Read the given passage and answer the following questions :
Besides acting as ‘conduits’ for energy transfer across trophic levels, predators play other important roles. They keep prey populations under control. But for predators, prey species could achieve very high population densities and cause ecosystem instability. When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have its natural predators. The prickly pear cactus introduced into Australia in the early 1920’s caused havoc by spreading rapidly into millions of hectares of rangeland. Finally, the invasive cactus was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced into the country. Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. If a predator is too efficient and overexploits its prey, then the prey might become extinct and following it, the predator will also become extinct for lack of food. This is the reason why predators in nature are ‘prudent’. Prey species have evolved various defenses to lessen the impact of predation. Some species of insects and frogs are cryptically-coloured (camouflaged) to avoid being detected easily by the predator. Some are poisonous and therefore avoided by the predators. The Monarch butterfly is highly distasteful to its predator (bird) because of a special chemical present in its body. For plants, herbivores are the predators. Plants have evolved an astonishing variety of morphological and chemical defences against herbivores. Thorns (Acacia, Cactus) are the most common morphological means of defence. Many plants produce and store chemicals that make the herbivore sick when they are eaten, inhibit feeding or digestion, disrupt its reproduction or even kill it. You must have seen the weed Calotropis growing in abandoned fields. The plant produces highly poisonous cardiac glycosides and that is why you never see any cattle or goats browsing on this plant. A wide variety of chemical substances that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.,) are produced by them actually as defences against grazers and browsers.
The Monarch butterfly is highly distasteful to its predator (bird). How does it get the chemical responsible for this?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → It acquires the chemical during its caterpillar stage by feeding on a poisonous weed
Prey species have many defences against predators. Some are distasteful, so predators avoid them. The Monarch butterfly does not make its bad-tasting chemical itself. It takes it in while it is a caterpillar, by feeding on a poisonous weed, and keeps it in its body as an adult.
Option (1) It makes the chemical in its adult body from flower nectar — Incorrect. The butterfly does not make the chemical from nectar. It gets it by feeding on a poisonous weed in the caterpillar stage.
Option (2) It acquires the chemical during its caterpillar stage by feeding on a poisonous weed — Correct. The chemical is taken in during the caterpillar stage from a poisonous weed and stays in the body of the adult.
Option (3) It inherits the chemical from its mother through the egg — Incorrect. The chemical is not passed down from the mother through the egg. Each butterfly gets it by its own feeding as a caterpillar.
Option (4) It sucks the chemical from the nectar of a poisonous weed as an adult — Incorrect. The weed is the right source, but the stage is wrong. The chemical is taken in by the caterpillar, not by the adult feeding on nectar.
NCERT Extract:
Population Interactions – “… Prey species have evolved various defenses to lessen the impact of predation. Some species of insects and frogs are cryptically-coloured (camouflaged) to avoid being detected easily by the predator. Some are poisonous and therefore avoided by the predators. The Monarch butterfly is highly distasteful to its predator (bird) because of a special chemical present in its body. Interestingly, the butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.”