Ecological Concepts and Ecosystem Dynamics

Grassland is a classic example of a disclimax or plagioclimax community, that is, a community held short of the theoretical climatic climax by recurrent disturbance and by a limiting resource rather than allowed to proceed to woodland. The two agencies that do the holding are moisture and fire. Rainfall in grassland belts is sufficient for grasses, whose root systems are shallow, fibrous and dense, but insufficient and too seasonal to sustain the deeper and longer term water demand of tree seedlings through the dry period. Fire, whether natural or set by graziers, kills woody seedlings, whose growing points are exposed, while leaving grasses to regenerate from basal meristems and underground rhizomes protected below the soil surface, and this asymmetry in the position of the growing point is the mechanism that makes fire selectively anti woody. Grazing works in the same direction.

• Option (c) names both agencies and is correct.
• Option (b) is incorrect on both limbs: grasslands are characteristically open and receive abundant insolation, which is why they are highly productive, and their soils, particularly the chernozems and mollisols of the temperate grasslands, are among the most fertile in the world.
• Option (a) is incorrect; insects and fungi act on grassland vegetation as they do everywhere and are not the controlling factor in arresting succession.
• Option (d) fails once (c) is established. The elimination route is option (b), which any candidate who knows grassland soils can reject outright, after which (c) is the only mechanism on offer.

• Statement 1 is correct and is close to the textbook definition: a food chain is the sequence of organisms through which energy and matter pass by consumption, running from producers through successive levels of consumers to decomposers, and the word order carries the essential idea of a directed sequence of trophic levels.
• Statement 2 is incorrect. A food chain is an inter specific relationship; it links different species occupying different trophic levels, and it cannot exist within the population of a single species, whose members by definition share a trophic position. Cannibalism within a population is a distinct phenomenon and is not what constitutes a food chain.
• Statement 3 is incorrect, and this is the sharper of the two errors. A food chain shows which organism is eaten by which, that is, the direction of energy flow, and says nothing quantitative. Numbers belong to a different construct, the pyramid of numbers, which is one of the three ecological pyramids alongside biomass and energy; conflating the chain with the pyramid is the mistake the statement is designed to elicit. Since only statement 1 survives, the official answer is (a). The governing distinction, worth stating to a class, is that a food chain is a qualitative map of trophic relations, a food web is the realistic branched version of it, and an ecological pyramid is the quantitative representation at each level.

The question tests the ammonification limb of the nitrogen cycle, that is, the return of organic nitrogen to the soil pool.

• Item 1 is correct. Urea excreted by animals is hydrolysed by soil urease to ammonia and carbon dioxide, and the ammonium so released is either taken up directly or nitrified to nitrate; this is the same chemistry that makes urea the principal manufactured nitrogenous fertilizer in India.
• Item 3 is correct. Dead vegetation is decomposed by soil bacteria and fungi, and the nitrogen locked in its proteins and nucleic acids is mineralised to ammonium, which is the largest natural input to the soil nitrogen pool in most terrestrial ecosystems.
• Item 2 is treated as incorrect. Coal combustion emits oxides of nitrogen, but the emission is to the atmosphere and the nitrogen leaves the system as a gaseous pollutant rather than entering the soil; the process the question is asking about is addition to the soil, and combustion of the fuel adds nothing to the ground it is burnt above. The official answer (c), one and three only, follows. The governing distinction is between mineralisation, which returns organic nitrogen to the soil, and combustion, which volatilises it.

The ecological niche is the sum of an organism's relations to its environment: not only where it lives but what it does there, what it eats and is eaten by, when it is active, what it tolerates in temperature, moisture and pH, and how it interacts with competitors and mutualists. Hutchinson's formulation as an n dimensional hypervolume captures exactly the combination of physical space and functional role that the question describes, and the distinction between the fundamental niche, the full range an organism could occupy, and the realised niche, the narrower range it actually occupies in the presence of competitors, is the standard elaboration.

• Option (b) is correct.
• Option (c), habitat, is the principal distractor and is defeated by the second half of the question: habitat is the physical address, the place where an organism lives, and carries no information about its role, the classical formulation being that the habitat is the address and the niche is the profession.
• Option (a), ecotone, is incorrect; an ecotone is a transition zone between two adjacent ecosystems, such as a mangrove between land and sea or a treeline between forest and alpine meadow, and is characterised by edge effect and elevated species richness.
• Option (d), home range, is incorrect; it is the area an individual animal traverses in its normal activities of foraging, mating and caring for young, which is a spatial measure only. The elimination route is the phrase functional role, which only the niche supplies.

Decomposers, also called saprotrophs, obtain energy by secreting enzymes onto dead organic matter and absorbing the products of extracellular digestion, and in doing so they mineralise organic compounds back to inorganic nutrients that producers can reuse.

• Item 2, fungi, qualifies and is the principal decomposer of the lignin and cellulose of plant litter, which few bacteria can attack.
• Item 3, bacteria, qualifies and dominates the decomposition of proteins and simpler compounds and of aquatic detritus.
• Item 1, virus, does not qualify. A virus has no metabolism of its own, no enzymes for extracellular digestion and no capacity to obtain energy; it is an obligate intracellular parasite that can replicate only by commandeering the machinery of a living host cell, so it cannot act on dead organic matter at all. Viruses do return nutrients to the water column by lysing living plankton, the process called the viral shunt, but that is predation on the living rather than decomposition of the dead. The official answer (b) follows. The governing principle is that decomposition is a metabolic activity performed on dead matter, which excludes viruses on both counts, and this is the same structural fact that decided option (d) in Q27 of this paper.

All three are recognised xerophytic adaptations directed at the same problem, that transpiration through the leaf is the principal route of water loss and the leaf is therefore the organ that must be modified.

• Statement 1 is correct: a thick cuticle of waxy cutin over the epidermis is largely impermeable to water and reduces cuticular transpiration, and the associated hardening, sclerophylly, resists wilting; sunken stomata and a hairy surface belong to the same suite.
• Statement 2 is correct: reducing the leaf area reduces the surface available for transpiration, and the extreme form is loss of the leaf altogether, in which case photosynthesis is transferred to a green stem, as in the phylloclade of Opuntia and the cladode of Asparagus, so the plant trades photosynthetic capacity for water conservation.
• Statement 3 is correct and is the same strategy carried further: in cacti the leaves are reduced to spines, which have almost no transpiring surface, and they perform two additional functions, deterring herbivores that would otherwise seek the plant's stored water and trapping a boundary layer of still air that reduces evaporative loss near the stem surface. All three being correct, the official answer is (d). The unifying principle is that every modification reduces the transpiring surface or the permeability of it, and the question tests whether the candidate recognises the common purpose behind three superficially different structures.

A mycorrhiza is the symbiosis between a fungus and a plant root in which the fungus receives photosynthate and the plant receives an enormously extended absorptive system, the external hyphae reaching soil volumes and pore sizes that root hairs cannot enter.

• Statement 1 is correct on both limbs and they are connected: the hyphal network increases the absorptive area by orders of magnitude, and that improved access to water and to immobile nutrients, phosphorus above all, is what confers drought tolerance, assisted by hyphal water transport and by improved soil aggregation around the root.
• Statement 2 is correct. Mycorrhizal association buffers the plant against soil chemical extremes, improving nutrient acquisition in strongly acidic and strongly alkaline soils and restricting the uptake and translocation of aluminium and of heavy metals by immobilising them in fungal tissue, which is why mycorrhizal inoculation is standard practice in the revegetation of mine spoil, overburden dumps and industrial wasteland, exactly the degraded sites the stem refers to.
• Statement 3 is correct. Mycorrhizal colonisation confers a measure of protection against soil borne root pathogens, by physical occupation of the infection sites on the root surface, by competition for photosynthate and root space, by the production of antifungal metabolites, and by inducing systemic resistance in the host. All three being correct, the official answer is (d). The organising idea worth giving a class is that a single symbiosis addresses simultaneously the three constraints that make a degraded site hostile, water shortage, chemical extremity and pathogen pressure, which is precisely why it is used in restoration.

• Statements 1, 2 and 4 are correct and statement 3 is incorrect, so the official answer is (c). Volcanic action releases carbon dioxide from the deep carbon reservoir through degassing at volcanic vents and mid-ocean ridges, and is the principal geological source that closes the long-term carbonate-silicate cycle. Respiration, whether in plants, animals or micro-organisms, oxidises organic carbon to carbon dioxide to release the energy stored in chemical bonds, and is a source by definition. Decay of organic matter is decomposer respiration under another name, in which saprotrophic bacteria and fungi mineralise dead tissue and return its carbon to the atmosphere as carbon dioxide, or as methane where conditions are anaerobic. Photosynthesis is the sole sink among the four, fixing atmospheric carbon dioxide into carbohydrate using light energy, and is therefore the planted false item. The governing principle is the distinction between a source and a sink, and the item requires nothing beyond it.
• Statement 3 alone decides the answer, and (c) is the only option that omits photosynthesis while retaining the other three.

• Option (b) is correct. A lichen is a mutualistic association between a fungal partner, the mycobiont, usually an ascomycete, and a photosynthetic partner, the photobiont, which is a green alga or a cyanobacterium or both. The fungus supplies the thallus structure, anchorage and protection from desiccation and ultraviolet radiation, and absorbs mineral nutrients; the photobiont supplies carbohydrate by photosynthesis.
• Options (a) and (c) are incorrect because neither omits nor supplies the correct pair, each dropping one of the two partners and substituting bacteria in its place.
• Option (d) is incorrect: mosses are bryophytes and are not lichen partners, although they commonly follow lichens in the same successional sequence, which is why the option is placed here. The ecological point in the stem is worth holding separately. Lichens are the pioneer community of a lithosere, the primary succession on bare rock, because they can survive extreme desiccation and secrete organic acids that chemically weather the rock surface. Their action, with their accumulated dead tissue, generates the first thin soil on which mosses and then herbs can establish.

• Option (a) is correct. A food chain runs from producer to primary consumer to secondary consumer, energy flowing in one direction and diminishing at each transfer. Diatoms are unicellular photosynthetic algae with silica frustules and constitute a large part of marine phytoplankton, so they are the producers and must come first. Crustaceans in this context are the zooplankton, principally copepods and krill, which graze on phytoplankton and are therefore primary consumers. Herrings are small pelagic fish that feed on zooplankton and are the secondary consumers, occupying the third trophic level.
• Options (b), (c) and (d) are all excluded by the single requirement that the producer must be at the base: (b) and (d) place crustaceans first, which would require an animal to be the producer, and (c) places herrings before crustaceans, which inverts predator and prey. The chain described is the classic short marine chain, and its shortness matters, since only about ten per cent of energy passes to the next level and marine chains with few links therefore support large fish populations.

• Option (c) is correct. An ecosystem, in Tansley's formulation of 1935, is the biotic community together with its physical environment considered as one interacting system, the essential point being the coupling of the living and the non living through energy flow and nutrient cycling. Remove the abiotic component and the concept collapses, because the transfer of energy through trophic levels and the cycling of carbon, nitrogen and phosphorus all pass through the physical environment.
• Option (a) describes the biotic community alone, which is a component of an ecosystem and not the whole of it.
• Option (b) describes the biosphere, the global sum of all ecosystems, and is a matter of scale rather than of definition.
• Option (d) describes the biota of an area, a taxonomic inventory that captures neither the interactions nor the environment. The elimination route is to look for the option that names both the organisms and their surroundings, since only one does, and the word best in the stem signals that more than one option will be partially true.

• Statement 1 is correct. Butterflies are pollinators, and although they are less efficient than bees for most crops, a number of wild plants with long tubular corollas and diurnal nectar presentation depend substantially on lepidopteran pollination. A collapse in butterfly numbers would depress reproduction in those plants.
• Statement 2 is incorrect and is the eliminating statement. There is no mechanism by which fewer butterflies would produce more fungal disease in cultivated plants. Butterflies are not fungal vectors, and if anything the relationship runs the other way, since caterpillar feeding damage creates entry wounds through which fungal pathogens invade, so fewer larvae would tend to reduce rather than increase infection.
• Statement 3 is correct. Butterflies and their larvae sit low in the food web and are prey and host to a large assemblage of predators and parasitoids. Parasitoid wasps in particular are often specialists on lepidopteran larvae, and many insectivorous birds and spiders depend heavily on caterpillar biomass, especially during the nestling period. Removal of the prey base propagates upward as a decline in those consumers. Rejecting statement 2 eliminates (b) and (d), and statement 1 being uncontroversial disposes of (a), leaving (c). Governing principle: trace the organism's two ecological roles, as pollinator and as prey, and test each proposed consequence against a stated mechanism rather than against plausibility.

All three are standard xerophytic adaptations and all three reduce transpirational water loss.

• Statement 1 is correct: a thickened cuticle and waxy coating raise the diffusive resistance of the leaf surface, and hard sclerophyllous tissue resists wilting at low water potential.
• Statement 2 is correct: reducing leaf area reduces the total transpiring surface, and small leaves also shed sensible heat more efficiently, so leaf temperature and hence the leaf to air vapour pressure difference stay lower.
• Statement 3 is correct: in Opuntia and many Acacia and Euphorbia species the leaf is reduced to a spine and photosynthesis shifts to the stem, which is the most extreme form of the same strategy. Since no statement can be rejected, only option (d) survives. The governing principle is that transpiration scales with the product of surface area and the conductance of that surface, so a desert plant can economise on either factor or on both.

Hence (d).

• Statement 1 is correct. The green sea turtle, Chelonia mydas, is herbivorous as an adult and grazes on seagrass and algae, and several freshwater and terrestrial chelonians are likewise plant feeders.
• Statement 2 is correct. Numerous reef fishes are herbivores, parrotfish and surgeonfish being the standard examples, and their grazing on algae is what keeps coral reefs from being overgrown.
• Statement 3 is correct. The sirenians, that is the dugong and the manatees, are marine mammals and are obligate herbivores feeding on seagrass, the dugong of the Gulf of Mannar and the Andaman waters being India's own example.
• Statement 4 is correct. Viviparity is widespread among snakes, sea snakes and vipers including the Russell's viper and the saw scaled viper giving birth to live young rather than laying eggs. Since all four hold, the official answer (d) follows. The item is easy because each statement is hedged by the word some, so a single counterexample validates it, and the dugong is sufficiently well known to carry statement 3, which is the only one a candidate is likely to hesitate over.

The savannah is a disturbance maintained biome, held below its climatic potential for forest by recurring suppression of tree seedlings.

• Item 2 is correct: periodic fire in the dry season kills woody saplings while grasses regenerate from protected basal meristems, which is the classic explanation for the persistence of savannah.
• Item 3 is correct: browsing and trampling by large herbivores, elephants especially, destroy young trees and maintain open ground.
• Item 4 is correct: a pronounced dry season creates the moisture deficit that trees cannot tolerate but deep rooted perennial grasses can, and it also supplies the dry fuel that makes fire possible.
• Item 1 is not accepted: burrowing animals and termites modify soil structure and nutrient distribution but do not control forest development at biome scale.
• Item 5 is likewise not accepted as a general controlling condition, since savannah occurs across widely differing soils. Hence 2, 3 and 4, giving (c). The unifying idea is that fire, grazing and seasonal drought are mutually reinforcing, which is why they are grouped in the key.

The fungus growing ants of the tribe Attini, of which the leafcutters are the best known, practise an obligate agriculture that is among the most striking mutualisms in nature. They cut and carry vegetation not to eat it but to use as substrate, on which they cultivate a fungus in underground gardens, tending it, weeding out competing moulds, applying antibiotic secretions from bacteria they carry on their bodies, and feeding on the specialised structures the fungus produces. Neither partner survives without the other, and the association is tens of millions of years old.

• Option (a) is therefore correct.
• Option (b) is wrong: cockroaches are generalist detritivores and, though they harbour gut symbionts that assist digestion, they cultivate nothing.
• Option (c) is wrong: some crabs tend algal gardens, but fungus cultivation is not a known crab habit.
• Option (d) is wrong: spiders are obligate predators of live prey. A candidate can reach the answer by recalling that fungus farming among insects is confined to attine ants, certain termites and some ambrosia beetles.

• Statement I is incorrect, and this is one of the most productive counterintuitive facts in ecology. Tropical rain forest soils are typically poor, being deeply weathered, acidic and heavily leached oxisols and ultisols. Almost the entire nutrient capital of the system is held in the living biomass rather than in the soil, and it is recycled rapidly through a thin litter layer by a dense mat of surface roots assisted by mycorrhizal fungi. This is why clearance for agriculture yields only a few seasons of fertility.
• Statement II is correct. Continuously high temperature and moisture drive very rapid microbial decomposition, so litter does not accumulate as it does in temperate or boreal forests. Note that Statement II is in fact part of the reason Statement I is false, since rapid decomposition combined with heavy rainfall leaches released nutrients out of the profile before they can accumulate.
• Statement I false and Statement II true gives (d).

The fig, genus Ficus, has an obligate mutualism with fig wasps of the family Agaonidae, in which each fig species is typically pollinated by its own specific wasp species. The wasp enters the enclosed inflorescence, the syconium, through the ostiole, pollinates the flowers and lays eggs, and the fig in turn provides the only site where the wasp can complete its life cycle. Neither partner can reproduce without the other, which is the textbook case of coevolved obligate pollination.

• Option (b) is wrong because Mahua, Madhuca longifolia, is pollinated by a range of generalist visitors including bats and insects.
• Option (c) is wrong because Sandalwood, Santalum album, is a hemiparasitic tree pollinated by various generalist insects.
• Option (d) is wrong because Silk cotton, Bombax ceiba, is chiefly bird pollinated, notably by sunbirds and mynas, and is not dependent on any single insect.

• Statement I is incorrect. Oceans, not rainforests, are the larger source of photosynthetic oxygen production. The popular description of the Amazon as the lungs of the planet is misleading in a further respect: a mature rainforest is close to carbon and oxygen neutral, because the oxygen released by photosynthesis is very largely consumed again by plant respiration and by microbial decomposition of litter on the forest floor.
• Statement II is correct: marine phytoplankton together with photosynthetic bacteria, notably cyanobacteria such as Prochlorococcus and Synechococcus, account for roughly half of global oxygen production, and by some estimates more.
• Statement III is incorrect, and the magnitudes are worth being precise about. Atmospheric air is about twenty-one per cent oxygen by volume, whereas even well-oxygenated surface water holds only about eight to ten milligrams of dissolved oxygen per litre, a concentration lower than air by a large factor rather than several folds higher. This scarcity of dissolved oxygen is exactly why aquatic organisms need gills with large surface areas and countercurrent exchange, and why warming water, which holds still less oxygen, threatens aquatic life. Only II holds, giving (b).
• Options (a) and (c) each retain a false statement, and option (d) discards the accurate one.