Plants produce food and oxygen through the photosynthesis and these are used up by animals for nutrition and respiration. They are hence called producers and occupy the first trophic level. These animals use oxygen and the food prepared by plants as sources of energy and release carbon dioxide. Carbon dioxide is used by plants for photosynthesis. Also when animals excrete or are degraded after death, the minerals and nutrients from their bodies are absorbed by the plants again. Hence they are inter dependent on each other.