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The distribution of organisms in a habitat may be affected by physical factors, such as temperature and light.

The Sun is the original source of energy for almost all organisms on the Earth. The way energy passes through an ecosystem is described as energy flow.

Within an ecosystem, nutrients (such as carbon or nitrogen) are continually recycled.

There is increasing evidence that the level of carbon dioxide in the Earth’s atmosphere is rising. There is also evidence that humans are responsible for this rise.

Nitrogen gas makes up 78% of Earth’s atmosphere and is an essential component in amino acids and proteins.