Ecology: Levels of organisation and species types

Both statements are correct and follow NCERT closely. NCERT says that in the successive seral stages there is a change in species diversity, 'an increase in the number of species and organisms as well as an increase in the total biomass'. It also notes that descriptions focus on vegetation, but these vegetational changes alter food and shelter, so 'the numbers and types of animals and decomposers also change'.

A trophic cascade is the chain of indirect effects that a change at one trophic level — usually the top predator — causes at lower levels. Predators suppress herbivores, which releases plants from grazing pressure; removing the predators reverses this. Such top-down effects have been shown in lakes, kelp forests and grasslands, and they are why loss of apex predators can change whole ecosystems.

Statements 1 and 2 are taken from NCERT. The prickly pear cactus, introduced into Australia in the early 1920s, caused havoc by spreading over millions of hectares until a cactus-feeding moth (Cactoblastis) from its natural habitat was introduced. The monarch butterfly acquires distasteful chemicals by feeding on a poisonous weed as a caterpillar, a defence against predators. Statement 3 is wrong: invasive species often spread because they have escaped the predators of their native range, so biological control usually introduces a natural enemy from the pest's native range, not from the invaded region.

Two — items 1 and 2. A tiger eating a chital is classic predation; NCERT points out that herbivores eating plants are, in a broad ecological sense, not very different from predators, so a grasshopper eating leaves also counts. Item 3 is brood parasitism: the koel's eggs resemble the crow's and the crow rears the koel's young. Item 4 is commensalism: barnacles gain a habitat and the whale is largely unaffected.

All three are correct. NCERT describes how removing the predatory starfish Pisaster from an enclosed area led to the extinction of more than ten invertebrate species within a year, because of interspecific competition — mussels in particular took over the space. The predator had been keeping the dominant competitor in check and so maintaining diversity. Robert Paine, who ran the experiment on the Washington coast from the 1960s, introduced the term 'keystone species' in 1969 for a species whose effect on the community is far larger than its abundance suggests.

Features 1, 2 and 3 are correct. In secondary succession the area has lost its community but keeps its soil, its seed bank and buried propagules (called residuals), and the organic matter and nutrients of the earlier community. Colonisers can therefore start growing at once, and NCERT notes that 'since soil is already there, the rate of succession is much faster'. Feature 4 describes primary succession on a new surface such as lava or a glacial moraine, where every propagule must arrive from outside, so it slows succession down.

All three are correct. In bottom-up control, resources (nutrients, light) set productivity, which limits every level above. In top-down control, predators regulate herbivores and, through a trophic cascade, plants or algae. Biomanipulation of lakes uses top-down control: adding piscivorous fish reduces planktivorous fish, which lets large zooplankton such as Daphnia multiply and graze algae, improving water clarity — an approach studied since the 1980s as a complement to cutting nutrient inputs.

All three statements are correct. In autotrophic succession green plants dominate from the start on a mainly inorganic substrate, so production (P) exceeds respiration (R) and organic matter piles up. In heterotrophic succession bacteria, fungi and other consumers dominate a substrate already rich in organic matter (sewage, leaf litter, a fallen log), so R exceeds P at first and stored energy declines. Odum (1969) showed that in both cases the P/R ratio moves towards 1 as the system matures, which is why it is used as an index of ecosystem maturity.