Answer:
Energy
Step-by-step explanation:
In ecosystem, energy flows from lower tropic level to higher tropic level.
In other words we can say that energy flows in one direction through an ecosystem. Plants are autotrophs, as they synthesize their food own-self with the help of sunlight, so they convert sunlight into chemical energy. As plants are called producers, and they are also known as primary tropic level. Energy is passed from primary tropic level to secondary tropic level (primary consumer) and so on. This energy is transferred from producers up through each level of consumers, and is not recycled.