In almost all ecosystems, animals exhale carbon dioxide and plants absorb carbon dioxide thus creating a system in which carbon is exchanged back and forth between the environment and the organisms within it, which could be easily measured by examining the pH if it's an aquatic environment. It was hypothesized that if carbon cycles in a closed aquatic community, carbon levels will remain relatively stable. By filling four test tubes with an equal amount of water and 2% bromothymol blue, a closed aquatic environment was created in which the pH could be measured in order to track the carbon levels since pH level and carbon levels are inversely related.
In the first test tube, a snail and cabomba plant was placed, in the second, a single snail, in the third, a single cabomba plant, and lastly in the fourth just the plain mixture of water and bromothymol blue.
With all the test tubes beginning with a pH of 7.5, after testing, the first snail and cabomba test tube had a reported pH of around 7.
2 (indicating a 0.3 change), the snail test tube had a pH of around 6.25 (indicating a 1.25 change), the cabomba plant test tube had a pH of around 8 (indicating a 1.0 change), and the control test tube had a pH of 7.5 (indicating no change). This data clearly supports the hypothesis since the test tube with only the snail had a decrease in pH indicating an increase in carbon and the test tube with only the cabomba plant had an increase in pH indicating a decrease in carbon, but the test tube with both the snail and the cabomba plant only suffered a small decrease in pH indicating a relatively stable amount of carbon exchange.
This is because as the snail performs cellular respiration since it's an animal, it releases carbon dioxide and as the cabomba plant performs photosynthesis since well it's a plant, it absorbs carbon dioxide and when the two are combined in a closed aquatic community, one releases and the other absorbs, creating a community in which there are relatively stable carbon levels.
Thus with the stable relationship of carbon exchange between animals and plants, it creates an ecosystem in which carbon levels are relatively stable, but when animals start to produce more carbon dioxide than the plants can absorb or if there aren't enough plants to absorb the carbon created by animals, this relationship begins to break down, similar to what's happening with the world right now.