Read the following passage and answer the questions that follow:
A farmer applied large quantities of pesticides to his paddy fields over several seasons to improve yield. After heavy rains, the water carrying dissolved pesticides drained into a nearby pond. The pond supported a food chain: Algae → Small fish → Large fish → Fish-eating birds. After a few years, scientists noticed that the fish-eating birds in the area were dying in large numbers. Chemical analysis showed that these birds had very high levels of pesticides in their bodies, even though the concentration of pesticides in the pond water itself was very low.
(i) Name the phenomenon responsible for the high pesticide concentration in the fish-eating birds. [1]
(ii) Arrange the four organisms — Algae, Small fish, Large fish, Fish-eating birds — in increasing order of pesticide concentration in their bodies. [1]
(iii) Explain why the concentration of pesticides is highest in the fish-eating birds even though the water contains only a very small amount of the chemical. [2]
Generated by claude-sonnet-4-6 · 2026-06-26 01:00 · grounding rag
Model Answer
(i) The phenomenon responsible is Biological Magnification.
(ii) Increasing order of pesticide concentration:
Algae < Small fish < Large fish < Fish-eating birds
(iii) Pesticides are non-biodegradable chemicals. When algae absorb them from water, small fish eat large amounts of algae, accumulating more pesticides. Large fish eat many small fish, concentrating the chemical further. Fish-eating birds, being at the top trophic level, consume the most large fish, so the maximum concentration accumulates in their bodies. Since the pesticides cannot be broken down or excreted, they build up progressively at each trophic level — a process called biological magnification — resulting in the highest levels in fish-eating birds, even though the water has very little of the chemical.
Source: Chapter 13, Section 13.1.1 — Food Chains and Webs
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Explanation
- (i) is a straight one-mark recall — just name the term correctly.
- (ii) is one mark for the correct sequence; the food chain order directly gives the order of increasing pesticide concentration.
- (iii) is 2 marks — examiners expect two key ideas: (a) non-biodegradable nature causes accumulation at each trophic level, and (b) organisms at higher levels eat many organisms below, so concentration keeps increasing. Mentioning "biological magnification" again here strengthens the answer. Keep it concise — roughly 50–60 words for 2 marks.