Trace the journey of a pesticide molecule from a farmer's field all the way to a human body, explaining the role of each trophic level and the ecosystem process that makes the final concentration in humans the highest.
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Model Answer
Journey of a Pesticide Molecule — Biological Magnification
A farmer sprays pesticides on crops to protect them from pests. These chemicals are washed into the soil and water bodies by rain.
- 1st Trophic Level (Producers): Plants absorb the pesticide along with water and minerals. A small concentration accumulates in plant tissues.
- 2nd Trophic Level (Primary Consumers/Herbivores): Animals like insects or cattle eat large quantities of plants. Since the pesticide is non-biodegradable, it is not broken down and accumulates in their bodies at a higher concentration.
- 3rd Trophic Level (Secondary Consumers): Small carnivores eat many herbivores, concentrating the pesticide further.
- 4th Trophic Level (Humans/Tertiary Consumers): Humans, being at the top of the food chain, consume organisms from all lower levels. The pesticide accumulates to its maximum concentration in the human body.
This progressive increase in concentration of non-biodegradable chemicals at each successive trophic level is called biological magnification.
Source: Chapter 13, Section 13.1.1 — Food Chains and Webs
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Explanation
- The key concept examiners look for is biological magnification — name it and define it.
- Explain why concentration increases: the chemical is non-biodegradable (does not break down) and accumulates because organisms consume large amounts from the level below.
- Mention all 4 trophic levels and link each to the food chain: producers → herbivores → small carnivores → humans.
- The phrase "humans occupy the top trophic level" is directly from the textbook — use it.
- Do not confuse biological magnification with energy loss (10% rule); these are opposite concepts — energy decreases, pollutant concentration increases up the food chain.