Q.86 Trees in the equatorial region of earth supply oxygen into the atmosphere that sustains species
living in distant polar regions. This relationship is called as
(A) mutualism (B) symbiosis
(C) commensalism (D) parasitism
The correct answer is (C) commensalism. Equatorial trees release oxygen through photosynthesis that benefits polar species via atmospheric mixing, while polar organisms provide no reciprocal benefit to the trees—one-sided benefit defines commensalism.
Question Context
This tests ecological relationships across global scales. Oxygen from equatorial rainforests (“lungs of Earth”) disperses worldwide, sustaining aerobic respiration in distant polar ecosystems.
Option Analysis
(A) Mutualism
Mutualism requires reciprocal benefits (both species gain). Trees benefit polar species with O₂, but polar organisms provide no return benefit to equatorial trees. Incorrect.
(B) Symbiosis
Symbiosis means close physical association (e.g., lichens, corals). Trees and polar species have no direct contact—only atmospheric O₂ diffusion. Too broad/wrong term here.
(C) Commensalism
Correct. One species benefits (polar organisms use O₂), host unaffected (equatorial trees neither gain nor lose). Classic global-scale example: rainforest O₂ → polar life support.
(D) Parasitism
Parasitism harms host, benefits parasite. Polar species don’t harm equatorial trees; O₂ production is their normal metabolism. Incorrect.
Introduction
Trees in the equatorial region of earth supply oxygen sustaining species living in distant polar regions exemplifies commensalism in GATE Life Sciences ecology. Equatorial rainforests produce ~20% global O₂, benefiting distant ecosystems unidirectionally.
Global Oxygen Cycle
Equatorial rainforests (Amazon, Congo) photosynthesize year-round due to sunlight/water, releasing O₂ that mixes globally via atmospheric circulation. Polar regions (Arctic/Antarctic) lack vegetation, relying on this input for aerobic respiration.
Net flow: Tropics → Poles (no reverse benefit).
Ecological Relationship Types
| Relationship | Benefit to A | Benefit to B | Physical Contact | Example |
|---|---|---|---|---|
| Commensalism | + (O₂ use) | 0 (unaffected) | None | Trees → Polar species |
| Mutualism | + | + | Often | Mycorrhizae |
| Symbiosis | +/0/- | +/0/- | Required | Corals + Zooxanthellae |
| Parasitism | – | + | Yes | Tapeworm-host |
GATE Exam Strategy
Key differentiator: Commensalism = one-way benefit, no harm, no reciprocity. Links to prior questions: N₂-fixing bacteria (Q.82), cyanobacterial heterocysts (Q.84). Rainforests = “lungs of Earth” mnemonic.
