What Is an Ecosystem?

An ecosystem is all the living things in one area, together with the non-living parts of their environment that they depend on. A rainforest, a coral reef, a freshwater pond, a meadow, even a small patch of garden are all ecosystems. Living things in an ecosystem are constantly interacting: eating each other, hiding from each other, helping each other, sharing the same air, water and soil. The study of how ecosystems work is called ecology, and it is one of the most important branches of modern biology.

  • Living partsBioticPlants, animals, microbes, fungi
  • Non-living partsAbioticAir, water, soil, sunlight, temperature
  • Smallest ecosystemA puddleOr even a single rotting log
  • LargestThe biosphereEvery ecosystem on Earth combined
  • Major biomes on land13Each with its typical ecosystems
  • Source of all energyThe SunCaptured by plants in photosynthesis

Biotic and abiotic factors

Every ecosystem has two main parts.

  • Biotic (living) factors: the plants, animals, fungi and microbes that live in the area, plus the dead organic material left behind by them.
  • Abiotic (non-living) factors: the air, water, soil, rocks, sunlight, temperature, weather and chemistry of the place.

The two parts shape each other. Plants need particular sunlight, water and soil to grow; once they grow, they change the soil chemistry, affect the local microclimate and provide habitat for animals. Animals eat the plants, fertilise the soil with their droppings, and so on. An ecosystem is the constant interplay of all these.

Ecosystems come in all sizes

Ecosystems can be enormous or tiny.

  • The Amazon rainforest, covering 5.5 million km2, is a huge ecosystem with millions of species.
  • A back garden pond is a smaller ecosystem with maybe 100 species.
  • A single rotting log on a forest floor is a small ecosystem of mosses, fungi, beetles, woodlice and microbes.
  • A drop of pond water is an ecosystem too, full of single-celled microbes hunting each other.

Scientists often divide ecosystems into biomes: large regions with similar climates and plant communities. Earth has around 13 major biomes, including desert, rainforest, grassland, tundra, freshwater and marine.

How energy flows

Almost every ecosystem on Earth is powered by the Sun. The energy flows through the ecosystem in a chain.

  • Producers (plants and algae) use sunlight to make their own food through photosynthesis.
  • Primary consumers (herbivores) eat the producers.
  • Secondary consumers (small predators) eat the herbivores.
  • Tertiary consumers (top predators) eat the smaller predators.
  • Decomposers (bacteria and fungi) break down dead bodies at every level, recycling the nutrients back into the soil.

About 90% of the energy is lost at each step, used up by movement, breathing and heat. That is why there are always far fewer top predators than producers.

How matter is recycled

Unlike energy (which flows through the system and is lost as heat), matter is recycled within an ecosystem. The same atoms of carbon, nitrogen, oxygen and water move endlessly between living things and the environment in biogeochemical cycles:

  • The water cycle: water moves between ocean, atmosphere, land and living things.
  • The carbon cycle: carbon moves between the air (CO2), plants (sugars), animals (food) and back to the air or into rocks.
  • The nitrogen cycle: nitrogen is fixed from the air by bacteria, used by plants, eaten by animals, returned to the soil and eventually back to the air.
Fact Almost every atom in your body has cycled through many ecosystems before becoming part of you. The water you drank this morning has been around for billions of years and has probably passed through ancient seas, prehistoric clouds, swamps full of dinosaurs and the rivers of every civilisation in history. The carbon in your DNA was made inside an ancient star and may have been part of dozens of plants and animals before reaching you.

Why ecosystems matter

Healthy ecosystems do enormous amounts of work that we take for granted. They:

  • Produce all the oxygen we breathe.
  • Pollinate roughly 75% of our food crops.
  • Filter and purify our water.
  • Recycle waste and dead material.
  • Build and maintain healthy soil.
  • Regulate the climate.
  • Prevent floods, soil erosion and landslides.
  • Provide food, materials and medicines.

The estimated total value of all these "ecosystem services" to humanity is many tens of trillions of dollars per year, vastly more than the entire global economy. Yet ecosystem services do not show up in any country's GDP, which is one reason ecosystems are so often taken for granted.

Did you know? A teaspoon of healthy garden soil contains more living organisms than there are people on Earth. Bacteria, fungi, protozoa, nematodes, mites and many other tiny creatures all interact in a single teaspoon. Soil is one of the densest ecosystems on the planet, even though we never see most of it.
Deeper dive: keystone species and the wolves of Yellowstone

Most ecosystems are surprisingly delicate. The loss (or arrival) of a single species can ripple through the whole system in unexpected ways. The most important species in this sense are called keystone species: species that have a much bigger effect on an ecosystem than their numbers alone would suggest.

The most famous example is the reintroduction of wolves to Yellowstone National Park in 1995. Wolves had been hunted to extinction in the park in the early 1900s. Without wolves, the elk population had exploded; the elk ate so much willow and aspen along the rivers that hardly any new trees grew up. River banks eroded. Beavers vanished. Songbirds declined. Without realising it, the park's ecosystem had been gutted by the loss of a single predator.

When 31 wolves were reintroduced in 1995-96, the change was dramatic. Wolf predation reduced elk numbers, and the surviving elk avoided the river valleys where they were most vulnerable. Willows and aspen started growing back. Beavers returned, building dams that created new ponds. Birds came back. Even the rivers themselves changed shape as plant roots stabilised their banks. The whole ecosystem reorganised, all because of about 30 wolves.

This kind of cascading effect is called a trophic cascade, and it shows how interconnected ecosystems really are. The lesson for conservation is that protecting the right species, especially top predators, can sometimes do far more good than a hundred other interventions combined.

For the chain of who-eats-who, see food chains and food webs. For the variety of life, see biodiversity.