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.

🧠 First Principles — Read This First

Where you find a particular set of plants and animals is decided, above all, by the climate — so the living world is arranged into great belts called biomes that mirror the climate map. A biome is a large community of life (a distinctive vegetation and the animals that depend on it) adapted to a particular climate: tropical rainforest where it is hot and wet, desert where it is dry, tundra where it is cold. Because climate is arranged by latitude, biomes are too — travel from the equator to the poles and you pass through rainforest, savanna, desert, temperate forest, grassland, taiga and tundra in a predictable order. The single key idea is that climate determines vegetation, and vegetation determines the animals — so the biome map is essentially the climate map brought to life, and the previous chapter's Köppen types and this chapter's biomes are two views of the same thing.

Life does not just sit on the Earth — it actively cycles the materials it needs, again and again, through the ecosystem. The carbon in a leaf, the nitrogen in a protein, the water in a cell are all borrowed from the environment and returned to it, to be used again — the biogeochemical cycles (carbon, nitrogen, water and others) that keep the elements of life in perpetual circulation. Crucially, energy flows one way (from the Sun, through plants, to animals, and out as heat) while matter cycles round and round. Understanding that an ecosystem is a system of energy flow and nutrient cycling is the foundation for grasping how life sustains itself — and how human disruption of these cycles (especially the carbon cycle) drives environmental crises.

Why UPSC cares: biomes, ecosystems, food chains, the biogeochemical cycles (especially carbon and nitrogen) and biodiversity hotspots are direct Prelims and GS3 (environment) content, and the carbon cycle is the link to climate change.

PART 1 — Quick Reference

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 — Concepts & Narrative

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

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.

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.

Biomes — The Living World Arranged by Climate

The most useful way to hold the world's biomes is, again, as a journey by latitude, because each biome is the living expression of a climate zone and the sequence explains itself. At the equator lies the tropical rainforest — hot and wet all year, with the richest biodiversity on Earth packed into a multi-layered evergreen canopy (the Amazon, Congo, Southeast Asia). Poleward, where a dry season appears, the forest gives way to tropical savanna — grassland dotted with drought-resistant trees and home to the great grazing herds and their predators (the African savanna, the Indian Deccan). Around 30°, under the dry subtropical highs, spread the hot deserts with their sparse, water-hoarding xerophytes and heat-adapted animals (Sahara, Thar). In the temperate mid-latitudes come the Mediterranean shrublands (fire-adapted scrub of hot-dry-summer coasts), the temperate grasslands (the prairies, pampas and steppes — the world's breadbaskets), and the temperate deciduous forests (oak and maple that shed their leaves in winter). Further poleward stretches the vast boreal forest (taiga) of conifers across Canada and Siberia, and finally the treeless tundra of mosses and lichens over permafrost, giving way to polar ice. The unifying insight is that this whole procession of life tracks the climate procession of the previous chapter — wet biomes where air rises and rains, desert biomes where it sinks, cold biomes toward the poles — so the biome map and the climate map are the same map seen through different eyes. Master the latitudinal sequence and you can predict the natural vegetation, and much of the wildlife, of anywhere on Earth.

Ecosystems — Energy Flow and Nutrient Cycling

The chapter's core scientific content is how an ecosystem actually functions, and the two complementary processes — energy flow and nutrient cycling — are worth understanding properly because they recur throughout environmental studies. Energy flow is one-directional: it enters as sunlight, is captured by producers (plants) through photosynthesis, and passes up the food chain to herbivores and carnivores, with about 90% lost as heat at each step (the 10% law), so it dwindles to nothing at the top and must be constantly resupplied by the Sun. Nutrient cycling, by contrast, is circular: the chemical elements of life (carbon, nitrogen, phosphorus, water) are taken up by organisms, passed along the food chain, and returned to the environment by decomposers when organisms die — to be used again and again, forever. The two processes interlock in the food web (the realistic, interconnected tangle of many food chains) and produce the ecosystem's structure: many plants at the base, fewer herbivores above, fewer still carnivores at the top — the ecological pyramid. This framework explains a great deal: why top predators are rare and vulnerable, why removing one species ripples through the whole web, why pollutants like pesticides concentrate up the chain (biomagnification) to poison the top predators, and why decomposers, though unglamorous, are indispensable (without them, nutrients would lock up in dead matter and life would grind to a halt). For an aspirant, the energy-flow-and-nutrient-cycling model is the master key to every ecology question in the GS3 environment syllabus.

The Biogeochemical Cycles — Carbon and Nitrogen

Among the nutrient cycles, two deserve particular attention because they are both heavily tested and central to contemporary crises: the carbon and nitrogen cycles. The carbon cycle moves carbon between the atmosphere (as CO₂), living organisms, soils, rocks and the oceans: plants remove CO₂ from the air by photosynthesis and lock it in their tissues; respiration and decomposition return it; the oceans absorb and release vast amounts; and over geological time, buried organic carbon becomes the fossil fuels. This cycle was roughly in balance for millennia — until humans began burning fossil fuels and clearing forests, injecting carbon that had been locked away for millions of years back into the atmosphere far faster than the cycle can reabsorb it, which is the very root of climate change. The nitrogen cycle is just as vital: although the air is 78% nitrogen, plants cannot use it in gaseous form, so it must be "fixed" into usable compounds — by lightning, by nitrogen-fixing bacteria (notably in the root nodules of legumes), and, hugely, by the industrial Haber process that makes synthetic fertiliser. Nitrogen then passes through plants and animals and is eventually returned to the air by denitrifying bacteria. Human interference here is also profound: the flood of synthetic nitrogen fertiliser has boosted food production but, as runoff, it pollutes water and causes eutrophication (algal blooms that suffocate aquatic life), a major GS3 theme. The lesson the examiner wants is that these cycles are the plumbing of the living planet, that they were balanced before industrialisation, and that human disruption of them — carbon driving climate change, nitrogen driving water pollution — is among the defining environmental problems of the age.

Biodiversity Hotspots — Where Life Concentrates

The chapter introduces a concept that bridges into the next and is a guaranteed exam topic: the biodiversity hotspot. Life is not spread evenly across the Earth — it concentrates spectacularly in certain regions, and a hotspot is a precisely-defined such region: an area that holds an exceptional concentration of endemic species (species found nowhere else) and has lost at least 70% of its original habitat (Norman Myers' criteria — so a hotspot is both irreplaceable and threatened). India is exceptionally rich, containing within or along its borders four of the world's three-dozen-odd hotspots: the Western Ghats (with their endemic frogs, plants and the source of peninsular rivers), the Himalaya (high-altitude endemism), the Indo-Burma region (the mega-diverse northeast), and Sundaland (the Nicobar Islands). It is no coincidence that these are India's wettest, most forested and most varied terrains — high rainfall, undisturbed habitat and varied topography are exactly what allow endemic species to evolve and accumulate. The reason the concept matters is conservation priority: because the world cannot protect everywhere at once, hotspots identify where protecting a small area saves a disproportionate share of the planet's unique biodiversity. For an aspirant, the hotspot idea links this chapter's ecology to the next chapter's conservation, explains why India is a global biodiversity treasure-house, and reframes the wettest, most "remote" parts of the country as the most irreplaceable — a perspective that recurs across the environment syllabus.

Why Life on Earth Is the Bridge to the Environment Syllabus

It is worth closing by recognising what this chapter does within the arc of physical geography: it is where the subject turns from the physical Earth to the living Earth, and so becomes the foundation of the entire environment-and-ecology syllabus. Every earlier chapter built the stage — the structure of the Earth, its landforms, its atmosphere, its climate, its oceans — and this chapter populates that stage with life, showing how the biosphere drapes itself across the physical world in patterns dictated by climate and how it sustains itself through energy flow and nutrient cycling. From here the syllabus opens directly onto its most pressing contemporary concerns: biodiversity and its conservation (the next chapter), the disruption of the carbon and nitrogen cycles that drives climate change and pollution, the loss of forests and species, and the whole framework of environmental governance. For an aspirant the realisation is that the apparently academic study of biomes and ecosystems is in fact the scientific bedrock of the environment paper and of some of the gravest challenges facing India and the world — the sixth mass extinction, climate-driven habitat loss, the collapse of pollinators and fisheries. Life on Earth is a delicate, interconnected system running on borrowed energy and recycled matter; understanding how it works is the prerequisite for understanding what is now threatening it, and for the responsibility of protecting it that the final chapter takes up.

PART 3 — UPSC Integration

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.

Practice 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.

📦 Revision Capsule