Q.Which among the following always decreases in a Food chain across tropic levels?

DDT biomagnifies up the aquatic food chain, reaching its highest concentration in top-level fish-eating birds such as the seagull.

DDT, being fat-soluble and non-biodegradable, is not readily excreted once absorbed by an organism and instead accumulates in body tissues. As one organism is eaten by the next in a food chain (phytoplankton → zooplankton/small fish → larger fish → fish-eating birds), the DDT concentration is progressively magnified at each trophic level — a phenomenon called biomagnification. Since fish-eating birds like seagulls occupy the top of this aquatic food chain and consume large quantities of contaminated fish over their lifetime, they accumulate the highest DDT concentration, historically causing eggshell thinning and reproductive failure in such birds. O …