Question

two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below.
Assertion (A): Greater numbers of individuals are present in lower trophic levels of all food chains.
Reason (R): As the amount of energy decreases at each successive trophic level, larger numbers of individuals are required at the lower trophic levels to support higher/next trophic levels.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true and Reason (R) is false.
• D. Assertion (A) is false and Reason (R) is true.

Answer 1

The Correct Option is A

Step 1: Understanding the Assertion (A):
The assertion states that in a typical food chain, the number of individuals decreases as you move up the trophic levels. This is a general observation in ecological systems, where producers form the base of the food chain and support primary consumers, secondary consumers, and so on. As energy is transferred from one trophic level to the next, a significant portion of the energy is lost, limiting the number of individuals that can be supported at higher levels.

Step 2: Understanding the Reason (R):
The reason provided supports the assertion. It explains that a large number of producers are needed to support a smaller number of primary consumers. Similarly, fewer primary consumers support an even smaller number of secondary consumers. This is due to energy loss at each trophic level, approximately 90%. The energy pyramid illustrates this, showing that energy is lost as heat at each level due to metabolic processes like movement, growth, and reproduction.

Step 3: Energy Loss in the Trophic Levels:
At each trophic level, energy is passed from one organism to another. However, only about 10% of the energy at one level is passed on to the next level. The remaining 90% is lost as heat, used for respiration, or released during metabolic activities. This is known as the "10% rule" in ecology, which explains why the number of individuals decreases as we move up the food chain.

Step 4: Connecting Assertion and Reason:
- The assertion claims that the number of individuals decreases at higher trophic levels due to the energy pyramid. - The reason elaborates on this by explaining that a large number of producers are required to support fewer primary consumers, and this continues for secondary and tertiary consumers. - The decrease in the number of individuals at each level is a direct result of the energy lost (about 90%) as you move up the pyramid.

Step 5: Conclusion:
Both the assertion and the reason are true and the reason correctly explains the assertion. The energy loss at each trophic level limits the number of individuals that can be supported at higher levels, and this is why fewer individuals exist at higher trophic levels in the food chain.