Global Biomes and Their Distribution

An ecosystem is a community where plants and animals interact with each other and their environment. When these ecosystems occur on a large scale globally, we call them biomes. Examples include tropical rainforests, deserts, and temperate deciduous forests.

Climate is the main factor determining biome distribution. Latitude plays a crucial role—polar regions experience constant cold, while equatorial areas maintain high temperatures with limited variation. The mid-latitude or temperate regions (like the UK) experience distinct seasons with temperature variations.

Did you know? Even at the equator, you can find alpine environments similar to polar regions if you climb high enough mountains, like Mount Kilimanjaro. This is because temperature decreases with increasing height!

Rainfall patterns also significantly impact biome distribution. Some regions receive precipitation evenly throughout the year, while others experience seasonal variations. The Mediterranean climate, for instance, receives most of its rainfall during winter rather than summer.

Major Global Biomes

Tundra covers about 6% of Earth's land surface, found in extreme high latitudes $55-70°N$ including northern Alaska, Canada, Greenland, and Russia. It's characterized by extremely cold temperatures where only low-growing hardy plants survive—no trees can grow here!

The temperate grasslands (7% of Earth's surface) occupy mid-latitudes between 30-50°N and 30-40°S. These include the North American prairies, Russian steppes, and the South African Veld. They feature varied grasslands with few trees due to low rainfall, natural fires, and grazing herbivores.

Challenge yourself: Can you explain why trees are rare in both tundra and grasslands despite their very different temperatures?

Hot deserts (12% of Earth's surface) exist around 20-30° north and south of the equator, including the Sahara, Mojave, and Great Australian Desert. These regions experience extreme aridity, limiting biological diversity.

The tropical rainforests (13% of Earth's surface) thrive between 10°N and 10°S of the equator in regions like the Amazon Basin and Southeast Asia. They're the most biodiverse terrestrial biomes, with scientists discovering new species regularly!

Understanding Soil Composition

Soil forms on Earth's surface through weathering of rock and the addition of organic material from biological activity. It's the foundation of terrestrial ecosystems and has a complex composition.

A typical soil consists of approximately 45% mineral particles, 25% water, 25% air, and 5% organic matter. The organic component includes decomposed plant and animal matter (humus), living organisms, and plant roots—these elements contribute to soil fertility.

Soil typically has three distinct layers or horizons. The topsoil (A horizon) contains decomposed organic material rich in plant nutrients. The subsoil (B horizon) consists mainly of inorganic rock fragments ranging from fine clays to sand. The C horizon is the parent material of weathering rock.

Remember this: Good soil takes an incredibly long time to form—approximately 1,000 years to create just 1cm of topsoil. This makes soil conservation extremely important!

The structure and composition of soil vary significantly between biomes, affecting what can grow there and how ecosystems function.

Soil Formation Factors: CROPT

Soils develop through the influence of five key factors, easily remembered as CROPT:

Climate determines moisture and temperature conditions—arguably the most influential factor. Relief (landscape) affects soil development; mountainous areas typically have thin, infertile soils, while lowlands tend to develop deeper, more fertile soils.

Organisms (plants and animals) contribute to the organic component, influencing soil pH and fertility. Parent material (the underlying rock) affects soil texture and mineral content. Time allows soil profiles to develop—younger soils show more influence from parent material than older ones.

Top tip: When studying soils, think about DCATS—Depth, Colour, Acidity, and Texture—as key characteristics that help identify and understand different soil types!

Several processes shape soil development. Weathering of parent material and decomposition of organic matter create the basic soil structure. Within established soil, leaching (the downward movement of dissolved compounds in water) significantly affects soil profile development.

Temperate Grassland Climate and Soils

The temperate grassland biome experiences dramatic seasonal variations. Summers can reach scorching 35°C, while winters plunge to -30°C, creating an average annual temperature range of about 30°C. This extreme variation occurs because of the continentality effect—being far from oceans means the land heats and cools rapidly without the moderating influence of large water bodies.

Annual rainfall in temperate grasslands is relatively low $400-800mm$, occurring mainly in spring and summer. Winter precipitation falls as snow, providing moisture when it melts in spring. However, summer evaporation often leads to drought conditions.

Fascinating fact: The rich, dark soil of temperate grasslands (called Mollisol or Chernozem) is among the most fertile on Earth, which is why these regions have become the world's breadbaskets!

The soil profile shows a deep (1.5m), dark brown topsoil with a crumb structure that's excellent for plant growth. A thick sod layer of dense grass roots protects against erosion. The parent material is often loess—wind-blown, calcium-rich deposits that contribute to soil fertility.

Soil Processes in Temperate Grasslands

Two key soil processes shape the temperate grassland soils: leaching and capillary action. Leaching occurs when dissolved minerals move downward through the soil after spring snowmelt or summer downpours. This process is limited during other times of year due to low precipitation.

Capillary action works in the opposite direction—it's the upward movement of water and minerals. During summer drought conditions, water is drawn upward, bringing calcium from the loess parent material. This forms hardened calcium carbonate nodules at the bottom of the A horizon.

The resulting soil characteristics make temperate grassland soils incredibly productive. They're deep (1.5m), have a rich dark brown colour indicating fertility, and contain mull humus—well-decomposed organic material providing nutrients. The soil has a good mix of sand, silt, and clay, creating a balanced texture.

Exam tip: Understanding these soil processes explains why temperate grasslands have been converted to some of the world's most productive agricultural regions!

The soil teems with organisms that mix soil layers, aerate it, and facilitate decomposition. This biological activity, combined with the chemical processes, creates ideal conditions for grasses and agricultural crops.

Tundra Climate and Soils

The tundra biome experiences extremely harsh conditions with bitterly cold winters lasting 6-10 months, when temperatures average -25°C and can drop to -30°C. These conditions halt plant growth completely. Summers are brief and cool $4-15°C$, allowing only 50-60 days of potential plant growth annually.

This extreme climate results from the tundra's high latitude position. During winter, the sun barely rises above the horizon, providing minimal warmth. Even in summer when the sun never sets, its low angle in the sky delivers limited heat energy.

Precipitation in the tundra is remarkably low—just 130-250mm annually—mostly falling as snow. High atmospheric pressure dominates these latitudes, causing sinking air that produces few clouds and little precipitation.

Visualise this: During the brief summer, the top layer of soil thaws while the ground beneath remains permanently frozen, creating a waterlogged surface where your boots would sink into soggy moss!

The soil type found in tundra regions is called Gelisol. Its most distinctive feature is the permafrost layer—permanently frozen ground that begins about 0.5m below the surface. Above this lies the "active layer" that thaws during summer, becoming waterlogged as water cannot penetrate the impermeable permafrost below.

Tundra Soil Characteristics

Tundra soils are quite young—having developed only over the last 10,000 years since the last ice age—and remain shallow (about 0.5m) due to the limiting permafrost layer. The extremely cold conditions severely restrict biological activity, slowing soil development processes.

The soil's fertility is poor, with acidic, waterlogged conditions in summer and frozen soil in winter limiting plant diversity and productivity. This restricted plant growth means there's little organic material returning to the soil, leading to minimal humus formation and nutrient release.

Unlike other biome soils, tundra soils lack distinct horizons because few organisms are present to mix materials, and soil processes occur very slowly. The humus that does form is mor humus—acidic and poorly decomposed due to the cold conditions.

Critical insight: The permafrost layer explains why trees cannot grow in the tundra, as their roots cannot penetrate this frozen barrier. This is why tundra vegetation consists primarily of shallow-rooted plants like mosses, lichens, and small shrubs.

Angular rock fragments appear throughout the soil, originating directly from the bedrock below, with minimal weathering due to the cold conditions.