AIIMS 2003 Botany Food Chains Assertion Reason Question

Explanation of the Question

In the study of ecology, food chains illustrate the flow of energy and nutrients through different levels of organisms in an ecosystem. The assertion and reason provided in the question relate directly to the structure of these food chains.

Assertion:

"In a food chain, members of successive higher levels are fewer in number."

This assertion is true. In any given food chain, the number of organisms decreases as you move up the trophic levels. This is often represented visually in an ecological pyramid, commonly referred to as a pyramid of numbers.

Reason:

"Number of organisms at any trophic level depends upon the availability of organisms which serve as food at the lower level."

This reason is also true. The population size at each trophic level is inherently linked to the availability of energy and nutrients from the lower levels. For example, primary producers (e.g., plants) provide energy for consumers (e.g., herbivores), and the number of herbivores is limited by the amount of plant life available. Consequently, fewer herbivores can support fewer carnivores (secondary consumers).

Correct Answer:

A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

The reason explains the assertion by showing the dependency of higher trophic levels on lower ones, establishing a direct relationship between the availability of food and the population size at each level.

Why Other Options are Incorrect

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.

  • This option is incorrect because the reason directly explains why the assertion is true. The decrease in numbers at higher trophic levels is indeed due to the limited availability of food from the lower levels.

• C) Assertion is true but Reason is false.

  • This option is incorrect as both the assertion and reason are true. The reason does not contradict the assertion; rather, it supports it.

• D) Both Assertion and Reason are false.

  • This option is clearly incorrect since both the assertion and the reason are true statements about ecological food chains.

Additional Concepts

The decline in number of organisms as we ascend trophic levels can also be understood through the concept of energy transfer efficiency. According to the 10% Law of Energy Transfer, only about 10% of the energy from one trophic level is converted into biomass at the next level. This inefficient transfer of energy further accounts for the smaller number of organisms at higher trophic levels.

Mathematically, if we denote:

• $N_1$ = number of primary producers
• $N_2$ = number of primary consumers
• $N_3$ = number of secondary consumers

Then we can say: $N_2 \approx 0.1 \times N_1$ $N_3 \approx 0.1 \times N_2$

Thus, as we move up the food chain: $N_3 \approx 0.1 \times (0.1 \times N_1) = 0.01 \times N_1$

This illustrates the diminishing numbers as you progress through the food chain, reinforcing the assertion that members of successive higher levels are fewer in number.

Conclusion

In summary, both the assertion and reason provided in the question are true, and they are interlinked, confirming that the correct answer is (A). Understanding these concepts is crucial for grasping the dynamics of ecosystems and the relationships between different organisms within a food chain.