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.
Why don't cattle or goats browse on the weed Calotropis?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → It produces highly poisonous cardiac glycosides.
The passage states that “The plant produces highly poisonous cardiac glycosides and that is why you never see any cattle or goats browsing on this plant.”
Plants cannot run away from herbivores, so they defend themselves in other ways. Some use morphological defences such as thorns. Many use chemical defences: they make and store chemicals that make the herbivore sick or even kill it. Calotropis uses a chemical defence.
Option (1) It has thorns that prevent browsing. — Incorrect. Thorns are a defence of plants such as Acacia and cactus. Calotropis is protected by a poison, not by thorns.
Option (2) It produces highly poisonous cardiac glycosides. — Correct. Calotropis makes highly poisonous cardiac glycosides, so cattle and goats do not browse on it.
Option (3) It grows in inaccessible areas. — Incorrect. Calotropis grows in abandoned fields, which are easy for cattle to reach.
Option (4) It produces nicotine, which repels browsing animals. — Incorrect. Nicotine is a defence chemical of the tobacco plant. It is not the chemical that protects Calotropis.