Plants make their own food by the process of photosynthesis. Because of this they are called producers or autotrophs (auto = self + troph = food).
Sunlight + carbon dioxide + water → food made in the green leaf, oxygen released
This is the single most important fact in the whole chapter. Every animal in the forest, and every decomposer too, is living on energy that a green plant captured from sunlight first.
Why it happens: energy cannot be made by any organism. It has to come in from outside, and the only doorway into an ecosystem is photosynthesis. That is why producers always occupy the first trophic level — nothing can be below them.
Q2.
How can we make linkages with the feeding relationship among organisms in a given ecosystem?
Answer
By drawing an arrow from the organism that is eaten to the organism that eats it. A row of such arrows is a food chain.
Grass → Grasshopper → Frog → Snake → Eagle
Read the arrow as "is eaten by", or better, as "passes its food and energy to". That is why it points from the grass to the grasshopper and not the other way round: the arrow shows the direction in which food and energy travel.
Check it yourself: if you find that an arrow in your diagram points from a hunter to its prey, you have drawn it backwards. Ask "which way is the energy going?" — the answer is always upward from the producer.