G12: Global Ecosystems and Biomes

📋 What is a Biome?

Biomes are determined primarily by climate, which is controlled by:

• Latitude — distance from the equator affects temperature and sunlight intensity
• Altitude — temperature decreases by approximately 6.5°C for every 1,000 m of height gained
• Distance from the sea — coastal areas have milder, wetter climates; inland areas are more extreme
• Ocean currents — warm currents raise coastal temperatures (e.g. Gulf Stream); cold currents lower them
• Prevailing winds — bring moisture from oceans or dry air from continental interiors

🌍 The Major Global Biomes

🌴 Tropical Rainforest Biome

Climate Characteristics

• Temperature: Hot all year round — typically 25–30°C with very little seasonal variation (less than 3°C difference between warmest and coolest months)
• Rainfall: Very high — 2,000–3,000 mm per year, often with rain every day. Convectional rainfall occurs daily as the Sun heats the moist forest, causing air to rise and condense into towering cumulonimbus clouds. Afternoon thunderstorms are common
• Humidity: Very high — typically 75–100%

Vegetation Structure

The rainforest has a distinctive layered structure:

• Emergent layer (40–50 m) — the tallest trees that emerge above the canopy; few in number; exposed to strong winds and intense sunlight
• Canopy layer (25–35 m) — a continuous layer of dense foliage forming a "roof" over the forest; intercepts most sunlight and rain; richest in wildlife
• Under canopy layer (10–20 m) — smaller trees adapted to low light; young canopy trees waiting for a gap
• Shrub layer (0–5 m) — shrubs, ferns and seedlings adapted to shade
• Forest floor — very dark; few plants; mainly decomposers breaking down leaf litter

🏜️ Hot Desert Biome

Climate Characteristics

• Temperature: Very hot by day (often exceeding 40°C; highest recorded 56.7°C in Death Valley) but can drop below 0°C at night due to lack of cloud cover — diurnal range can exceed 40°C
• Rainfall: Very low — less than 250 mm per year; often irregular and unpredictable; may go years without rain
• Other features: Very low humidity; high evaporation rates (often 20× the rainfall); strong winds causing erosion and sandstorms

Vegetation Characteristics

• Very sparse — plants are widely spaced with bare ground between them
• Plants are highly adapted to conserve water (see G16 for detailed adaptations)
• After rare rainfall, ephemeral plants germinate rapidly, flower and set seed before the soil dries out
• Soils are generally thin, sandy and low in organic matter but can be rich in minerals

🌲 Other Major Biomes

Temperate Deciduous Forest

• Found between 40° and 60° north — e.g. UK, Western Europe, eastern USA, eastern China, Japan
• Four distinct seasons with warm summers and cool winters
• Vegetation: broadleaf deciduous trees (oak, beech, maple, ash) that lose their leaves in autumn to conserve energy during winter
• Rich understorey of spring flowers that bloom before the canopy closes
• Brown earth soils — fertile, well-developed, good for agriculture

Coniferous Forest (Taiga)

• The world's largest biome, covering 17 million km² across Russia, Canada and Scandinavia
• Located between 50° and 70°N — long, cold winters and short, mild summers
• Vegetation: evergreen conifers (pine, spruce, fir) with needle-like leaves that reduce water loss and resist cold
• Podzol soils — acidic and nutrient-poor due to slow decomposition in cold conditions

Tundra

• Found in the Arctic north of the taiga, at 60–80°N — also at high altitude (alpine tundra)
• Extremely cold — winter temperatures can reach -50°C; summer temperatures rarely above 10°C
• Permafrost — permanently frozen subsoil that prevents deep root growth and water drainage
• Vegetation: low-growing mosses, lichens, dwarf shrubs, sedges; no trees
• Very low biodiversity — short growing season of only 6–10 weeks

Savanna (Tropical Grassland)

• Located between tropical rainforests and hot deserts (10–25°N/S)
• Distinct wet and dry seasons — rainfall concentrated in 4–6 months
• Vegetation: tall grasses (up to 3 m) with scattered drought-resistant trees (acacia, baobab)
• Very large herbivore populations (wildebeest, zebra, elephant) and their predators (lions, cheetahs)
• Fire plays an important ecological role — regular grass fires prevent tree encroachment

📊 Climate and Biome Distribution

However, other factors modify this pattern:

• Altitude — temperatures decrease with height, creating altitudinal zonation (e.g. Mount Kilimanjaro has rainforest at its base, then temperate forest, then alpine tundra near the summit)
• Continentality — areas far from oceans have more extreme temperatures (e.g. central Asia has grassland rather than forest)
• Ocean currents — the warm Gulf Stream keeps Western Europe warmer than equivalent latitudes in Canada, supporting deciduous forest instead of tundra
• Topography — mountain ranges create rain shadows, causing dry conditions on the lee side (e.g. the Atacama Desert is in the rain shadow of the Andes)

❓ Practice Questions

✅ Answers

1. A biome is a large-scale ecosystem covering a vast area of the Earth's surface, defined by its climate (temperature and rainfall), dominant vegetation type, and the wildlife it supports.
2. Global atmospheric circulation determines biome distribution through its pressure belts and wind patterns. The rising air at the equator (ITCZ) creates hot, wet conditions — producing tropical rainforests. The sinking air at 30°N/S (sub-tropical high) creates hot, dry conditions — producing hot deserts. The rising air at 60°N/S (sub-polar low) creates mild, wet conditions — producing temperate deciduous forests. The sinking air at the poles creates cold, dry conditions — producing tundra. The prevailing winds also bring moisture or dryness, further shaping biome distribution.
3. Climate: Temperature is hot year-round (25–30°C) with very little seasonal variation. Rainfall is very high (2,000–3,000 mm/year) and occurs nearly every day as convectional rainfall. Humidity is consistently high (75–100%). Vegetation: The rainforest has a distinctive layered structure — emergent trees (40–50 m), a dense canopy (25–35 m) that intercepts most sunlight, an under canopy (10–20 m), shrub layer (0–5 m), and a dark forest floor. The dense canopy supports extremely high biodiversity (approximately 50% of all species).
4. Hot deserts are located at approximately 30°N/S because this is where the Hadley Cell's air sinks back to the surface. This sinking air warms adiabatically and can hold more moisture, preventing cloud formation and rainfall. The result is the sub-tropical high-pressure belt, which creates persistently hot, dry conditions ideal for desert formation. The Sahara, Arabian, Kalahari and Australian deserts all lie near 30°N/S.
5. Tropical rainforests and hot deserts are very different biomes. Temperature: Rainforests are hot year-round (25–30°C) with minimal variation; deserts are hotter by day (often 40°C+) but can drop below 0°C at night, giving a huge diurnal range. Rainfall: Rainforests receive 2,000–3,000 mm/year with daily rain; deserts receive less than 250 mm/year, often with years between rainfall events. Vegetation: Rainforests have dense, multi-layered vegetation with enormous biomass; deserts have very sparse, widely spaced vegetation with bare ground between plants. Biodiversity: Rainforests have the highest biodiversity on Earth (50% of species); deserts have low biodiversity, but species are highly specialised. Soils: Rainforest soils are surprisingly nutrient-poor (nutrients stored in biomass); desert soils are thin and sandy but can be mineral-rich. Both biomes are located in the tropics, but at different latitudes — rainforests near the equator (0–10°) where air rises, and deserts at 15–35° where air sinks.
6. Altitude modifies biome distribution because temperature decreases by approximately 6.5°C for every 1,000 m of altitude. This means that as you climb a mountain, you pass through different climate zones equivalent to moving towards the poles — from tropical forest at the base, through temperate forest, to tundra near the summit. Mount Kilimanjaro demonstrates this altitudinal zonation. Distance from the sea (continentality) modifies biome distribution because oceans moderate temperature — coastal areas have smaller temperature ranges and higher rainfall. Inland areas have more extreme temperatures (hotter summers, colder winters) and lower rainfall. For example, at 50°N, the UK (coastal) has temperate deciduous forest, while central Asia (inland) has temperate grassland or even semi-desert due to the more extreme, drier climate.

🎯 Exam Tips

• Know the climate data for each major biome — temperature and rainfall ranges
• Always link biome distribution back to global atmospheric circulation — it's the most common link question
• Use specific named examples of biomes and their locations (e.g. "Amazon rainforest near the equator")
• Remember that tropical rainforest soil is NUTRIENT-POOR despite lush vegetation — nutrients are stored in biomass
• Hot deserts have extreme diurnal temperature range — explain why (no cloud cover)
• For comparison questions, make direct point-by-point comparisons, don't describe each separately

📝 Exam Technique

⚠️ Common Errors

✍️ Model Answer

📊 AO Deep Dive

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