Here's why:

* Carbon dioxide (CO2) is the primary form of carbon in the atmosphere. Plants take in CO2 during photosynthesis and use it to build their tissues.

* This carbon then passes through the food web: When animals eat plants, they consume the carbon stored in those plants. This carbon continues to move through the food web as animals consume other animals.

* Decomposers break down dead organisms: When organisms die, decomposers break down their tissues, releasing some carbon back into the atmosphere as CO2.

* Carbon cycles through the ecosystem: This continuous cycle of carbon moving from the atmosphere to organisms and back again is what makes carbon a vital element for life on Earth.

While there are other sources of carbon, like rocks and fossil fuels, the atmosphere is the primary source that fuels the carbon cycle within a given ecosystem.