Life on Earth

Life on Earth is concentrated in a thin zone — the biosphere — extending from the deep ocean floor to the upper atmosphere. Within this zone, the distribution of life is not random but follows clear patterns shaped by climate, soils, topography, and evolutionary history. Understanding biomes — the large-scale ecosystems that characterise different climatic zones — is fundamental for UPSC questions on natural vegetation, biodiversity conservation, climate change impacts, and environmental geography.

This chapter bridges physical geography (climate, soils) and environmental science (biodiversity, ecosystem services), making it relevant across GS Papers 1, 3, and 4.

PART 1 — Quick Reference Tables

Table 1: Major World Biomes

Table 2: Biodiversity Hotspots (Selected)

(A biodiversity hotspot must have ≥1,500 endemic plant species AND have lost ≥70% of its original habitat — Norman Myers' criteria)

Table 3: Biogeochemical Cycles — Carbon

Table 4: Biogeochemical Cycles — Nitrogen

Table 5: India's Biomes and Natural Vegetation Types

PART 2 — Detailed Notes

The Biosphere

The biosphere (from Greek: bios = life, sphaira = sphere) is the zone of life on Earth — a thin shell extending from deep ocean vents (~10 km below surface) to the lower stratosphere (~10 km above surface). Life is most concentrated in the troposphere–surface zone.

The biosphere is the fourth of Earth's major spheres, interacting intimately with the other three:

• Lithosphere provides physical substrate, nutrients from weathered rock
• Hydrosphere provides water (universal solvent for biochemistry)
• Atmosphere provides CO₂ (for photosynthesis), O₂ (for respiration), and regulates temperature

💡 Explainer: Biomes and Climate

Biomes are large-scale ecological communities defined primarily by their climate and dominant plant form. The key insight is that climate determines vegetation type, which in turn determines which animals can survive.

The distribution of biomes follows a clear latitudinal pattern from equator to poles, which largely mirrors the Köppen climate classification:

• Equator → Tropical Rainforest (Af)
• 5°–15°N/S → Tropical Savanna (Aw)
• 15°–30°N/S (west coasts) → Desert (BWh)
• 30°–40°N/S (west coasts) → Mediterranean Shrubland (Cs)
• 40°–60°N/S → Temperate Forest (Cfb/Cfa)
• 50°–60°N → Boreal Forest/Taiga (Dfc)
• 60°–90°N → Tundra (ET) and Ice Cap (EF)

However, altitude creates a vertical replication of latitudinal biomes — as you climb a mountain from the base to the summit in the tropics, you pass through vegetation zones similar to what you'd encounter travelling from the equator to the poles.

Tropical Rainforest: The Biodiversity Crown

The tropical rainforest biome receives >2,000 mm of rainfall distributed year-round and experiences consistently high temperatures (25–30°C). These stable, warm, moist conditions have supported diversification of species over millions of years.

Key characteristics:

• Vertical stratification: 4–5 layers — emergent trees (up to 60 m), main canopy (25–45 m), sub-canopy, shrub layer, ground layer
• High biodiversity: ~50% of all known species in ~6% of land area
• Fast nutrient cycling: Decomposition is rapid; nutrients are held in living biomass, not soil — cleared rainforest soils are surprisingly infertile
• Epiphytes and lianas: Many plants use other plants for support (not parasitically)
• Carbon storage: Tropical forests store ~250 Gt of carbon — deforestation is a major source of CO₂ emissions

India's tropical rainforest: Western Ghats (Silent Valley, Agasthyamalai), Andaman & Nicobar Islands, NE India (Meghalaya, Mizoram) — part of the Western Ghats–Sri Lanka biodiversity hotspot.

Savanna: Fire and Drought Adapted

The tropical savanna biome has a distinct dry season (3–6 months) with annual rainfall 750–1,500 mm. Vegetation is a mix of grassland and scattered drought-deciduous trees (acacia, baobab in Africa).

Fire plays a critical ecological role — periodic burning maintains the grassland by preventing tree encroachment. Many savanna species (acacia, certain grasses) are fire-adapted.

India's Deccan Plateau has a savanna-like dry tropical climate. The classic dry deciduous forests of central India (tiger reserves: Kanha, Pench, Bandhavgarh) occupy this climatic zone.

Tundra and Alpine Ecosystems

The tundra is characterised by:

• Very short growing season (6–10 weeks)
• Permafrost — permanently frozen subsoil that prevents water drainage and deep root growth
• Low productivity; low diversity
• Vulnerability to climate change: The Arctic is warming 3–4 times faster than the global average; permafrost thaw releases stored methane (CH₄) — a powerful positive feedback

Alpine tundra (above treeline in mountains) has similar conditions. India's Himalayan alpine meadows (bugyals) in Uttarakhand — Auli, Bedni, Valley of Flowers — are seasonal alpine ecosystems of outstanding beauty and vulnerability.

🎯 UPSC Connect: Biogeochemical Cycles and Environment

The Carbon Cycle is central to climate change understanding. Key points:

• Atmospheric CO₂ is both source (for photosynthesis) and product (of respiration, decomposition, combustion)
• Human activities have increased atmospheric CO₂ from ~280 ppm (pre-industrial) to ~421 ppm (2024) — a 50% increase
• Oceans and forests act as carbon sinks (absorbing net CO₂); human activities make them net sources in degraded areas
• REDD+ (Reducing Emissions from Deforestation and Forest Degradation) — UN programme to make forests financially competitive with agriculture by valuing carbon storage

The Nitrogen Cycle matters for:

• Agricultural productivity (nitrogen is the primary limiting nutrient for plant growth)
• Eutrophication — excess nitrogen from fertilisers enters water bodies, promotes algal blooms, depletes oxygen, kills fish
• Nitrous oxide (N₂O) from fertilisers — potent greenhouse gas and ozone depleter
• India's heavy use of nitrogenous fertilisers (urea) has led to soil acidification and water pollution in the Indo-Gangetic Plain

The Water Cycle (covered in detail in Ch. 11) links climate, vegetation, and freshwater availability. Forest cover significantly increases evapotranspiration, maintains local rainfall patterns, and regulates river flow.

Biodiversity Hotspots and India

India is one of the world's 17 megadiverse countries — countries that together contain >70% of the world's biodiversity. India's contribution:

• 7–8% of world's biodiversity in <2.5% of land area
• ~91,000 animal species, ~45,000 plant species
• 4 biodiversity hotspots: Western Ghats–Sri Lanka, Himalayas, Indo-Burma, Sundaland (Nicobar Islands)

However, India faces severe threats: habitat loss (urbanisation, agriculture), poaching and wildlife trade, invasive species, pollution, and climate change. India's Biological Diversity Act, 2002 and National Biodiversity Action Plan address these issues.

PART 3 — Frameworks & Analysis

Biome Productivity Comparison

Biogeochemical Cycle: Key Linkages

Exam Strategy

Prelims Traps:

• Biodiversity hotspot criteria: ≥1,500 endemic plant species AND ≥70% habitat lost. India has 4 hotspots — Western Ghats–Sri Lanka, Himalayas, Indo-Burma, Sundaland.
• Tropical rainforest soils are NOT fertile (nutrients in biomass, not soil) — counterintuitive but important.
• Tundra has permafrost; taiga does not (has cold soil but not permanently frozen).
• Nitrogen fixation converts atmospheric N₂ to ammonia — done by bacteria (Rhizobium, Azotobacter), not plants themselves.
• Eutrophication = excess nutrients (N, P) in water → algal bloom → oxygen depletion → fish die. Caused by agricultural runoff, sewage.

Mains Frameworks:

• Biodiversity conservation: threats (habitat loss, poaching, invasive, climate) → in-situ vs ex-situ → CBD, CITES, NBSAP → India-specific (Project Tiger, EIA, Protected Areas).
• Climate–ecosystem linkage: biome shifts due to warming → species displacement → ecosystem services loss.
• Agricultural sustainability: nitrogen cycle disruption by fertiliser overuse → eutrophication → soil health → sustainable agriculture.

Previous Year Questions

1. UPSC Prelims 2021: What are biodiversity hotspots? Which of the following regions in India qualifies as a biodiversity hotspot?
2. UPSC Prelims 2019: Which of the following processes is responsible for converting atmospheric nitrogen into a form usable by plants? (Nitrogen fixation)
3. UPSC Mains GS3 2020: Examine the role of carbon sequestration in forests in addressing the climate crisis. What are the challenges in implementing REDD+?
4. UPSC Mains GS3 2018: What is eutrophication? Discuss its causes, effects, and remedial measures.