UPSC Prelims 2011 Q23: Environment & Ecology – Ecosystems & Ecology Concepts
If a tropical rain forest is removed, it does not regenerate quickly as compared to a tropical
deciduous forest. This is because
- (a) the soil of rain forest is deficient in nutrients
- (b) propagules of the trees in a rain forest have poor viability
- (c) the rain forest species are slow - growing
- (d) exotic species invade the fertile soil of rain forest
Answer: (a) the soil of rain forest is deficient in nutrients
Explanation
Why tropical rain forests regenerate slowly after clearing
| Option | Verdict | Reason |
|---|---|---|
| (a) Soil of the rain forest is deficient in nutrients | Correct | Most nutrients are locked in the standing vegetation and leaf litter, not the soil; once trees are cleared, the thin nutrient-poor soil cannot support quick regrowth |
| (b) Propagules have poor viability | Incorrect | Rain forest seeds are generally viable; the problem is the soil, not seed quality |
| (c) Rain forest species are slow-growing | Incorrect | Many rain forest trees actually grow fast; slow regeneration is a soil issue, not a growth-rate issue |
| (d) Exotic species invade the fertile soil | Incorrect | Rain forest soil is not especially fertile, and this describes a secondary problem, not the root cause |
Nutrient cycling in a rain forest:
| Where nutrients are held | In a rain forest | In a temperate/deciduous forest |
|---|---|---|
| Mainly in | Living biomass and litter (fast recycling) | Soil (more nutrient reserve) |
| Effect of clearing | Nutrients lost with the vegetation; soil left poor | Soil retains more nutrients, so recovery is easier |
Answer: (a) the soil of rain forest is deficient in nutrients.
Why each option is right or wrong
- (a) CORRECT. Rain forest soil is naturally nutrient-poor; most nutrients are tied up in the living vegetation, which is lost when the forest is cleared.
- (b) Incorrect. The seeds (propagules) of rain forest trees are generally viable; soil quality is the real issue.
- (c) Incorrect. Many rain forest trees are actually fast-growing; slow recovery owes to poor soil, not slow species.
- (d) Incorrect. Rain forest soil is not particularly fertile, so this does not explain the slow regeneration.
Key point
Tropical rain forest soil is nutrient-poor because nutrients are stored mainly in the biomass/litter, not the soil; once cleared, there is little nutrient reserve left for quick regrowth.
Remember
Rain forest soil is nutrient-poor; nutrients live in the biomass, not the soil.
Common mistakes
- Assuming thick, green rain forests must have rich, fertile soil.
- Blaming seed viability or growth speed instead of the soil's nutrient content.
- Overlooking where tropical forest nutrients are actually stored, in the living plants, not the ground.
Exam tip
In rain forests, the 'wealth' is in the trees, not the soil. Clear the trees and the nutrient bank is gone.
Sources
- NCERT Class XII Biology — Ecosystem (nutrient cycling)
- P. W. Richards — The Tropical Rain Forest: An Ecological Study (Cambridge University Press)