(1) Producers refer to green plants, which use light energy to change carbon dioxide and water into organic matter through photosynthesis, and convert light energy into chemical energy to be stored in organic matter, and release oxygen. Therefore, in this ecosystem, energy flows from the photosynthesis of grass, and solar energy is fixed. (2) Energy decreases step by step in the process of flowing along the food chain. The longer the food chain, the more energy it consumes. The shortest food chain in which the eagle lives is grass → rabbit → eagle, so the food chain in which the eagle gets the most energy in this ecosystem is grass → rabbit → eagle. (3) In the ecosystem, harmful substances can accumulate in organisms through the food chain, and their concentration gradually increases with the improvement of consumers' level, which is called biological concentration. In these food chains, the eagle ranks the highest, and its heavy metal content is the highest according to the phenomenon of biological enrichment.

(4) Ecosystem includes biological components and abiotic components, and abiotic components include sunlight, air, moisture, soil, etc. Biological components include producers, consumers and decomposers. It can be seen that there are plants-producers, animals-consumers, but there are also bacteria and fungi decomposers.

So the answer is: (1) grass fixes solar energy (2) grass → mouse → eagle? (3) eagle

(4) decomposer