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What are ecological pyramids?

Class 12 BiologyNCERT chapter: Ecosystemइसे हिंदी में पढ़ें

Exam-ready answer

Ecological pyramids are graphic representations of the relationship between organisms at different trophic levels in terms of number, biomass or energy. The base is always formed by producers (the first trophic level) and the apex by top-level consumers. The three types are pyramids of number, biomass and energy; the pyramid of energy is always upright.

Explanation

In a food chain, organisms occupy trophic levels: producers, herbivores (primary consumers), carnivores (secondary consumers) and so on. When the number, biomass or energy at each level is drawn as a horizontal bar, with producers at the bottom, the bars usually get shorter going up and the figure looks like a pyramid. Any calculation of these values has to include all organisms at that trophic level, not just a sample.

In a pyramid of numbers, the individuals at each level are counted. In a pyramid of biomass, the dry weight of organisms per unit area is used. In most ecosystems both are upright, but there are exceptions. Many insects feeding on one big tree give an inverted pyramid of numbers, and in the sea the pyramid of biomass is generally inverted because the biomass of fishes far exceeds that of phytoplankton.

A pyramid of energy shows the energy at each trophic level. It is always upright and can never be inverted, because some energy is lost as heat at every step. On average only about 10 per cent of the energy passes from one trophic level to the next, the 10 per cent law. Ecological pyramids have limitations: they ignore a species that belongs to two or more trophic levels, assume a simple food chain and give no place to saprophytes.

Points to remember

  1. Ecological pyramid: base formed by producers (first trophic level); apex formed by top-level consumers.
  2. Three types: pyramid of number, pyramid of biomass and pyramid of energy.
  3. Pyramid of biomass in the sea is generally inverted because fish biomass far exceeds that of phytoplankton.
  4. Pyramid of energy is always upright because energy is lost as heat at each trophic level (10 per cent law).
  5. Limitations: a species at two or more trophic levels is not shown, a simple food chain is assumed and saprophytes get no place.

Exam tip

Draw all three pyramids neatly with each trophic level labelled and producers at the base, and give one example each of an inverted pyramid of numbers and an inverted pyramid of biomass.

Questions students also ask

Why can a pyramid of energy never be inverted?
When energy flows from one trophic level to the next, part of it is always lost as heat in respiration and other life activities. So each higher level receives less energy than the level below it, and the bar for top consumers is always narrower than the base.
Why is the pyramid of biomass inverted in the sea?
In the sea, the producers are tiny phytoplankton with a small standing biomass at any moment, though they multiply very fast. The biomass of fishes far exceeds that of the phytoplankton, so the consumer bar is wider than the base and the pyramid looks inverted.
What is the 10 per cent law?
The 10 per cent law states that, on average, only about 10 per cent of the energy at one trophic level is transferred to the next higher level. The rest is used in life processes or lost as heat, which is why food chains usually have only a few trophic levels.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.