Global Distribution and Factors Affecting Biomes

A biome is a very large-scale ecosystem characterised by distinct climate, vegetation, and adapted wildlife. Together, all biomes make up the biosphere, which is the living layer of the Earth. Biomes follow clear spatial patterns based on latitude:

• Tropical Rainforests: Found in the equatorial belt at low latitudes ($0^\circ$ to $10^\circ$ N/S). Key locations include the Amazon Basin, the Congo (Zaire) Basin, Southeast Asia, and New Guinea.
• Tropical Grassland (Savanna): Located between $5^\circ/15^\circ$ and $30^\circ$ N/S. These act as transition zones between rainforests and deserts (e.g., the Serengeti in Tanzania).
• Hot Deserts: Found at mid-latitudes, approximately $15^\circ$ to $30^\circ$ N/S, near the Tropics of Cancer and Capricorn. Key areas include the Sahara, Kalahari, Arabian Desert, and Central Australia.
• Temperate Grasslands: Located at $25^\circ$ to $55^\circ$ N/S. These have a continental distribution, meaning they are found in continental interiors (like the North American Prairies and Eastern European Steppes) away from ocean moisture.
• Temperate Forest (Deciduous): Found at mid-latitudes ($40^\circ$ to $60^\circ$ N and $30^\circ$ to $40^\circ$ S) in regions like the UK, Western Europe, and North-east USA.
• Boreal Forest (Taiga): Found at high latitudes ($50^\circ$ to $70^\circ$ N) as a continuous belt across Scandinavia, Canada, and Northern Russia. They are exclusive to the Northern Hemisphere.
• Tundra and Polar: Located at extreme high latitudes. Tundra sits north of the Arctic Circle ($60^\circ$ to $70^\circ+$ N) and on the fringes of Antarctica, while Polar biomes are found exactly at $90^\circ$ N/S.

How Climate Influences Ecosystems

Biome distribution relies heavily on interdependence with the climate. The primary global factor is insolation — the intensity of solar radiation. The sun's rays are highly concentrated at the equator (averaging $26^\circ\text{C}$ to $28^\circ\text{C}$) but spread over a much larger surface area at the poles (dropping below $0^\circ\text{C}$ for most of the year).

Global atmospheric circulation (the Hadley, Ferrel, and Polar cells) creates distinct pressure belts that control precipitation:

• Low-Pressure Belts (Equator & Mid-latitudes): Here, warm air rises, cools, and condenses to form heavy rain clouds. This high rainfall (over $2000\,\text{mm}$ annually) supports lush Tropical Rainforests and Temperate Deciduous Forests.
• High-Pressure Belts (Tropics & Polars): Here, air descends and warms. Descending air can hold more moisture, which explicitly prevents cloud formation. This extreme lack of rain (under $250\,\text{mm}$ annually) creates Hot Deserts and Polar/Tundra biomes.

The Role of Altitude (Lapse Rate)

While latitude is the main global driver, altitude acts as a powerful local factor that can disrupt global patterns. For every $100\,\text{m}$ of altitude gained, the temperature drops by $1^\circ\text{C}$ (or $6.5^\circ\text{C}$ per $1000\,\text{m}$). This is known as the lapse rate.

As air rises up a mountain, atmospheric pressure decreases. The air expands (adiabatic cooling) and uses up energy, causing the temperature to fall. Thinner air also cannot retain heat effectively. This creates altitudinal zonation — where ecosystems change in distinct layers as elevation increases, mirroring latitudinal changes. Trees experience stunted growth until the tree line, above which only hardy grasses and mosses survive. Altitude also creates relief rainfall, leaving the leeward side of a mountain dry in a rain shadow.

Worked Example: Calculating Lapse Rate

If the temperature at sea level ($0\,\text{m}$) is $30^\circ\text{C}$, what is the temperature at the summit of a $3000\,\text{m}$ mountain?

Step 1: Calculate the height increase in $100\,\text{m}$ units.

• $\text{Units} = 3000\,\text{m} / 100\,\text{m} = 30$ units

Step 2: Multiply by the lapse rate ($1^\circ\text{C}$ drop per unit) to find the total temperature drop.

• $\text{Temperature Drop} = 30 \times 1^\circ\text{C} = 30^\circ\text{C}$

Step 3: Subtract the drop from the starting temperature.

• $\text{Summit Temperature} = 30^\circ\text{C} - 30^\circ\text{C} = 0^\circ\text{C}$

Local Factors: Soils, Geology, and Drainage

Ground conditions dictate exactly what plant life can thrive in a specific area.

• Soils:
  • Tropical Rainforest soils (known as latosols) are extremely nutrient-poor due to heavy leaching — continuous heavy rain washes minerals out of the soil. The vast majority of nutrients are stored in the biomass, not the soil.
  • Temperate Deciduous soils are highly fertile. Annual leaf fall creates a thick litter layer that decomposes into rich humus.
  • Taiga soils (known as podzols) are highly acidic due to the slow breakdown of coniferous needles, which limits plant variety.
  • Tundra features permafrost (permanently frozen ground). This prevents deep root growth and restricts drainage, causing boggy waterlogging in the brief summer.
• Geology and Drainage: Underlying rock determines soil pH and drainage. Alkaline soils (from limestone) support beech trees, while acidic soils (from granite) support oak. The rock's permeability matters: impermeable rocks like clay cause waterlogging, cutting off oxygen to plant roots, while permeable sandstone allows for quick drainage and drier soils.