(1) Producers refer to green plants 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 make organic matter by photosynthesis and belongs to the producer. Animals such as rabbits, rats, foxes, eagles and snakes directly or indirectly feed on plants and belong to consumers.

(2) The food chain is the relationship between consumers and producers, excluding abiotic parts and decomposers. The food chain in the picture is: ① grass → rabbit → eagle, ② grass → rabbit → fox, ③ grass → mouse → fox, ④ grass → mouse → eagle, ⑤ grass → mouse → snake → eagle, which are intertwined to form a food web. Therefore,

(3) decomposers can decompose organic matter into carbon dioxide, water and inorganic salts for green plants to use. Decomposers include bacteria and fungi. Therefore, in an ecosystem, bacteria and fungi are decomposers. The basic structure of bacteria is cell wall, cell membrane, cytoplasm and DNA-rich region, but there are no formed nuclei and chloroplasts in cells. The basic structure of fungi is cell wall, cell membrane, cytoplasm and nucleus, and there is no chloroplast in cells. Therefore, the structure of fungi is more complicated than that of bacteria, because there is a formed nucleus in fungal cells.

(4) The length of 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 which are the longest. The longest food chain is grass → mouse → snake → eagle.

(5) Toxic substances accumulate along the food chain, and the higher the nutritional level, the higher the content of toxic substances in the body; The lower the nutritional level, the less harmful substances in the body. Eagle has the highest nutritional level at the end of the food chain and the highest toxic substances in its body. Therefore, the answer is:

(1) grass;

(2)5;

(3) decomposers; Formed nucleus;

(4) Grass → Rat → Snake → Eagle;

(5) eagle.