Write a short note on pyramid of numbers and pyramid of biomass.

NCERT Exemplar · Q49

Q. Write a short note on pyramid of numbers and pyramid of biomass.

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Concept understanding — Ecological Pyramid

Picture a real pyramid -- wide at the base, narrowing all the way up to a single point at the top. Ecologists borrow exactly this shape to describe something that has nothing to do with stone: how numbers, weight (biomass), or energy are distributed across the feeding levels of a food chain. This visual model is called an ecological pyramid.

Every ecological pyramid works the same way. In your NCERT textbook (Class 12 Biology, Chapter 12, Ecosystem, section 12.5), the base always represents the producers -- the first trophic level -- and each successive tier above it represents the next trophic level up (herbivores, then primary carnivores, then secondary or top carnivores), with the apex representing the top-level consumer. Three distinct types are studied, because the same food-chain relationship can be measured in three different ways:

Pyramid of numbers -- plots the count of individual organisms at each trophic level. NCERT's own grassland example is dramatic: nearly 6 million producer plants are needed to support just three top-carnivores at the apex.
Pyramid of biomass -- plots the standing crop (usually expressed as dry weight, which is more accurate than fresh weight) of organisms at each level, rather than headcount.
Pyramid of energy -- plots the amount of energy present at each trophic level, measured per unit area, usually annually.

Important

In most ecosystems, all three pyramids come out upright -- producers are more numerous and have more biomass than herbivores, and herbivores more than carnivores -- because energy shrinks at every step up the food chain (the same 10 per cent law behind energy flow: only about 10 per cent of the energy at one trophic level is transferred to the next).

When a pyramid turns upside down -- numbers and biomass pyramids don't always come out upright, and NCERT points to genuine exceptions you're expected to reason through, not just memorise:

A single large tree can support so many feeding insects that a pyramid of numbers built around it is inverted right at the base -- one producer, many primary consumers.
The pyramid of biomass in the sea is commonly inverted: at any given moment the standing crop of tiny, fast-reproducing phytoplankton is small, yet it is turning over fast enough to support a much larger standing crop of the zooplankton feeding on it.

Note

The pyramid of energy is the one type that is never inverted -- it is always upright, in every ecosystem, without exception. Energy is lost as heat at every transfer between trophic levels (unlike nutrients, it is never recycled back), so the level below always has to hold more usable energy than the level feeding on it. …

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Ecological pyramids graphically represent the relationship between different trophic levels in an ecosystem, with the pyramid of numbers showing individual counts and the pyramid of biomass showing total mass, both of which can be upright or inverted depending on the ecosystem.

Ecological interactions within an ecosystem are complex, involving the flow of energy and nutrients through various trophic levels. To simplify and visualize these relationships, ecologists use "ecological pyramids," also known as Eltonian pyramids, named after Charles Elton who first proposed them. These pyramids are graphical representations that illustrate the quantitative relationships between different trophic levels, such as producers, primary consumers, secondary consumers, and tertiary consumers. They can depict the number of individuals, the total biomass, or the energy content at each successive trophic level.

Pyramid of Numbers

The pyramid of numbers illustrates the number of individual organisms at each trophic level. The base of the pyramid represents the producers, and successive levels represent primary, secondary, and tertiary consumers.

Upright Pyramid: In most ecosystems, such as grasslands or pond ecosystems, the pyramid of numbers is upright. This means that the number of producers (e.g., grasses) is far greater than the number of primary consumers (e.g., deer), which in turn are more numerous than secondary consumers (e.g., wolves), and so on. Each successive trophic level has fewer individuals than the one below it.
Inverted Pyramid: In some specific cases, the pyramid of numbers can be inverted. A classic example is a parasitic food chain or a single large tree ecosystem.
- Consider a single large tree (producer). It can support numerous herbivorous insects (primary consumers) feeding on it, and those insects in turn support a smaller number of birds (secondary consumers). Because the base — one tree — is narrower than the insect level above it, the pyramid is inverted at its base.
Limitations: While simple to construct, the pyramid of numbers has limitations. It does not account for the size of the organisms; a single large tree is counted as one individual, just like a single blade of grass, despite their vast difference in contribution to the ecosystem. It can also be challenging to accurately count all individuals, especially for microscopic organisms or those with complex life cycles.

Pyramid of Biomass

The pyramid of biomass represents the total mass of living organisms (biomass) at each trophic level in an ecosystem. Biomass is typically measured as dry weight to avoid variations due to water content.

Upright Pyramid: In most terrestrial ecosystems, such as forests or grasslands, the pyramid of biomass is upright. The total biomass of producers (e.g., trees, grasses) is significantly larger than the total biomass of primary consumers (e.g., herbivores), which is greater than the biomass of secondary consumers (e.g., carnivores), and so forth. This reflects the general decrease in biomass as energy is transferred up the food chain.
Inverted Pyramid: The pyramid of biomass can be inverted in certain ecosystems, most notably in aquatic ecosystems.
- In a pond or ocean, the producers are microscopic phytoplankton. Their individual biomass is very small, and their total biomass at any given time might be less than that of the zooplankton (primary consumers) that feed on them. The zooplankton, in turn, might have less total biomass than the small fish (secondary consumers) that feed on them, and so on. This inversion occurs because phytoplankton have a very high reproductive rate and a short lifespan, meaning they are consumed rapidly and have a high turnover rate, even though their standing crop biomass at any moment is low. …

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Q.Write a short note on pyramid of numbers and pyramid of biomass.

Pyramid of Numbers

Pyramid of Biomass

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