Chapter 13 · Question 6

What is biological magnification? Explain with an example how harmful chemicals enter the food chain and accumulate at each trophic level. Will the levels of magnification be the same at all trophic levels?

Answer Revealed

Direct Answer:

Biological magnification (biomagnification) is the phenomenon where non-biodegradable chemicals like pesticides accumulate progressively in higher concentrations at each higher trophic level in a food chain. For example, when pesticides are sprayed on crops, they wash into the soil or water bodies. In water, aquatic plants absorb small amounts of these chemicals. When small fish eat many aquatic plants, the pesticide accumulates in their bodies. Larger fish that eat several small fish concentrate even more pesticide. Since humans occupy the top level in many food chains, the maximum concentration accumulates in human bodies. The levels of magnification are not the same — they increase at each higher trophic level.

Simple Explanation

Imagine a pond sprayed with pesticide. The water has a tiny amount of pesticide. Aquatic plants absorb it — they now have slightly more pesticide than the water. Small fish eat many aquatic plants, collecting all their pesticide — the fish's body now has even more. Big fish eat many small fish, packing in all their pesticide. By the time a human eats the big fish, the pesticide has been concentrated many times over. This is why even small amounts of pesticides in the environment can become dangerous high up the food chain. The level increases at each step — it is lowest in the water, higher in plants, even higher in fish, and highest in humans.

Exam-Ready Structure

Biological magnification is one of the most serious ecological consequences of modern agricultural and industrial practices, detailed in NCERT Class 10 Chapter 13 under the discussion of how harmful chemicals enter and move through food chains. 1. Definition: Biological magnification (also called biomagnification) refers to the progressive increase in the concentration of non-biodegradable toxic substances in organisms at successively higher trophic levels of a food chain. 2. Mechanism — how chemicals enter the food chain: (a) Harmful chemicals, such as pesticides (DDT, organophosphates) and other crop-protection chemicals, are used in agricultural fields. (b) These chemicals are either washed down into the soil with rainwater or carried into nearby water bodies like ponds, lakes, and rivers. (c) From the soil, terrestrial plants absorb the chemicals along with water and minerals. (d) From water bodies, aquatic plants (phytoplankton) and animals take up these chemicals. (e) Once inside an organism's body, these chemicals are not broken down (non-biodegradable) and persist in the tissues. 3. Accumulation at successive trophic levels: (a) A primary consumer (herbivore) eats many producer organisms — each containing a small amount of the chemical. The chemical accumulates in the consumer's body. (b) A secondary consumer eats several primary consumers, concentrating the chemical further. (c) At each step, the concentration multiplies because the consumer retains the chemicals from all the organisms it has eaten. (d) Since humans occupy the highest trophic level in many food chains, the maximum concentration of these harmful chemicals accumulates in human bodies. 4. Evidence: Food grains (wheat, rice), vegetables, fruits, and even meat contain varying levels of pesticide residues, which cannot always be removed by washing or other means. This is why Activity 13.3 in the textbook asks students to debate the need for banning ready-made food items found to have high pesticide levels. 5. Are levels the same at all trophic levels? No. The concentration is lowest at the producer level and increases at each higher trophic level. Top carnivores (and humans at the top of food chains) bear the highest concentrations of these toxic chemicals.

Key Points

Biological magnification: progressive increase in concentration of non-biodegradable chemicals at each higher trophic level
Non-biodegradable chemicals (e.g., pesticides, DDT) enter food chains via soil and water
Chemicals are absorbed by producers, then concentrated in herbivores, then carnivores, then top carnivores
Humans occupy the top trophic level in many food chains and accumulate the highest concentrations of toxic chemicals
Levels are NOT the same — concentration increases at each higher trophic level
Pesticide residues in food grains, vegetables, fruits, and meat cannot always be removed by washing

Common Mistakes

Confusing biological magnification with biomagnification in the context of population size (which decreases at higher levels) — concentration of toxins increases even as population size decreases
Assuming washing food removes all pesticide residues — some pesticides are absorbed into the tissues and cannot be washed off

What is biological magnification? Explain with an example how harmful chemicals enter the food chain and accumulate at each trophic level. Will the levels of magnification be the same at all trophic levels?