The amount of carbon dioxide in the atmosphere would greatly increase if they were fewer

The correct answer is - plants.

Explanation:

Plants are the producers which mean they are autotrophs that generate their food by themselves. of the ecosystem and uses photosynthesis. In the process of photosynthesis, the plants convert atmospheric carbon dioxide and water into glucose and oxygen in the presence of light.

This process helps in lowering the carbon dioxide in the atmosphere and if the plants were fewer carbon dioxide would increase a great extent.

Thus, the correct answer is - plants.

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