UPSC Prelims 2010 Q23: Environment & Ecology – Pollution & Waste Management
A pesticide which is a chlorinated hydrocarbon is sprayed on a food crop. The food chain is:
Food crop - Rat - Snake - Hawk. In this food chain, the highest concentration of the pesticide
would accumulate
in which one of the following?
- (a) Food crop
- (b) Rat
- (c) Snake
- (d) Hawk
Answer: (d) Hawk
Explanation
Biomagnification in a food chain
Chlorinated hydrocarbon pesticides are known for bioaccumulation and biomagnification up the food chain.
| Point | Reasoning |
|---|---|
| Mechanism | Such pesticides are fat-soluble and non-biodegradable, so they accumulate in tissue rather than being excreted |
| Effect up the chain | At each trophic level (Food crop → Rat → Snake → Hawk), the pesticide concentration increases, since predators eat many prey, concentrating the toxin further |
| Highest concentration | The Hawk, as the top (apex) predator in this four-level chain, accumulates the highest concentration of the pesticide |
Biomagnification basics:
| Point | Detail |
|---|---|
| Cause | Chlorinated hydrocarbons are fat-soluble, resist breakdown |
| Pattern | Concentration increases at each successive trophic level |
| Highest point | Apex predator (here, the Hawk) |
Answer: (d) Hawk.
Why each option is right or wrong
- (a) Incorrect. The food crop is the starting point with the lowest concentration, not the highest.
- (b) Incorrect. The rat, being an early consumer, has a lower concentration than predators further up the chain.
- (c) Incorrect. The snake has a higher concentration than the rat but still less than the apex predator above it.
- (d) CORRECT. The hawk, as the apex predator in this chain, accumulates the highest concentration through biomagnification.
Key point
Chlorinated hydrocarbon pesticides biomagnify up the food chain — concentration increases at each trophic level, reaching its HIGHEST level in the apex predator (Hawk) in this food crop → rat → snake → hawk chain.
Remember
Biomagnification of chlorinated hydrocarbon pesticides: highest concentration accumulates in the apex predator (here, the Hawk).
Common mistakes
- Assuming pesticide concentration stays constant rather than increasing up the food chain.
- Not identifying the apex/top predator as the point of maximum biomagnification.
- Confusing bioaccumulation (within one organism over time) with biomagnification (increasing across trophic levels).
Exam tip
Biomagnification: the apex predator in any food chain always has the HIGHEST pesticide concentration. A standard, recurring ecology fact.
Sources
- NCERT Class XII Biology — Environmental Issues (biomagnification)
- Rachel Carson — Silent Spring (Houghton Mifflin, 1962)