Explain why energy in most ecosystems must flow from the sun, then to autotrophs and finally to heterotrophs, in that order.

Answer:

Most of the energy in ecosystems enters as sunlight, so it is the principal source of energy for the biosphere. Autotrophs are organisms that can convert sunlight, solar radiation, into chemical energy that they and other organisms can use. Heterotrophs cannot utilize sunlight directly to make their food, so energy can only flow to them after passing through autotrophs.