Ecosystems and Food Chains

An ecosystem is all the living things in one area (plants, animals, fungi, microbes) together with the non-living parts of their environment (air, water, soil, sunlight). A rainforest, a coral reef, a pond and even a small patch of garden are all ecosystems. Inside any ecosystem, living things depend on each other for food, shelter and survival. Food chains trace the flow of energy from one living thing to another, all the way from the Sun to the top predator.

  • Major biomes13Different climates and plant types
  • Source of energyThe SunCaptured by plants in photosynthesis
  • Energy lost per stepapprox. 90%Only 10% passes up the food chain
  • Trophic levels (typical)3 to 5Producer to top predator
  • First ecosystemsapprox. 3.7 billion years agoMicrobes around hot vents and pools
  • Top ocean predatorOrca (killer whale)Hunts even great white sharks

The three jobs in a food chain

In any ecosystem, every living thing falls into one of three groups based on how it gets its energy.

  • Producers (mostly plants and algae) use sunlight, water and carbon dioxide to make their own food in a process called photosynthesis. They are the foundation of almost every food chain on Earth.
  • Consumers get their energy by eating other living things. Herbivores eat plants; carnivores eat other animals; omnivores (like humans) eat both.
  • Decomposers (bacteria, fungi and some animals like worms) break down dead plants and animals, recycling their nutrients back into the soil for new plants to use.

Food chains and food webs

A food chain is a simple line: grass → rabbit → fox. Each arrow means "is eaten by". Energy flows from the producer all the way up to the top predator. But most real ecosystems are far more tangled than that. Every plant might be eaten by dozens of different animals, and every predator might eat dozens of different things. A more accurate picture is a food web: many overlapping food chains crisscrossing each other.

Energy and the 10% rule

Each time energy passes up a food chain, around 90% of it is lost as heat from movement, breathing and digestion. Only about 10% makes it to the next level. This is why food chains rarely have more than 5 levels: by then there is so little energy left that no animal could survive on what is available. It is also why predators are always rarer than their prey, and why there are many more rabbits than foxes.

Fact Some ecosystems do not depend on the Sun at all. Around deep-sea hydrothermal vents 3,000 m down on the ocean floor, no sunlight ever reaches. Yet whole communities of strange creatures (tube worms, blind crabs, giant clams) thrive there, fed by bacteria that get their energy from the chemicals in the boiling water instead of from sunlight. Scientists think life on Earth may have first appeared in places like this.

Pick a topic below to explore ecosystems and food chains in more depth.

What Is an Ecosystem?A community of living things together with the non-living things (water, soil, weather) around them, all working as one system.
Food ChainsA line that shows who eats what in an ecosystem, from plants at the start to top predators at the end.
Food WebsA diagram showing how lots of food chains interconnect. Most real-world ecosystems are webs, not chains.
Producers, Consumers and DecomposersThe three jobs in any food web: producers make food (plants), consumers eat it (animals), decomposers recycle it (fungi, bacteria).
Predator and PreyThe hunter and the hunted. Their populations rise and fall together over time, keeping ecosystems in balance.
SymbiosisTwo different species living closely together. Sometimes both benefit, sometimes one benefits and one is harmed.
BiodiversityThe variety of life in an ecosystem. More biodiversity usually means a healthier and more resilient system.