What is an ecosystem? Food webs, energy flow and balance

An ecosystem is all the living things in a place, together with the nonliving surroundings they depend on and change: sunlight, air, water, soil and climate. A pond, a forest and a coral reef are ecosystems, and so is a rotting log. What makes it a system is the flow between the parts: energy enters as light and leaves as heat, while carbon, nitrogen and water are used and recycled over and over.

In short

• An ecosystem is a community of organisms plus its physical environment, linked by flows of energy and nutrients.
• Energy moves one way through a food web and only about a tenth of it passes from each level to the next, which is why food chains are short.
• Nutrients cycle; energy doesn't. Remove a key species and the whole web can rearrange itself, though how much is often argued.

What are the parts of an ecosystem?

The word was first used in print in 1935 by the British ecologist Arthur Tansley, who wanted a term for living things and their physical setting treated as one unit, with materials passing between them. Every ecosystem has the same two halves and the same three working roles.

Part	What it is	Examples in a temperate forest
Abiotic (nonliving)	Light, temperature, water, air, soil, minerals	Sunlight, rain, clay soil, winter frost
Producers	Organisms that make their own food, mostly by [[how-does-photosynthesis-work	photosynthesis]]
Consumers	Organisms that eat other organisms	Deer and caterpillars (herbivores), foxes and owls (carnivores), bears (omnivores)
Decomposers	Organisms that break down dead matter and waste, releasing nutrients	Fungi, bacteria, earthworms

Ecosystems have no fixed size. Ecologists draw the boundary to suit the question: a single tide pool, a lake and its shoreline, or the whole Serengeti. The edges are usually blurry, because water, animals and seeds cross them. The planet as a whole, every ecosystem together, is called the biosphere.

How does energy flow through a food chain?

A food chain is one path that energy takes: grass is eaten by a mouse, the mouse by a snake, the snake by a hawk. Each step is a trophic level, from the Greek for "nourishment". Real ecosystems are food webs, many crossing chains, because most animals eat more than one thing and are eaten by more than one thing. OpenStax's biology textbook treats both.

At every step most of the energy is lost. An animal uses most of what it eats to stay alive, to move, keep warm and repair itself, and that energy ends up as heat. Some of what it eats is not digested at all. Only the part built into new body tissue is available to whatever eats it next. The ecologist Raymond Lindeman described this in 1942, and it became the rule of thumb known as the 10% rule.

Worked example: 10,000 kJ of grass

Start with grass that has stored 10,000 kilojoules (kJ) of energy from sunlight over a season, and apply 10% at each step.

1. Grass: 10,000 kJ.
2. Mice that eat the grass build 10% of that into their bodies: 10,000 × 0.1 = 1,000 kJ.
3. Snakes that eat the mice: 1,000 × 0.1 = 100 kJ.
4. Hawks that eat the snakes: 100 × 0.1 = 10 kJ.

The hawk level holds a thousandth of the energy the grass started with. That is why ecosystems have far more plants than plant-eaters and far more plant-eaters than predators, a shape ecologists draw as a pyramid. It is why food chains rarely run past four or five levels: there isn't enough energy left at the top to support another. And it is why large predators need large territories: a hawk needs a wide hunting ground because each square meter supports so little of it.

The 10% is an average, not a law. Real efficiency depends on the animals. In a classic study of Silver Springs, a Florida river, OpenStax reports that the transfer between plants and plant-eaters was about 14%. Warm-blooded animals spend so much energy keeping warm that they usually pass on less than cold-blooded ones. The same arithmetic applies to people: a field of grain feeds more people eaten as grain than fed to animals first, though diets turn on far more than energy alone.

How do nutrients cycle?

Energy flows through an ecosystem once; matter goes round and round. The carbon in a leaf was in the air as carbon dioxide; a caterpillar eats the leaf; a bird eats the caterpillar; the bird dies; fungi and bacteria break it down, releasing the carbon as CO₂ again, which a plant takes up. Nitrogen, phosphorus and water cycle the same way.

Decomposers make this possible, and they are the part most people forget. Without them, dead matter would pile up and the nutrients locked inside it would never return to the soil. The difference between the two flows is the most common point of confusion: people sometimes say an ecosystem "recycles energy". It cannot. Every bit of energy that comes in as sunlight eventually leaves as heat radiated to space, so an ecosystem needs a constant supply of new light, while the same atoms can be used for millions of years.

What is a keystone species?

Some species matter far more than their numbers suggest. The ecologist Robert Paine showed this in the 1960s on the rocky shore of Washington state: he kept removing one predator, the ochre sea star, from a stretch of coast. Without it, mussels crowded out almost everything else, and the number of species there fell sharply. He called such species keystones, after the stone at the top of an arch that holds the rest in place.

A predator's effects can pass down a food chain in steps, called a trophic cascade: fewer predators means more plant-eaters, which means fewer plants, and the reverse. The best-known case is in Yellowstone National Park. Wolves had been killed off there by the 1920s. According to the National Park Service, 41 wild wolves from Canada and northwest Montana were released in the park from 1995 to 1997. Elk numbers fell over the next two decades, and in some places willow and aspen, which elk browse, grew taller.

Still argued

How much the wolves themselves caused the plants to recover is one of the longest-running arguments in ecology. On one side, William Ripple and colleagues argued in 2025 that the return of carnivores produced one of the strongest trophic cascades known, citing an estimated 1,500% rise in willow crown volume. On the other, Daniel MacNulty and colleagues at Utah State and Colorado State universities replied in a comment released in October 2025 that the figure was an artifact of circular modeling, because crown volume was calculated from the same height data it was used to predict, and that a 2024 study by N. Thompson Hobbs's team, using two decades of field experiments, found only a weak cascade. A summary published by the National Park Service of the debate, written in 2016 by park biologist Douglas Smith and colleagues including MacNulty, notes other causes in play: cougars and bears also returned or increased, elk were hunted outside the park, beavers lost early in the twentieth century had changed the streams, and water supply limits willow growth. It also notes there has been no increase in the area covered by willow and aspen, only taller existing stands. What is widely agreed is that wolves changed elk numbers and behavior and supplied carcasses to scavengers; the size of their effect on plants is not settled.

What happens when an ecosystem changes?

Ecosystems are always changing. After a fire or a flood, a sequence of species moves back in, called succession, from fast-growing weeds to shrubs and eventually a forest. Many ecosystems absorb shocks and recover, a property called resilience. Ones with many species, a property called biodiversity, tend to recover more reliably, because if one species is lost, another can often fill a similar role.

Pushed far enough, though, an ecosystem can flip into a different state and stay there: a clear lake turned green by fertilizer runoff, or a coral reef overgrown by seaweed after its fish are overfished. Human activity drives many such changes now, through land clearing, pollution, overfishing, invasive species and climate change. Shared resources such as fisheries are especially easy to drain, the problem known as the tragedy of the commons. Studying ecosystems is how ecologists work out which changes can be undone, and which cannot. It is also a way of seeing life not one organism at a time but as a web of dependence.

Check yourself

• Without looking back, explain why energy flows through an ecosystem but nutrients cycle, and which organisms make the cycling possible.
• Starting from 10,000 kJ of plants, how much energy reaches the fourth level, and what does that imply about the number of predators?
• What is a keystone species, and why is the Yellowstone wolf case still argued?

Sources

• 46.1 Ecology of Ecosystems — OpenStax, Biology 2e, 2018
• 46.2 Energy Flow through Ecosystems — OpenStax, Biology 2e, 2018
• Wolf ecology — US National Park Service, Yellowstone National Park
• The big scientific debate: trophic cascades — Smith, Peterson, MacNulty and Kohl, US National Park Service, Yellowstone Science, 2016
• Flawed analysis invalidates claim of a strong Yellowstone trophic cascade after wolf reintroduction — Utah State University, via EurekAlert, 21 October 2025
• Ecosystem — Wikipedia