Question

Explain biomagnification of DDT in an aquatic food chain. How does it affect the bird population?

Answer 1

Hint: Nutrients and the other compounds are broken down by decomposers and hence, they do not accumulate in the ecosystem. Decomposition drives nutrient cycling in nature. However, decomposers cannot metabolize some compounds and these compounds are accumulated in the tissues of the living beings.

 Complete answer:
We will first learn about the food chain and then move to biomagnification.
To understand biomagnification, we should know about the interdependence of living beings for food and energy. This interdependence is represented in the form of the food chain when organisms at one step serve as a source of energy and food for the ones present at the next step. Each step of a food chain is called a trophic level.
As the name suggests, aquatic food chains represent the interrelationship of animals present in aquatic ecosystems. Besides passing the nutrients, the food chain also passes the harmful substances stored in the tissues of the organism at the lower trophic levels to the ones present at the higher trophic levels.
For example, DDT is a toxic substance that cannot be metabolized easily and hence, accumulates in the adipose tissues of the organisms. The food chain moves DDT from producers to consumers. Its levels are built up from the producers to the top consumers. For instance, the 0.04 ppm DDT in planktons can magnify up to 0.16 ppm in the primary consumer (zooplanktons), 0.28 ppm in the secondary consumer (small fishes), 2.07 in the tertiary consumer (big fishes), and finally reaching 75.5 ppm DDT in the fish-eating birds.
The accumulation and increased concentration of the persistent toxic metabolites in the tissues of the top consumers of food chains are called biomagnification.
Since DDT is slowly decomposed, it exhibits biomagnification. One of the effects of biomagnification of DDT on birds is that they lay eggs with very thin and fragile shells. As we know that shells protect the developing embryo, the eggs with weak shells usually break during incubation and cause the death of the developing young ones. The overall effect is a decline in the bird population.

Note: Here, we should know that the effect of DDT on the eggshells is due to disturbed calcium metabolism. Calcium carbonate is the main compound of the eggshells and makes them strong. Biomagnification also shows the role of natural decomposers in the ecosystem. The biomagnified toxins are not decomposed by natural decomposers such as bacteria and therefore, are accumulated at the toxic levels.