Ecology & Ecosystems — PPSC previous year questions

A keystone species has an effect on the ecosystem far larger than its numbers suggest. It holds the food web together. So removing it can trigger a cascade of extinctions and collapse the ecosystem. Robert Paine coined the term in 1969 after studying the starfish Pisaster. Sea otters, elephants and tigers are other examples. 'Limiting', 'vital' and 'top' species are not recognised ecological terms.

The biosphere is the narrow zone where land, water and air meet and support life. It includes the hydrosphere. It includes the upper part of the lithosphere — the soil and crust. It also includes the lower atmosphere, the troposphere. The stratosphere, above about 12 km, is too cold and thin to support life, so III is excluded. So I, II and IV.

Tropical rain forests have the highest annual net primary productivity of any terrestrial ecosystem (about 2,200 g/m²/year on average) because they enjoy high temperature, heavy rainfall and year-round sunlight. Tropical deciduous and temperate forests are less productive owing to dry or cold seasons; among all ecosystems only coral reefs, estuaries and swamps rival the rain forest.

Both statements are correct. As a community passes through the stages of ecological succession (seral stages) towards the stable climax stage, the number of species generally increases. In real landscapes this process is often stopped midway, either by natural events like fire and floods or by human activity such as farming, grazing and construction.

Both statements are correct definitions. Bioaccumulation is the gradual build-up of a persistent, non-biodegradable substance (such as DDT or mercury) within a single organism over its lifetime. Biomagnification is the increase in concentration of such a substance as it passes up successive trophic levels (feeding levels) of a food chain. R describes a different (though related) phenomenon and does not explain why bioaccumulation occurs, so R is not the correct explanation of A.

Statement I is incorrect. Photosynthesis did turn the early carbon-dioxide-rich atmosphere into an oxygen-rich one. But oxygen, at about 21%, is only the second most abundant component. Nitrogen, at about 78%, is the most common gas in the atmosphere. Statement II is correct. Today's oxygen level stays steady because photosynthesis releases oxygen while respiration and combustion use it up.

Repeated burning to drive game or clear land favours grass over trees. Grazing herbivores then multiply. They destroy tree seedlings by nibbling or trampling, so forest turns permanently into grassland. A is correct. It is also true that such human intervention weakens the normal biome–climate link. Savannah, prairie and steppe are believed to have arisen at least partly this way. So R is correct too. But R gives the general result of anthropogenic activity. It does not explain why burning turns forest into grassland. So R is not the explanation of A.

Assertion A is correct: invasive species typically establish themselves in habitats already disturbed by human activity, such as cleared land, ports and degraded wetlands. Reason R is incorrect because high reproductive rate, rapid dispersal and high genetic variability are the traits of invasive species themselves, not of the native species they displace.

Decomposition is fastest where it is warm and wet all year. Bacteria and fungi work fastest in high heat and moisture. The tropical evergreen rainforest has exactly these conditions. There, litter breaks down and nutrients return within weeks. A subtropical desert lacks moisture. The savanna has a long dry season. In the taiga (boreal forest) the cold slows decomposers badly. A thick layer of undecomposed litter builds up there, so it is the slowest of the four.

In the chain grass → grasshopper → frog → snake → hawk, each animal eats the one before it, so the feeding order is grasshopper, frog, snake, hawk. Only option B keeps that order, with the hawk at the top level. Strictly, green plants occupy the 1st trophic level and the grasshopper the 2nd; the question counts the four animals from the grasshopper as level 1.