Ecology and Ecosystems — Practice Questions
UPSC Prelims-pattern practice questions on ecology with full explanations: succession, edge effect, nutrient cycles, keystone species and decomposition.
Questions
These are original UPSC-pattern practice questions. Attempt each one before revealing the answer.
Consider the following statements about ecological succession:
- Primary succession begins in areas where no soil exists.
- Secondary succession is generally faster than primary succession.
- Lichens are the pioneer species in hydrarch succession.
Which of the statements given above is/are correct?
Solution
Statement 1 is correct — primary succession starts on bare rock, cooled lava or sand. Statement 2 is correct — secondary succession begins where soil already exists (an abandoned farm, a burnt forest), so it proceeds faster. Statement 3 is incorrect — lichens pioneer xerarch succession on bare rock; in hydrarch succession the pioneers are phytoplankton.
Answer: B
Which one of the following best describes the “edge effect” in ecology?
Solution
An ecotone is a transition zone between two ecosystems (a mangrove, an estuary, a forest–grassland boundary). The edge effect is its tendency to support more species, and sometimes higher densities, than either neighbouring ecosystem. Option B is the ten per cent law, option C is biomagnification, and option D is ecological succession.
Answer: A
Which of the following nutrient cycles are sedimentary cycles?
- Carbon
- Phosphorus
- Nitrogen
- Sulphur
Select the correct answer using the code given below.
Solution
In a sedimentary cycle the main reservoir is the Earth’s crust. Phosphorus (no significant gaseous phase; released by weathering of rocks) and sulphur are sedimentary. Carbon and nitrogen have their main reservoirs in the atmosphere (and, for carbon, the ocean), so they are gaseous cycles.
Answer: A
Which one of the following best defines a keystone species?
Solution
A keystone species has an influence on community structure far greater than its abundance would suggest — Robert Paine showed that removing the predatory starfish Pisaster let mussels dominate and diversity fell. Option A describes an endemic species, option B a flagship species and option C an umbrella species. All four terms appear together in Prelims options.
Answer: D
How many of the following conditions generally slow down the rate of decomposition?
- Detritus rich in lignin
- Low temperature
- Anaerobic, waterlogged conditions
- Detritus rich in nitrogen and water-soluble sugars
Solution
Lignin (and chitin) resist microbial breakdown; low temperature reduces microbial activity; anaerobic, waterlogged conditions inhibit decomposers — this is why peat builds up in bogs. Conditions 1, 2 and 3 therefore slow decomposition. Detritus rich in nitrogen and water-soluble sugars decomposes faster. That makes three.
Answer: C
What is a keystone species? With examples, discuss why the loss of a keystone species can trigger changes across an entire ecosystem, and what this implies for conservation policy in India.
Solution
Introduction: A keystone species is one whose effect on its ecosystem is disproportionately large relative to its abundance. The concept emerged from Robert Paine’s starfish experiments in the 1960s, where removing a single predator collapsed local diversity.
Body:
- How keystone effects work:
- Top-down control: predators regulate herbivores, which in turn shapes vegetation (a trophic cascade, as studied after wolves returned to Yellowstone in 1995).
- Ecosystem engineering: species that modify habitat, such as elephants opening up forest and dispersing seeds.
- Mutualist hubs: species such as figs that provide food when other fruit is scarce, supporting many frugivores.
- Consequences of loss: herbivore overpopulation and overgrazing, loss of regeneration, shifts in community composition, reduced ecosystem services such as pollination and seed dispersal.
- Implications for India’s policy:
- Landscape-level protection and corridors rather than isolated reserves.
- Using flagship and umbrella species (tiger, elephant) to protect whole habitats.
- Attention to less charismatic keystone groups — pollinators, scavengers such as vultures, and figs.
- Monitoring of trophic interactions, not only headcounts.
Conclusion: Conserving keystone species is conserving the processes that hold an ecosystem together; Indian policy gains most when species projects are designed as ecosystem projects.