Because the green plants have chlorophyll which makes the
plants absorb carbon dioxide from the atmosphere and water
from the earth. This water and carbon dioxide combine to
from compounds like sugar. Animals cannot produce sugar as
they ate having no chlorophyll. So they have to depend upon
the green plants for sugar and hence for food.
Animals depend upon green plants for their food because plants produce oxygen, absorb carbon dioxide, and food source. Every animal eat other animals, this animals depend on plants because the other animal or prey eats green plants. Without green plants animals can have source of energy because the prey that they eat eats green plants. Without the energy of the prey that the other animals eat the other animal can have energy that comes from the prey.