Ecosystems & Biomes
What is an Ecosystem?
An ecosystem is a community of plants and animals interacting with the non-living parts of their environment — soil, water, sunlight and climate. Ecosystems work at every scale, from a rock pool a metre across to a tropical rainforest covering several million square kilometres. The largest ecosystems are called biomes. Biomes follow climate zones, so on a map their boundaries roughly track latitude and rainfall: rainforests sit around the equator, deserts lie along 30° latitude, and tundra wraps the polar regions.
Six major biomes wrap the planet from the equator to the poles — tropical rainforest, savanna, hot desert, temperate forest, taiga (boreal forest) and tundra. Each is defined mainly by its climate, which controls what plants can grow.
Global Distribution of Biomes
Around the equator, where the sun is most direct year-round and air rises and cools, you get heavy rainfall — this drives the wet, warm tropical rainforest belt of the Amazon, Congo and Indonesia. At about 30° north and south, descending dry air gives the world’s deserts (Sahara, Atacama, Australian Outback). Higher latitudes get cooler temperate forests, then taiga (huge coniferous forests of Russia and Canada) and finally tundra in the polar margins. Climate is the master switch: change the rainfall or temperature, and the biome changes too.
The Tropical Rainforest in Detail
The tropical rainforest covers around 6% of the Earth’s land surface but holds an estimated 50% of all species. Average temperatures sit at 25–30°C all year, and annual rainfall typically exceeds 2,000 mm. The forest grows in distinct layers: the emergents (50 m tall trees breaking above the canopy), the canopy (the 30–40 m roof where most photosynthesis happens), the understory of younger trees, and the dim, fern-rich forest floor. Because conditions are warm and wet all year, plants grow continuously and there is fierce competition for light.
Tropical rainforest soils are surprisingly poor. Nutrients are stored in the living biomass — the trees themselves — not the soil. When a leaf or animal dies, decomposers (bacteria, fungi, termites) break it down within weeks. Tree roots are shallow and grab the nutrients before heavy rain can wash them away. Cut the trees down and the cycle collapses: rain leaches nutrients straight through the thin soil, and within 3–5 years the land is too poor to farm. This is why deforestation is so much more damaging in the tropics than in temperate forests, where soils are deeper and more fertile.
Plant and Animal Adaptations
Rainforest species have evolved precise adaptations to thrive in heat, humidity and competition. Trees grow buttress roots — flared structures up to 5 m tall — to support enormous trunks in shallow soil. Leaves have drip tips so that heavy rain runs off, preventing fungal growth. Lianas climb other trees to reach the canopy without growing their own thick trunks. Animals show equally specialised adaptations: sloths hang upside down with hooked claws and grow algae on their fur for camouflage; toucans have lightweight beaks to reach fruit on thin branches; poison dart frogs advertise toxicity with bright colours.
A UK Small-Scale Ecosystem: Hedgerows
You need to know one small-scale UK ecosystem. UK hedgerows are an excellent choice because they are everywhere and easy to study. A typical hawthorn hedgerow forms a thin strip of woodland edge habitat between fields. Soil at the base is rich and dark from years of leaf litter; mosses and ferns grow at ground level, brambles and bracken fill the middle, and hawthorn or blackthorn forms the canopy at 2–4 m. The hedge supports more than 600 plant species, 1,500 insect species and 65 nesting bird species across the UK as a whole — an extraordinary biodiversity in just a few metres of width.
Hawthorn flowers and berries feed insects (butterflies, hoverflies) and birds (blackbirds, redwings). Insects feed shrews, hedgehogs and bats; small mammals are eaten by tawny owls, foxes and kestrels. Decomposers in the leaf litter return nutrients to the soil. The hedgerow also delivers ecosystem services that benefit people: it shelters crops from wind, slows surface water runoff to reduce field flooding, stores carbon in its woody biomass, and supports pollinators that fertilise nearby crops — about £500m worth of crop pollination every year in the UK.
UK hedgerows are vital wildlife corridors — they connect woodlands, support over 2,100 plant and animal species, and provide ecosystem services worth hundreds of millions of pounds a year through pollination, flood reduction and carbon storage.