(1) The food chain reflects the relationship between producers and consumers of eating and being eaten, so decomposers should not appear in the food chain. The correct way to write the food chain is: producer → primary consumer → secondary consumer... Note that the starting point is the producer. There are four food chains in the picture: grass → rabbit → owl; grass → rat → owl; grass → rat → snake → owl; grass → rabbit → snake → owl. The longest one is: grass → rat → snake → owl, or grass → rabbit → snake → owl.

(2) If the number of snakes in Figure 1 is greatly reduced, and the natural enemies of rabbits and rats are greatly reduced, and there is sufficient food, they will breed in large numbers, resulting in a large increase in the number of rabbits and rats, and therefore the number of grasses. It will be greatly reduced, thus affecting the ecological balance.

(3) The grass in Figure 1 belongs to the producer, and the various animals belong to the consumers. As a complete ecosystem, the missing components in Figure 1 are abiotic parts and decomposers.

(4) Toxic substances accumulate step by step along the food chain. The higher the trophic level, the more toxic substances accumulate in the body. Owls are at the top of the food chain and have the largest accumulation of toxic substances in their bodies.

(5) In a food chain, the higher the trophic level, the smaller the relative quantity, and the lower the trophic level, the greater the relative quantity. In Figure 2, C has the largest relative quantity and the lowest trophic level, followed by A. B has the smallest relative quantity and the highest trophic level. Therefore, the food chain composed of organisms and the direction of energy flow in the picture are: C → A → B.

So the answer is: (1) 4; grass → rat → snake → owl, or grass → rabbit → snake → owl.

(2) Reduction;

(3) Decomposer;

(4) Grass; Eagle;

(5) C →A→B