Questions & explanations
1. Consider the following statements about the oceanic carbon cycle:
1. The biological pump transfers carbon from surface waters to the deep ocean.
2. Cold water can hold more dissolved CO2 than warm water.
3. Ocean acidification reduces the ocean's ability to absorb CO2.
Which of the statements given above is/are correct?
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (d) 1, 2 and 3
Statement 1 (biological pump transfers carbon to deep ocean) is correct. Statement 2 (cold water holds more dissolved CO2) is correct — CO2 solubility increases at lower temperatures. Statement 3 is incorrect: ocean acidification (lowering of pH) actually reduces the ocean's buffering capacity and can slow CO2 absorption, but the mechanism stated is debated and overstated as a blanket fact; the more accurate statement is that acidification reduces carbonate ion concentration which affects shell-forming organisms, not that it reduces overall CO2 absorption capacity straightforwardly. However, looking carefully: the standard NCERT/UPSC understanding accepts statements 1 and 2 as clearly correct; statement 3 is a nuanced claim. At UPSC level the accepted answer involves 1 and 2 only as unambiguously correct, making (a) the best answer.
2. Consider the following statements:
1. Denitrification returns nitrogen to the atmosphere as N2.
2. Ammonification releases ammonium from organic matter.
3. The Haber process mimics biological nitrogen fixation.
Which of the statements given above is/are correct?
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (a) 1 and 2 only
Statement 1 is correct: denitrification by bacteria (e.g., Pseudomonas) returns nitrogen to the atmosphere as N2 (and N2O). Statement 2 is correct: ammonification is the breakdown of organic nitrogen in dead matter to release ammonium (NH4+). Statement 3 is partly misleading: the Haber process synthesises ammonia from N2 and H2 industrially, inspired by but not truly mimicking biological nitrogen fixation (which uses nitrogenase enzyme at ambient conditions). At UPSC level, stating it 'mimics' is considered incorrect because the Haber process requires high temperature (~450°C) and pressure (~200 atm), unlike biological fixation. Statements 1 and 2 only are correct; answer is (a).
3. A farmer in a rain-shadow region of India notices that his crops are stunted and leaves are yellowing. Soil test shows high pH and low organic matter. Which nutrient cycle is most likely disrupted, and what is the probable cause?
- (a) Nitrogen cycle – due to low denitrification
- (b) Sulphur cycle – due to reduced microbial activity in alkaline soil
- (c) Water cycle – due to low rainfall
- (d) Phosphorus cycle – due to fixation in alkaline soil
Answer: (d) Phosphorus cycle – due to fixation in alkaline soil
The scenario describes stunted crops with yellowing leaves, high soil pH, and low organic matter in a rain-shadow region. In alkaline soils, phosphorus precipitates as calcium phosphate and becomes unavailable — this is classic phosphorus fixation in alkaline conditions. The phosphorus cycle is disrupted by alkaline pH causing phosphorus to become insoluble. Option (b) claims sulphur cycle disruption, but the primary and most direct cause of these specific symptoms in alkaline soil is phosphorus unavailability. Phosphorus deficiency causes stunted growth and purpling/yellowing; in alkaline soils, phosphorus fixation is the primary nutritional problem. The answer is (d).
4. Consider the following statements about the Haber process:
1. It is an industrial method to fix atmospheric nitrogen.
2. It requires high temperature and pressure.
3. It has doubled the natural rate of nitrogen fixation globally.
Which of the statements given above is/are correct?
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (a) 1 and 2 only
Statement 1 is correct: the Haber-Bosch process industrially fixes atmospheric nitrogen into ammonia. Statement 2 is correct: it requires high temperature (~450°C) and high pressure (~150–200 atm). Statement 3 is incorrect in its phrasing: human industrial nitrogen fixation (including Haber process, fertiliser use, and legume cultivation) has approximately doubled or more than doubled total nitrogen fixation on Earth; however, stating specifically that the Haber process alone 'has doubled the natural rate' overstates what a single process does. Statements 1 and 2 only are unambiguously correct; answer is (a).
5. Consider the following statements:
1. In a forest ecosystem, the standing state of nitrogen in soil is usually higher than that in living biomass.
2. Residence time of phosphorus in a lake is shorter than in a forest soil.
3. Human activities can alter the standing state of nutrients by adding fertilizers.
Which of the statements given above are correct?
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (c) 1 and 3 only
Statement 1 is correct: in a forest ecosystem, most nitrogen is stored in soil organic matter (humus, decomposing matter) rather than in living plant biomass, so soil standing state > biomass standing state. Statement 2 is incorrect: phosphorus residence time in a lake tends to be longer than in forest soil because phosphorus becomes bound in lake sediments; forest soil phosphorus cycles more rapidly through plant uptake and decomposition. Statement 3 is correct: fertiliser additions directly increase the standing state of nutrients in the soil. Statements 1 and 3 are correct; answer is (c).
6. In the hydrological cycle, which of the following processes is responsible for the largest transfer of water from the land to the atmosphere?
- (a) Evaporation from lakes
- (b) Transpiration from forests
- (c) Evaporation from soil
- (d) Sublimation from ice
Answer: (b) Transpiration from forests
Globally, transpiration from vegetation (particularly forests) accounts for the largest fraction of land-to-atmosphere water flux. Studies (Schlesinger & Jasechko 2014) estimate that transpiration accounts for about 61% of terrestrial evapotranspiration. In forest-covered regions like the Amazon, transpiration is the dominant flux. Among the listed options, transpiration from forests is the largest single pathway. Note: 'evaporation from lakes' and 'evaporation from soil' together constitute soil/surface evaporation but are individually smaller than forest transpiration on a global scale.
7. Which of the following best describes an exception to the diversity-stability hypothesis?
- (a) A species-poor grassland is more resistant to drought than a species-rich one
- (b) A coral reef with high diversity collapses due to bleaching
- (c) A monoculture crop field has high resilience to pest attacks
- (d) A forest with many tree species recovers slowly after fire
Answer: (a) A species-poor grassland is more resistant to drought than a species-rich one
The diversity-stability hypothesis says more diversity leads to more stability. But sometimes species-poor ecosystems can be more resistant if they have species adapted to the disturbance. For example, a grassland with few drought-tolerant grasses may resist drought better than a diverse forest. Option (b) is not an exception; it shows that even diverse systems can collapse under extreme stress. Option (c) is false; monocultures are usually less resilient. Option (d) is false; diverse forests usually recover faster.
8. Consider the following pairs:
1. Carbon cycle - Gaseous cycle
2. Phosphorus cycle - Sedimentary cycle
3. Sulfur cycle - Gaseous cycle
Which of the pairs given above is/are correctly matched?
- (a) 1 only
- (b) 1 and 2 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer: (b) 1 and 2 only
Pair 1: Carbon cycle is a gaseous cycle (reservoir = atmosphere as CO2) — correctly matched. Pair 2: Phosphorus cycle is a sedimentary cycle — correctly matched. Pair 3: Sulfur cycle has BOTH sedimentary and gaseous components (SO2, H2S are gaseous; sulfate in rocks is sedimentary) — it is often classified as a gaseous cycle because of significant atmospheric SO2 and H2S phases. In standard ecology, the sulfur cycle is considered a gaseous cycle. All three pairs are correctly matched. correct answer is (d).
9. Which climax theory recognizes that factors like soil, topography, and fire can produce multiple stable communities in the same climate?
- (a) Monoclimax theory
- (b) Polyclimax theory
- (c) Climax-pattern theory
- (d) Both (b) and (c)
Answer: (d) Both (b) and (c)
Both polyclimax theory (proposed by A.G. Tansley) and climax-pattern theory (proposed by R.H. Whittaker) recognise that multiple stable climax communities can exist in the same climate, influenced by local factors like soil type, topography, fire regime, and moisture. The polyclimax theory identifies discrete climax types; the climax-pattern theory views the landscape as a continuous mosaic varying with environmental gradients. Both (b) and (c) are correct answers, making (d) the right choice.
10. Consider the following statements about carbon sinks:
1. The Amazon rainforest is a net carbon sink.
2. Boreal forests store more carbon per unit area than tropical rainforests.
3. Peatlands are significant terrestrial carbon stores.
Which of the statements given above is/are correct?
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (b) 2 and 3 only
Statement 1 is contested: large parts of the Amazon have become net carbon sources due to deforestation and degradation. Statement 2 is correct: boreal forests store more carbon per unit area (especially in soils and permafrost) than tropical rainforests. Statement 3 is correct: peatlands store approximately 550–650 Gt C globally, making them one of the largest terrestrial carbon stores. Therefore 2 and 3 are correct; answer is (b).
11. In a pond ecosystem, which of the following is likely to have the highest STANDING CROP (biomass at a given point in time)?
- (a) Phytoplankton
- (b) Zooplankton
- (c) Fish
- (d) Aquatic plants (macrophytes)
Answer: (d) Aquatic plants (macrophytes)
In a pond ecosystem, phytoplankton are the dominant primary producers and typically constitute the highest standing crop by numbers and often by biomass in open water. Aquatic macrophytes may be abundant in shallow margins but phytoplankton dominate open pond biomass in most productive ponds. However, in heavily vegetated shallow ponds, macrophytes can dominate. The standard ecology answer for a pond ecosystem is phytoplankton (a).
12. Consider the following statements about ecosystem restoration and rewilding:
1. Rewilding focuses on restoring natural processes and trophic complexity, often by reintroducing keystone species.
2. The UN Decade on Ecosystem Restoration prioritizes only forest ecosystems.
3. Passive restoration involves human intervention to accelerate natural recovery.
Which of the statements given above is/are correct?
- (a) 1 only
- (b) 1 and 2 only
- (c) 2 and 3 only
- (d) 1, 2, and 3
Answer: (a) 1 only
Statement 1 is correct: rewilding aims to restore self-sustaining ecosystems by reintroducing keystone species and restoring natural processes. Statement 2 is incorrect: the UN Decade covers all ecosystems, including forests, wetlands, grasslands, mountains, and oceans. Statement 3 is incorrect: passive restoration means letting nature recover without human intervention, while active restoration involves direct human actions.