A pyramid of numbers is constructed by

Step 1: Analyze the question The question asks how a pyramid of numbers is constructed. It is an ecology topic about trophic levels in a food chain.

Step 2: Explain the concept A pyramid of numbers shows the number of individual organisms at each trophic level of a food chain (producers → primary consumers → secondary consumers → and so on). Each level is drawn as a bar, and the bar's length or area is proportional to the number of organisms.

To get those numbers, you have to count the organisms at each level in a given area. This counting of a population is a census.

Step 3: Check each option

• A. Comparing living and non-living things: This describes the study of an ecosystem's biotic and abiotic components. It is not how the pyramid is built.
• B. Measuring productivity: This relates to a pyramid of energy or biomass, which measures energy or mass, not numbers.
• C. Conducting census: Counting the organisms at each trophic level is what produces a pyramid of numbers. ✔
• D. Labelling the living organisms: Labelling only names the organisms. It does not give their numbers.

Step 4: Answer The pyramid of numbers is built by conducting a census of the organisms at each trophic level.

Exam tip: Match the pyramid to what it measures:

• Numbers → counting (census)
• Biomass → mass of organisms
• Energy → productivity or energy flow

Note that a pyramid of numbers is not always pyramid-shaped. For example, one big tree supporting many insects gives an inverted pyramid.