If the carbon atoms fixed by producers already have passed through three species, the trophic level of the las

NCERT Exemplar · Q12

Q. If the carbon atoms fixed by producers already have passed through three species, the trophic level of the last species would be.

(a) scavenger

(b) tertiary producer

(d) secondary consumer

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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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When carbon atoms fixed by producers pass through three subsequent species in a food chain, the last species in that sequence is a tertiary consumer.

To understand the trophic level of the last species, we must first grasp the fundamental concept of trophic levels within an ecosystem. Trophic levels describe the position an organism occupies in a food chain, indicating how it obtains energy. Energy flows from one trophic level to the next, typically starting with producers.

Important

Trophic levels represent the feeding positions of organisms in an ecosystem, illustrating the flow of energy from producers to various levels of consumers.

Let's trace the path of the carbon atoms, which are essentially tracking the flow of energy in this scenario:

Producers: These are organisms like plants and algae that fix atmospheric carbon dioxide into organic compounds through photosynthesis. They form the base of almost all food chains and are considered the first trophic level. The carbon atoms originate here.

Now, let's follow the carbon atoms as they pass through subsequent species:

First Species (Primary Consumer): The carbon atoms are first consumed by a herbivore, an animal that eats the producer. This organism is called a primary consumer and occupies the second trophic level. Examples include deer eating grass or insects feeding on leaves.
Second Species (Secondary Consumer): Next, these carbon atoms are transferred when the primary consumer is eaten by a carnivore or an omnivore. This predator is known as a secondary consumer and occupies the third trophic level. For instance, a frog eating an insect, or a snake eating a mouse.
Third Species (Tertiary Consumer): Finally, the carbon atoms pass to a third species when the secondary consumer is eaten by another carnivore. This organism is called a tertiary consumer and occupies the fourth trophic level. An example would be an eagle preying on a snake, or a large fish eating a smaller fish.

The question states that the carbon atoms fixed by producers have already passed through three species. Based on our tracing:

The first species is the primary consumer.
The second species is the secondary consumer.
The third species is the tertiary consumer. …

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Q.If the carbon atoms fixed by producers already have passed through three species, the trophic level of the last species would be.

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