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Natural ecosystems are usually stable because the flows of materials and energy within them are mostly balanced. However, human activity can upset ecosystems and an understanding of the effects of this can help us to use natural resources in a sustainable way.

Closed loop systems

In a perfect closed loop system no materials enter or leave the system. Waste products from one organism or process are used by another organism or process. It can be said, therefore, that the output from one part of the system becomes the input to another part.

No ecosystem [ecosystem: A community of animals, plants and microorganisms, together with the habitat where they live. can be a perfect closed loop system because some output is always lost, eg some organisms migrate [migrate: To travel long distances in search of a new habitat., some nutrients are transported away by the wind or by rain and rivers.

Recycling waste products

Within a natural ecosystem, most waste materials are not lost. They can be used as food or reactants for animals, plants and microorganisms.

  • Oxygen is a waste product from photosynthesis [photosynthesis: A chemical process used by plants and algae to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis.. It is used in respiration [respiration: Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration..

  • Carbon dioxide is a waste product from respiration. It is used in photosynthesis.

  • Dead organic matter (eg fallen leaves, fruits, flowers, faeces, remains of bodies) is used directly as food or processed into useful nutrients by microorganisms [microorganism: Another name for microbe. Microscopic (too small to see) organisms such as bacteria and viruses.. These microorganisms use digestive enzymes to break down complex molecules into simpler nutrients.

Closed loop system diagrams

We can show how substances move through an ecosystem using diagrams.

The carbon cycle

The carbon cycle

There is only a certain amount of carbon on Earth. This carbon is cycled through a number of stages where it exists in different forms. For example:

  • Carbon exists in the atmosphere as carbon dioxide. This can be taken out of the atmosphere by the process of photosynthesis - the carbon then becoming part of glucose molecules.

  • These glucose molecules are then broken down during respiration. The process of respiration releases carbon back into the atmosphere as carbon dioxide.

  • Carbon dioxide could also be released into the atmosphere by combustion of wood and fossil fuels.

  • Microorganisms and some animals feed on dead organic matter. The carbon then becomes part of these microorganisms. This process is known as decomposition, the carbon has become part of new organisms in the system


Note that throughout the processes described above, carbon is always being recycled.



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