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.
In the rocky intertidal communities of the American Pacific Coast, the removal of which predator led to the extinction of more than 10 species of invertebrates within a year?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The starfish Pisaster
Predators do more than eat their prey. By keeping the numbers of prey species down, they reduce the competition among those prey species, and so help many species to live together. Remove the predator, and the prey species compete so strongly that many of them are wiped out, as an experiment on the American Pacific Coast showed.
Option (1) The barnacle Balanus — Incorrect. Balanus is a barnacle, not the predator in this experiment. It is the book’s example of competition: on the coasts of Scotland it pushes the smaller barnacle Chthamalus out of the intertidal zone.
Option (2) The starfish Pisaster — Correct. Pisaster is the starfish predator whose removal led to the loss of more than 10 invertebrate species.
Option (3) The barnacle Chthamalus — Incorrect. Chthamalus is the smaller barnacle that loses the competition with Balanus in Scotland. It is not a predator.
Option (4) Flamingos — Incorrect. Flamingoes are the book’s example of unrelated species competing: in some South American lakes they compete with fishes for zooplankton. They are not part of the Pacific Coast experiment.
NCERT Extract:
Population Interactions – “… Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. In the rocky intertidal communities of the American Pacific Coast the starfish Pisaster is an important predator. In a field experiment, when all the starfish were removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year, because of interspecific competition.”