Final answer:
A decrease in plant life in the biosphere would cause carbon dioxide levels to increase and oxygen levels to decrease in the atmosphere.
Step-by-step explanation:
The most likely effect of a decrease in plant life in the biosphere on the atmosphere is that carbon dioxide levels would increase and oxygen levels would decrease. This is because plants play a crucial role in photosynthesis, where they take in carbon dioxide and release oxygen. With a decrease in plant life, there would be less carbon dioxide being absorbed and less oxygen being produced, leading to an imbalance in the atmosphere.