Characteristics and Climate of Major Ecosystems

Recognising a large-scale ecosystem

An ecosystem includes living organisms and the non-living surroundings with which they interact. A biome is a large-scale ecosystem with a characteristic climate and vegetation. Forests, grasslands, deserts and tundra are broad groups, but the differences within them matter: a tropical rainforest is very different from a boreal forest.

Temperature and the availability of water help determine how much vegetation can grow and when it grows. The growing season is the part of the year when conditions allow plant growth. It may last all year in a warm, wet rainforest, but only a few weeks in Arctic tundra. These differences also affect the food and shelter available to animals.

The locations below describe broad patterns. Each biome varies between places, so example climate figures are guides rather than fixed boundaries.

Tropical rainforest: warm, wet and layered

Tropical rainforests occur around the equator, including the Amazon in Brazil. Temperatures remain high throughout the year, with little annual variation. Rainfall is abundant; an Amazon example receives about 2,000–2,200 mm annually, with temperatures around 26–30°C.

Warmth and plentiful water allow year-round growth. Most trees are evergreen: they replace individual leaves rather than losing all their foliage in one season. Tall emergent trees rise above a canopy, with an under-canopy and a lower shrub layer beneath. The canopy intercepts much of the sunlight, so relatively little reaches the forest floor.

Lianas climb trees towards the light. Epiphytes grow on other plants, using them for support. The forest's varied layers provide many habitats, supporting high biodiversity, including monkeys, sloths, snakes and insects.

Temperate forest: growth follows the seasons

Temperate forests occur in mid-latitude regions, including the UK, western Europe, eastern North America and eastern Asia. They have warmer summers and cooler winters, with precipitation through the year. A UK example has monthly temperatures around 4–18°C and annual rainfall of roughly 1,000 mm.

Deciduous trees, such as oak, shed their leaves in autumn and grow new ones in spring. Growth is therefore seasonal rather than continuous. A canopy, sub-canopy, herb layer and ground layer provide different habitats. Fallen leaves form litter which decomposes and returns nutrients to the soil. Deer and rabbits are examples of animals found in these forests.

Boreal forest: surviving long, cold winters

Boreal forest, also called taiga, forms a broad belt across Canada, Alaska, northern Europe and Russia, south of the Arctic tundra. Winters are long and cold; summers are short and relatively cool. A Russian example has monthly temperatures around −10 to 15°C and annual precipitation of about 500 mm.

Evergreen conifers such as pine, spruce and fir dominate. Their narrow needles limit water loss, while retaining foliage lets them begin photosynthesis when suitable conditions return without first growing a whole new set of leaves. Thick bark provides protection from cold.

The forest generally has a simpler structure than tropical rainforest, often with a tree layer and a ground layer. Cold conditions slow decomposition, and soils tend to be acidic and nutrient-poor. Wolves, deer and bears inhabit boreal forests.

Grasslands: tropical savanna and temperate grassland

Tropical grasslands, or savannas, occur in Africa, South America and northern Australia. East Africa provides familiar examples. They remain hot throughout the year, typically around 25–30°C, but have distinct wet and dry seasons.

Grasses dominate, with scattered trees such as acacia. The prolonged dry season restricts the development of dense forest. Fire also helps maintain grassland, and extensive root systems allow many plants to regrow afterwards. In East African savannas, animals such as wildebeest migrate in search of food; giraffes and elephants also occur.

Temperate grasslands include the North American prairies, Eurasian steppes and the Pampas of Argentina. Their temperatures vary more seasonally, especially in continental interiors where summers can be hot and winters cold. Annual precipitation is commonly about 250–750 mm.

Grasses dominate, with few trees except where additional water is available, such as beside rivers. Grass roots contribute organic matter to the soil, helping create fertile soils. These ecosystems support grazing animals, such as bison on North American prairies and guanacos in the Pampas.

Deserts: limited water, not necessarily constant heat

Hot deserts occur mainly around the tropics, roughly 15–30° north and south of the equator. Examples include northern African deserts and Australia's Great Sandy Desert. Desert precipitation is very low and unreliable. Some deserts are cold: lack of water, rather than high temperature alone, is their defining feature.

Vegetation is sparse. Plants may have small leaves, deep roots or water-storing stems; cacti are examples from American deserts. Other plants grow and reproduce rapidly after rain. Some desert animals avoid daytime heat by being nocturnal or sheltering in burrows.

Hot deserts can have large differences between daytime and night-time temperatures. This daily contrast is different from the annual temperature range shown by a monthly climate graph.

Tundra: low plants and a short growing season

Arctic tundra lies north of the boreal forest across the northern parts of North America, Europe and Asia. Winters are extremely cold, while summers are short and cool. Average summer temperatures are commonly around 3–12°C, and precipitation is low.

Trees are absent; low-growing shrubs, other small plants and lichens dominate. The growing season lasts only about 10–12 weeks, although long summer daylight allows rapid growth during that period.

Much tundra has permafrost beneath the surface. Frozen ground restricts deep rooting, while cold conditions slow decomposition and nutrient release. These conditions help explain its low-growing vegetation and relatively low biodiversity.

Reading and comparing climate graphs

A climate graph usually shows months along the horizontal axis, precipitation as bars in millimetres and mean monthly temperature as a line in degrees Celsius. Read each using its own vertical scale. Precipitation is a monthly total; temperature is a monthly mean.

Start with the temperature pattern: identify the warmest and coldest months, then subtract the lowest temperature from the highest to find the annual range. Next examine precipitation: identify wetter and drier months and add the monthly totals to calculate annual precipitation.

The rainforest-style graph stays between 26 and 28°C, giving an annual temperature range of 2°C. Its precipitation totals 2,160 mm, with substantial rain every month. The temperate-forest-style graph ranges from 4 to 18°C, a range of 14°C, and receives 1,000 mm annually. Its July–August temperature peak represents a northern-hemisphere seasonal pattern.

A useful comparison is therefore: ‘The rainforest-style climate is warmer throughout the year and has a smaller annual temperature range, 2°C compared with 14°C. It also receives more annual precipitation, 2,160 mm compared with 1,000 mm.’ This links the graphs to year-round rainforest growth and seasonal temperate-forest growth.

Apply the same method to other graphs. A hot climate with a marked dry season suggests tropical grassland rather than rainforest. Very low precipitation suggests desert; a long freezing winter and short cool summer suggests boreal forest or tundra. Climate graphs provide evidence about suitable growing conditions, but vegetation and location help distinguish biomes whose climates overlap.