What is an ecosystem?

Living things and their surroundings

Walk around a pond. You see water weeds, frogs, fish and insects. You also see water, mud, sunlight and stones. The living things depend on each other, and on the water, air and light around them.

All the living things of an area, together with the non-living parts of their surroundings, form an ecosystem.

• The living parts are the biotic components: plants, animals and microorganisms.
• The non-living parts are the abiotic components: temperature, rainfall, wind, soil, water and minerals.

Natural and artificial ecosystems

Some ecosystems form on their own. A pond, a forest, a grassland and a lake are natural ecosystems.

Others are made and looked after by people. A garden, a crop field and an aquarium are artificial ecosystems. In an aquarium we add fish food, clean the tank and change the water. Nature does these jobs in a pond.

Producers

Green plants and some bacteria can make sugar and starch from simple substances. They use the energy of sunlight, with the help of chlorophyll. This is photosynthesis. Such organisms are producers.

Every other living thing in the ecosystem depends on the food the producers make, directly or through other animals.

Consumers

Animals cannot make their own food. They eat the food made by producers, either straight from plants or by eating other animals. They are consumers. They are grouped by what they eat.

• Herbivores eat plants: a cow, a deer, a grasshopper.
• Carnivores eat other animals: a tiger, a frog, a snake.
• Omnivores eat both plants and animals: a crow, a bear, human beings.
• Parasites live on or inside another living thing and feed on it: a tapeworm, a louse, the dodder plant.

Decomposers

Plants shed leaves, and animals die. If nothing broke them down, dead matter would pile up everywhere. The nutrients locked in it would never return to the soil.

Decomposers do this job. Bacteria and fungi break down dead plants and animals into simple substances. These go back into the soil and water, where plants can use them again. So decomposers keep materials going round in the ecosystem.

Watch the litter on a forest floor below. Leaves keep falling, yet the litter does not pile up for ever. Decomposers break down a share of it each month, so it settles at a steady level.

Try it in Python

The program follows the litter on a forest floor for a year. Each month $25$ kg of leaves fall, and decomposers break down $40\%$ of all the litter.

Practise with new numbers

Common mistakes

• "An ecosystem is only the animals and plants." It also includes the non-living parts: soil, water, air, light and temperature.
• "Decomposers are consumers that eat living things." Decomposers break down dead matter and return nutrients to the soil.
• "An aquarium looks after itself." It is artificial, so people must feed the fish and clean the water.
• "Omnivores are animals that eat a lot." Omnivores eat both plants and animals.

Remember these

• An ecosystem is the living things of an area plus the non-living parts around them.
• Biotic components are living; abiotic components are non-living.
• Ponds, forests and grasslands are natural ecosystems; gardens, crop fields and aquariums are artificial.
• Producers make food by photosynthesis. Consumers are herbivores, carnivores, omnivores or parasites.
• Decomposers, bacteria and fungi, break down dead matter and return nutrients to the soil.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name two biotic and two abiotic components of a grassland.

2. Is a crop field a natural or an artificial ecosystem? Why?

3. A bear eats berries in summer and fish in autumn. What kind of consumer is it?

4. What would happen to a forest floor if all the bacteria and fungi died?

5. Why are green plants called producers?