on top. In contrast, in many aquatic ecosystems, the producers are tiny phytoplankton that grow and reproduce rapidly. Here, the pyramid of biomass can have a small base, with the consumer biomass at any instant actually exceeding the producer biomass. The phytoplankton are consumed about as fast as they reproduce, it is just that the survivors (they may be few) are reproducing at a phenomenal rate.

Pyramid of Energy

When we wish to know about the functional roles of the tropic levels in an ecosystem, an energy pyramid is probably the most informative, for the pyramid shape is not distorted by over emphasis on variations in the size and metabolic rates of the individuals. An energy pyramid more accurately, reflects the laws of thermodynamics, hence the pyramid is always right side up, with a large energy base at the bottom. A pyramid of energy must be based on a determination of the actual amount of energy that individuals take in, how much they burn up during metabolism, how much remains in their waste products, and how much they store in body tissue. The energy inputs and outputs are calculated so that energy flow can be expressed per unit of land or water per unit time. Let me explain this with an example.

An ecosystem receives 1000 calories of light energy in a given day. Most of the energy is not absorbed; some is reflected back to space; of the energy absorbed only a small portion is utilized by green plants, out of which the plant uses up some for respiration and of the 1000 calories, therefore only 100 calories are stored as energy rich materials.

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Now suppose an animal, say a deer, eats the plant containing 100 cal of food energy. The deer uses some of it for its own metabolism and stores only 10 cal as food energy. A lion that eats the deer gets an even smaller amount of energy. Thus usable energy decreases from sunlight to producer to herbivore to carnivore. Therefore, the energy pyramid will always be upright.