Q.Indiscriminate use of chemicals, pesticides and weedicides by humans are polluting our water bodies, which in turn are harming the living organisms. Study the flow chart and answer the questions based on it.
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Start your 14-day free trial to unlock the full solution →Persistent chemicals like DDT accumulate and magnify at each trophic level because they are fat-soluble and non-biodegradable; top predators suffer the highest toxicity. This is biomagnification.
The question asks us to trace the fate of a persistent pesticide like DDT as it moves through a food chain. The key insight is that not all substances dilute as they pass from prey to predator — some do the opposite.
DDT and similar organochlorine pesticides are fat-soluble (lipophilic) and chemically stable. When a small organism absorbs DDT from contaminated water or food, it cannot break the chemical down or excrete it efficiently. The DDT dissolves into the organism's fatty tissues and stays there. Now imagine a fish eating hundreds of these small organisms over its lifetime: the fish accumulates DDT from every meal, but never loses it. A bird that eats many such fish inherits the accumulated burden from all of them. At each step up the food chain, the concentration multiplies.
(a) Why does the concentration of DDT seem to be considerably high in the top consumer?
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Non-biodegradability: DDT is chemically stable; organisms lack enzymes to break it down into harmless products. Once absorbed, it persists.
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Lipid solubility: DDT dissolves in fats and oils, not water. It gets stored in adipose tissue and cell membranes rather than being flushed out in urine.
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Accumulation over time: A primary consumer (say, zooplankton) absorbs small amounts of DDT from the water and phytoplankton. Over its life, this builds up in its body.
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Magnification at each trophic level: A secondary consumer (small fish) eats many zooplankton. The DDT from all those prey items concentrates in the fish's tissues. A tertiary consumer (large fish or bird) eats many small fish, inheriting the cumulative DDT load from each.
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Concentration factor: If each trophic transfer involves eating, say, 10 kg of prey to gain 1 kg of body mass (a typical 10% energy efficiency), then the predator concentrates the toxin roughly tenfold. Across three or four trophic levels, this can mean a million-fold increase from water to top predator.
The top consumer thus carries the compounded burden of every organism below it in the food web.
(b) How would the organisms at the highest level be affected?
The effects are severe because the dose in top predators reaches toxic thresholds:
| Effect | Mechanism |
|---|---|
| Reproductive failure | DDT interferes with calcium metabolism in birds, leading to thin, fragile eggshells that break during incubation. This caused dramatic declines in raptors like the peregrine falcon and bald eagle. |
| Nervous system damage | DDT disrupts sodium–potassium ion channels in nerve cells, causing tremors, seizures, and behavioral abnormalities. |
| Immune suppression | High toxin loads weaken the immune system, making organisms more susceptible to disease. |
| Reduced fertility | Hormonal disruption (endocrine interference) lowers reproductive success even when adults survive. |
| Population decline | With fewer offspring surviving and adults suffering chronic toxicity, top-predator populations crash. This destabilizes the entire ecosystem. |
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