An ecosystem (short for ecological system) is a self-contained system of interdependent organisms and their natural environment.

Every ecosystem is driven by light energy from the sun which enters the system via the green plants and is then passed on to the animals as chemical energy when they eat the plants. For example, in a woodland ecosystem, the plants absorb sunlight and rainwater for photosynthesis, the animals feed on the plants and on each other. The dead remains of animals and plants, acted upon by fungi and bacteria, return nutrients to the soil.

Components of an ecosystem

An ecosystem has biological and physical components.

Physical components of an ecosystem

The physical components of an ecosystem are the nonliving or abiotic parts of an ecosystem.

These include:

  • air
  • soil
  • light
  • water
  • rock particles

Biological components of an ecosystem

The biological components of an ecosystem are the living or biotic parts of an ecosystem.

These include:

  • animals
  • plants
  • fungi
  • bacteria
  • humus

Biotic components are typically sorted into three main categories:

  • producers
  • consumers
  • decomposers

Soil as an ecosystem

Soil itself is also an ecosystem and it contains physical and biological components. In soil there is a community of organisms, which includes earthworms, springtails and other insects, mites,
fungi and bacteria.

Components of soil

The physical components of soil are:

  • air
  • water
  • mineral salts
  • rock particles

The biological components of soil are:

  • earthworms
  • nematodes
  • termites
  • fungi
  • bacteria
  • humus

Natural ecosystem

A natural ecosystem is an interaction between organisms and their natural environment without human intervention.

Relationships between organisms in an ecosystem can be represented by food chains and food webs.

Food chains

A food chain is a linear feeding relationship between organisms of an ecosystem. The general form of a food chain is as follows:

Sun (Source of energy) → Producers (green plants) → Primary consumers (herbivores) → Secondary consumers (carnivores) → Tertiary consumers (carnivores)

Examples of food chains are:

  • grass → cow → human
  • grass → locust → lizard

Different levels on a food chain are called trophic levels. Energy decreases from a lower trophic level to a higher trophic level through:

  • respiration
  • movement
  • excretion

Approximately 10% of the energy received at each trophic level is passed on to the next level.

Food webs

A food web shows the feeding interrelationships between organisms of an ecosystem. An organism feeds on several organisms and can itself be food for more than one type of organism as shown below:


Example of a food web

From the food web shown above we can extract the following food chains:

  • grass → rabbit → hawk
  • grass → rabbit → snake → hawk
  • grass → grasshopper → snake → hawk
  • grass → mouse → snake → hawk
  • seeds → mouse → snake → hawk
  • etc

The diagram below shows another example of a food web:

Food web

Example of a food web