3 marksShort AnswerSection B

What are trophic levels? Why are autotrophs considered to be at the first trophic level of all food chains? State the reason for limited number of trophic levels in nature.

Q: What are trophic levels? Why are autotrophs considered to be at the first trophic level of all food chains? State the reason for limited number of trophic levels in nature.

1. **Trophic Levels:** Each step or level of a food chain where the transfer of food and energy takes place is called a trophic level. 2. **Why Autotrophs are at the First Trophic Level:** Autotrophs (or producers) are at the first trophic level because they are the only organisms that can capture solar energy (using chlorophyll) and convert it into chemical energy (food) through photosynthesis, making this energy available for all heterotrophs (consumers). 3. **Reason for Limited Number of Trophic Levels:** According to the $10\%$ law of energy transfer, only an average of $10\%$ of the food eaten is turned into an organism's own body and made available for the next level of consumers. The remaining energy is lost as heat to the environment, used in digestion, growth, and reproduction. Because the loss of energy at each step is so great, very little usable energy remains after four trophic levels, limiting food chains to only three or four steps.

Our Environment

Food Chains and Webs

Official Answer

Trophic Levels: Each step or level of a food chain where the transfer of food and energy takes place is called a trophic level.
Why Autotrophs are at the First Trophic Level: Autotrophs (or producers) are at the first trophic level because they are the only organisms that can capture solar energy (using chlorophyll) and convert it into chemical energy (food) through photosynthesis, making this energy available for all heterotrophs (consumers).
Reason for Limited Number of Trophic Levels: According to the $10\%$ law of energy transfer, only an average of $10\%$ of the food eaten is turned into an organism's own body and made available for the next level of consumers. The remaining energy is lost as heat to the environment, used in digestion, growth, and reproduction. Because the loss of energy at each step is so great, very little usable energy remains after four trophic levels, limiting food chains to only three or four steps.

trophic levelautotrophsproducerssolar energyphotosynthesis10% lawenergy lossthree or four steps

Marking Scheme

1Definition of Trophic Levels (1 mark)
2Explanation of why autotrophs are at the first trophic level (fixing solar energy/photosynthesis) (1 mark)
3Reason for limited trophic levels (energy loss/10% law of energy transfer) (1 mark)

Hint

Recall the definition of trophic levels, how autotrophs produce food using sunlight, and the $10\%$ law of energy transfer.

Quick Oral Answer

Trophic levels are the steps in a food chain. Autotrophs are at the first level because they produce food from sunlight. Food chains are limited to 3-4 steps because only $10\%$ of energy is transferred to the next level, leaving too little energy at higher levels.

Analysis & Explanation

In an ecosystem, energy flows unidirectionally starting from autotrophs. Autotrophs capture about $1\%$ of the sunlight falling on their leaves and convert it into food energy. When herbivores eat these plants, a massive amount of energy is lost as heat, and only $10\%$ is stored as organic matter to be passed to the next level. This progressive decline in available energy at each successive trophic level limits the length of food chains to a maximum of 3 or 4 steps, as beyond that, the energy is insufficient to support another trophic level.

Common Mistakes

1Confusing the $1\%$ solar energy capture by plants with the $10\%$ energy transfer between trophic levels.
2Stating that $90\%$ of energy is transferred instead of stating that $90\%$ is lost and only $10\%$ is transferred.

Interesting Facts

Because of the progressive loss of energy, the number of individuals is generally greatest at the lower trophic levels (producers) and lowest at the top trophic levels (carnivores).

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2022?

This question carries 3 marks in the CBSE Class 10 Science 2022 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Our Environment" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Food Chains and Webs" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2022 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2022 paper.