Ecology, Biodiversity and Pollution MCQs with Explanations

Producers form the first trophic level, so a herbivore that eats them is the primary consumer at the second level. Choosing 'secondary consumer' is the usual error of counting the plant level as level zero.

Most assimilated energy is spent on metabolism and lost as heat, with some leaving in waste and uneaten parts; this is Lindemann's ten-per-cent law of energy flow. Undigested waste (option 4) is a real but much smaller route, and option 1 misstates where the energy goes.

The standard formulation is that a habitat is an organism's address and its niche is its profession: the total of its requirements and relationships. Option 3 is the trap because it reduces the niche to diet alone.

Denitrifiers such as Pseudomonas and Clostridium reduce nitrates to nitrogen gas under anaerobic conditions, removing fertility from soil. Nitrobacter completes nitrification from nitrite to nitrate, while Rhizobium fixes nitrogen — the opposite direction, and the tempting distractor.

Within nodules, the enzyme nitrogenase reduces inert atmospheric N2 to ammonia, which the plant uses to build amino acids in an energy-costly symbiosis. Option 2 describes denitrification, which reduces rather than adds soil nitrogen.

Endemism means restriction to a particular area, which is why endemic species are disproportionately threatened by habitat loss there. The Indus river dolphin is a familiar Pakistani example, confined to the Indus system. A keystone species is defined by its outsized ecological role, not by range.

Ex-situ conservation means conserving components of biodiversity outside their natural habitat, as in seed banks, zoological parks, botanical gardens and captive breeding. Parks, sanctuaries and biosphere reserves protect ecosystems in situ, which is why they appear together as distractors.

The threat categories run Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild and Extinct. Options 1 and 2 place Vulnerable and Endangered out of order, and option 3 is the sequence reversed.

CITES works through import and export permits listed in its appendices, so it governs cross-border trade rather than hunting inside a country. Option 3 describes the Ramsar Convention on wetlands, a different instrument.

Biochemical oxygen demand measures the oxygen micro-organisms need to oxidise biodegradable matter over five days at 20 degrees, so a high BOD means heavy organic pollution and oxygen depletion. Chemical oxygen demand (COD) covers chemically oxidisable matter too, biodegradable or not.

In the stratosphere, UV liberates chlorine from chlorofluorocarbons, and each chlorine atom catalytically destroys many ozone molecules before it is removed. Carbon dioxide and sulphur dioxide are involved in warming and acid rain respectively, not in catalytic ozone loss.

Sulphur dioxide and nitrogen oxides are oxidised and dissolved in cloud water to form sulphuric and nitric acid, falling as rain with pH well below the natural value of about 5.6. Carbon oxides form weak carbonic acid only, and chlorine compounds are not the main cause.