(1) Producer refers to a green plant that can carry out photosynthesis and provide organic matter (food) and oxygen for plants themselves, consumers and decomposers. There is grass in this picture, which can carry out photosynthesis to produce organic matter and belongs to a producer;

(2) Animals such as rats, foxes, eagles and snakes directly or indirectly feed on plants.

(3) Decomposers can decompose organic matter into carbon dioxide, water and inorganic salts for green plants to use. Decomposers include saprophytic bacteria and fungi. Therefore, in an ecosystem, Bacteria and fungi are decomposers.

4) The food chain is the relationship between consumers and producers, excluding abiotic parts and decomposers. The food chains in the figure are: ① grass → rabbit → eagle, ② grass → rabbit → fox, ③ grass → mouse → mouse → eagle, and the food chains are intertwined. A food web is formed. The length of the food chain mainly depends on the number of trophic levels. ①, ②, ③ and ④ have three trophic levels. ⑤ There are four trophic levels that are the longest. The longest food chain is: grass → mouse → snake → eagle. < P > (5) Toxic substances accumulate along the food chain. The higher the trophic level, the higher the toxic substances content in the body. The lower the nutritional level, the less harmful substances in the body. The eagle has the highest nutritional level at the end of the food chain, and the highest toxic substances in the body.

So the answer is:

(1) grass

(2) Any one of rabbit, mouse, fox, snake and eagle can be used?

(3) decomposition?

(4) Grass → Rat → Snake → Eagle;

(5) eagle