Ecosystems & Global Distribution
What Is an Ecosystem?
An ecosystem is a community of living things and the physical conditions they share. Every ecosystem on the planet is built from the same two types of components. Abiotic components are the non-living parts: weather, climate, soil, rock and water. Biotic components are the living parts: plants, animals, fungi, bacteria and humans.
The two are interdependent — remove one and the others change. Cut down a rainforest tree and the soil around it dries faster, the water cycle slows, animals lose their habitat and bacteria in the leaf litter starve. Raise sea-surface temperature by about one degree above the usual summer maximum and corals bleach, fish populations migrate, and entire reef ecosystems collapse. Understanding interdependence is the foundation of every ecosystem case study.
Every ecosystem has abiotic components (weather, climate, soil, rock, water) and biotic components (plants, animals, fungi, bacteria, humans). They are interdependent — a change in one affects all the others.
The Global Pattern of Ecosystems
The world's major ecosystems — the biomes — show a clear pattern from the equator to the poles. Polar regions sit at the highest latitudes (above the Arctic and Antarctic Circles), with extreme cold, low precipitation and minimal vegetation. Coral reefs form in shallow tropical seas where water is warm (23-29°C), clear and salty. Hot deserts like the Sahara, Atacama and Australian Outback occupy 20-30° latitude north and south, where descending air from the global atmospheric circulation produces dry conditions. Grasslands form in mid-latitudes (the Argentine Pampas, North American Prairies) where rainfall is too low to support forest but high enough for grasses.
Temperate forests dominate mid-latitudes with seasonal climates — UK woodlands, the eastern United States, parts of China and Japan. Tropical rainforests sit in a belt around the equator (Amazon, Congo Basin, South East Asia) where temperatures stay 25-28°C year-round and rainfall exceeds 2000mm. The pattern is not random — it follows the global circulation of the atmosphere, ocean currents and latitude-driven temperature.
At the equator, the sun is high overhead all year, warming air and ocean intensely. Warm air rises, cools and produces heavy convectional rainfall almost daily. The combination of constant warmth and heavy rainfall creates conditions for the most diverse ecosystems on Earth. The Amazon alone is home to an estimated 10% of all known species. Move 10° either way and the rainfall pattern becomes seasonal — rainforest gives way to seasonal forest and savannah.
Climate, Plants and Animals in Each Biome
Each biome has a distinctive climate, plant community and animal community:
- Polar regions — very cold (-50 to 5°C), little precipitation, slow-growing lichens and mosses, polar bears, seals, penguins (Antarctica only).
- Coral reefs — warm tropical seas, sunlit shallows; coral colonies form the structure that shelters thousands of fish species, sharks, turtles and molluscs.
- Tropical rainforests — warm, wet all year; multi-storey vegetation (emergents, canopy, under-storey, ground layer); jaguars, sloths, parrots, vast insect populations.
- Hot deserts — very hot days, cold nights; cacti, drought-resistant scrub; camels, scorpions, snakes, desert foxes.
- Temperate forests — four seasons; deciduous oak, beech and ash; deer, foxes, badgers, woodland birds.
- Grasslands — seasonal rainfall, fertile soils; grasses dominate; herds of bison, antelope, kangaroos.
Vegetation type follows climate. Where temperatures stay warm and rainfall is plentiful, dense forests grow with multiple layers. Where rainfall is seasonal, grasslands or savannahs dominate. Where it is consistently dry, deserts form with sparse, drought-tolerant vegetation. Where it is consistently cold, tundra mosses and lichens are the main plant life. The mapping of climate to vegetation is one of the most predictable relationships in geography.
The six global biomes — polar, coral reef, tropical rainforest, hot desert, grassland, temperate forest — form a predictable pattern from the equator to the poles, controlled by latitude, atmospheric circulation and ocean currents. Climate sets the framework; plants and animals adapt to fit.
Putting It Together
Ecosystems are not random — they sit in a global pattern shaped by latitude, atmosphere and ocean. The interdependence of abiotic and biotic components means a change in one part triggers changes everywhere else. The next two lessons explore two of the most threatened biomes — tropical rainforests and coral reefs — in case-study detail.