Q.102 The term biological magnification refers to the increased levels of a toxin seen in successive trophic levels in a food web.
Which one of the following options correctly states the reason(s) for the increment of a toxin in the ecosystem?
(A) The toxin is highly toxic to primary producers, relatively less toxic to primary consumers, and non-toxic to secondary consumers.
Thus, a higher level of toxin is seen in species representing higher trophic levels
(B) The toxin cannot be degraded by microorganism and consequently persist in the environment for years
(C) The toxin to begin with was not toxic or less toxic, but became more toxic by metabolism in the primary producers
(D) Both (B) and (C)
Biological magnification refers to increased toxin concentrations across successive trophic levels in food webs, primarily because persistent, non-degradable toxins accumulate and concentrate as biomass decreases up the chain.
Correct Answer
(B) The toxin cannot be degraded by microorganism and consequently persist in the environment for years
Option Analysis
(A) Toxin Highly Toxic to Producers, Less to Consumers
Incorrect. Toxicity variation doesn’t cause magnification; even equally toxic substances magnify if persistent. Magnification stems from unequal biomass transfer (10% rule) where predators ingest multiple contaminated prey, concentrating lipophilic toxins in fat.
(B) Toxin Not Degraded by Microorganisms, Persists
Correct. Primary reason—persistent organic pollutants (POPs) like DDT, PCBs resist microbial breakdown, remaining bioavailable. They bioaccumulate in organisms and biomagnify as predators consume many contaminated prey while excreting little toxin.
(C) Toxin Becomes More Toxic via Producer Metabolism
Incorrect. While some bioactivation occurs (e.g., plant cytochrome P450 converting pro-toxins), this explains metabolic toxification, not magnification pattern across trophic levels. Magnification requires persistence first.
(D) Both (B) and (C)
Incorrect. (C) is secondary; core driver is (B) persistence enabling food chain transfer.
| Reason | Causes Magnification? | Mechanism |
|---|---|---|
| Variable toxicity (A) | No | Doesn’t explain concentration increase |
| Microbial persistence (B) | Yes | Enables bioaccumulation + biomagnification |
| Metabolic activation (C) | Partial | Toxification ≠ trophic level buildup |
| Both B+C (D) | No | Overstates C’s role |
Mechanism Details
Toxins enter at producers (e.g., DDT via water), persist undegraded, bioaccumulate (organism body burden rises), then biomagnify: phytoplankton (1 ppm) → zooplankton (10 ppm) → fish (100 ppm) → eagles (25 ppm+). Fat-solubility and ~90% biomass loss per level drive 10x concentration jumps.
