A community plus its surroundings
An ecosystem is all the living things in an area together with the non-living parts of their surroundings, considered as one interacting system. The living part is called the biological community: plants, animals, fungi, bacteria and everything else that lives there. The non-living part includes sunlight, temperature, water, air, soil and minerals.
The key word is “interacting.” A list of the species in a meadow is not an ecosystem description. What makes it an ecosystem is the connections: rain soaks into soil, grass roots take up the water, rabbits eat the grass, foxes eat the rabbits, and when any of them die, microbes return their nutrients to the soil.
The term was introduced in the early twentieth century by the British ecologist Arthur Tansley, who wanted a word for living things and their physical environment viewed together. It is now one of the most common ideas in environmental science.
Living and non-living parts
Ecologists often split the living members into roles.
- Producers make their own food, usually from sunlight. Plants, algae and some bacteria are the main examples.
- Consumers get energy by eating other organisms. Herbivores eat plants, carnivores eat animals, and omnivores eat both.
- Decomposers break down dead material and waste. Fungi and bacteria do most of this work, releasing nutrients that producers can use again.
The non-living, or abiotic, factors set the stage. Temperature and rainfall decide whether a place is a desert, grassland or forest. Soil type affects which plants can root. In water, the amount of dissolved oxygen, salt and light shapes who can live where. Living things also change these factors in turn: trees shade the ground, cool the air beneath them and drop leaves that build soil.
Energy flows, nutrients cycle
Two things move through every ecosystem, and they behave differently.
Energy generally enters as sunlight, is captured by producers, passes to consumers when they eat, and leaves as heat. It does not recycle. The ecosystem needs a steady supply from outside, which is why the Sun is so central. How that energy is passed along is covered in more detail in the article on food webs.
Nutrients such as carbon, nitrogen and phosphorus, by contrast, are used over and over. A nitrogen atom in a leaf may be eaten by a caterpillar, end up in a bird, and return to the soil when the bird dies. The same logic applies to water, which moves through living things and the environment as part of the water cycle. Carbon moves between the air and living things through processes such as plant photosynthesis and respiration.
Ecosystems come in all sizes
There is no single correct size for an ecosystem. A rotting log with its insects, fungi and bacteria can be studied as an ecosystem. So can a pond, a coral reef, a forest, or, at the largest scale, the whole biosphere. Where the boundary is drawn depends on the question being asked. Boundaries are also usually leaky: birds fly between forests and wetlands, streams carry nutrients downhill, and wind moves seeds and dust.
Scientists often group large ecosystems into types, such as tropical rainforests, temperate forests, grasslands, deserts, tundra, freshwater and marine systems. Each has typical conditions and typical kinds of organisms, though real places vary and blend into each other.
What “ecosystem balance” means, and its limits
People often describe a healthy ecosystem as being in balance. There is something in this: a mature forest or lake tends to have populations that stay within a fairly steady range, because predators, food supply, disease and competition push back when a population grows too large or too small.
But ecologists today are cautious about picturing a perfect, fixed balance. Real ecosystems change constantly. Fires, storms, floods, droughts and population cycles are normal in many places, and some habitats depend on disturbance to stay as they are. A better picture is of a system that fluctuates around a typical pattern and can recover from moderate shocks, though not always from large or repeated ones.
Researchers use the word resilience for the ability to absorb disturbance and keep working in much the same way. Resilience is linked to the variety of species present, a point explored in the article on species biodiversity. When a system is pushed past a tipping point, it can shift to a different state, such as a clear lake becoming a murky, algae-filled one. Whether and when that happens in a given place is often hard to predict.
Why the idea is useful
Thinking in terms of ecosystems helps explain why removing one species can have effects that seem unrelated. Predators that keep grazers in check, for example, can indirectly shape how many plants grow and even how a stream flows. The details differ from place to place, and well-studied cases are not guaranteed to repeat elsewhere, but the general lesson is that species are connected.
It also helps in understanding what people get from nature. Ecosystems filter water, build soil, pollinate crops, store carbon and provide food and materials. Sometimes these are called ecosystem services. Human activities, from farming to building cities, change ecosystems, and the loss of habitat is among the main reasons that species go extinct.
A quick way to read any habitat
When you look at a pond, park or forest, you can ask a few simple questions. What is the energy source? Who are the producers, and who eats them? Where do the dead leaves and animals go? What non-living conditions, such as water, light or soil, limit what can live here? The answers will not be complete, but they show how the pieces connect, which is the heart of the ecosystem idea.