Some species if eliminated seriously affect the ecosystem. These are called

Question

UnlockIAS · Accepted Answer

Correct — option (4), Keystone. A keystone species is one whose effect on the structure of its community is out of all proportion to its abundance, so that its removal changes the ecosystem far more than its numbers or its biomass would lead anyone to expect. The name is an architectural metaphor and the metaphor is exact: the keystone is the wedge-shaped block at the crown of a stone arch, no larger than the other blocks and often the last to be put in place, but the one whose removal brings the arch down. The idea comes from an experiment. Robert Paine worked on the rocky shore of the Pacific north-west and repeatedly removed the predatory starfish Pisaster from a stretch of intertidal rock; freed from predation, mussels spread over the rock and crowded out the other occupants, and the number of species present fell sharply. The starfish had been holding the community open by keeping its dominant competitor in check, and that is what a keystone species does. Other familiar examples run the same way: sea otters keep sea urchins down and so allow kelp forests to persist; elephants open woodland and maintain grassland by their feeding and movement; fig trees fruit when little else does and carry a whole community of frugivores through the lean season. The practical point for conservation is that species cannot be ranked by abundance alone, since protecting a rare keystone may preserve more of an ecosystem than protecting a common species would. The stem's English trails off after 'These are called' with no blank printed, but its sense is plain.

Why the other options are wrong — (a) 'Blue stone' is not an ecological term at all. It is offered to make the option set look like a family of four colour-coded categories, which is a device the Commission uses when the real answer is a single technical word and the other three options have to be manufactured. The candidate's protection is to notice that only one of the four options is a term he has ever met in an ecology text, and that the other three vary only by a colour word. Where three options differ from one another by a single interchangeable element, they are almost always the invented ones, and the odd option out is the term being tested.  (b) 'Red stone' is likewise not an ecological category. It may draw a candidate who half-remembers the IUCN Red List or the Red Data Book, both of which are genuine and both of which concern threatened species, and who supposes that a 'red' term must belong somewhere in conservation vocabulary. But the Red List classifies species by their risk of extinction, which is an entirely different idea from the disproportionate ecological influence the stem describes; a keystone species may be common or rare, and its keystone status has nothing to do with its conservation status. Recognition of a colour rather than of a concept is what this option punishes.  (c) 'Yellow stone' is the sharpest of the three invented options, because Yellowstone is a real and famous place — the national park in the United States, and the one most often cited in ecology teaching for the return of wolves and the changes that followed in the elk population and the vegetation. That story is in fact a keystone-predator story, so the word is doing double duty as a distractor: it is close enough to the right idea to reward half-knowledge with the wrong answer. But the concept asked for is named after the stone in an arch, not after a park, and 'pyramid of Yellow stone' is not a category in any ecology text.

Concept: Ecology distinguishes several kinds of species that matter more than their numbers suggest, and MPSC expects the categories to be kept apart. A keystone species holds the structure of a community together through its interactions — usually predation or another form of regulation — so that removing it produces a cascade of change; the starfish on a rocky shore, the sea otter in a kelp forest and the elephant in a savanna are the standard examples. A foundation or dominant species, by contrast, shapes a community through sheer abundance, as corals build a reef and oaks make an oak wood. An ecosystem engineer physically alters habitat, as a beaver does by damming a stream. An indicator species signals the condition of an environment, as lichens do for air quality. A flagship species is chosen for its public appeal and used to raise support for conservation, and an umbrella species is one whose large habitat requirements incidentally protect many others when its range is conserved — the tiger in India is routinely described in both these ways. The categories overlap in practice, but they are defined by different criteria: influence out of proportion to abundance for the keystone, abundance itself for the dominant, habitat modification for the engineer, and human or administrative usefulness for the flagship and the umbrella.

State PCS 2023 Q20 — Environment & Ecology PYQ with Answer

State PCS 2023 Q20 — Environment & Ecology PYQ with Answer