What Is an Ecosystem?

🌍 What Is an Ecosystem?

An ecosystem is a community of living organisms interacting with each other and with their non-living environment. That single sentence contains three ideas that examiners test constantly, so let's unpack each one.

Living organisms = biotic factors. Everything alive: plants, animals, fungi, bacteria, insects, birds, algae. Every organism that eats, grows, reproduces, and eventually dies. In a temperate oak woodland, that includes the oak trees, the bluebells on the woodland floor, the caterpillars eating the oak leaves, the blue tits eating the caterpillars, the sparrowhawk hunting the blue tits, and the earthworms and fungi decomposing everything that dies.

Non-living environment = abiotic factors. Everything that is not alive but shapes what can live there: temperature, rainfall, sunlight, soil type, geology, topography (slope angle and aspect), pH, wind speed, and humidity. These factors determine which organisms can survive in a place. The tropical rainforest receives over 2,000 mm of rain a year near the equator — those abiotic conditions make it possible for the world's densest canopy to exist. The Sahara Desert receives less than 25 mm in some years — those same abiotic conditions make any life at all a remarkable feat of adaptation.

Interacting. This is the critical word. Biotic and abiotic components do not exist in isolation — they shape each other continuously. Rainfall (abiotic) determines which vegetation (biotic) grows. That vegetation holds the soil together (abiotic). The soil stores nutrients (abiotic) that feed the plants (biotic). Dead plants feed decomposers (biotic) that return nutrients to the soil (abiotic). Remove any one link and the whole system is affected. This is why geography is interested in ecosystems — not as lists of species, but as systems.

Scale: From Puddle to Planet

Ecosystems exist at every scale. A rock pool on a beach is a small-scale ecosystem: abiotic factors include salt concentration, water temperature, and tidal exposure; biotic factors include limpets, periwinkles, anemones, seaweed, and the bacteria decomposing dead material in the crevices. A tropical rainforest is a large-scale ecosystem stretching across multiple countries. The entire Amazon basin — 5.5 million km² — can be treated as a single ecosystem. And at the largest scale of all, the biosphere is the sum of all ecosystems on Earth.

For your exams, you need to be able to work at two scales: the small-scale ecosystem (typically a local UK example like a sand dune, heathland, or woodland) and the large-scale biome (tropical rainforest, hot desert, etc.). Both follow the same rules — the same energy flows, the same nutrient cycles, the same interdependence. The scale changes but the principles do not.