Ecosystem: The system resulting from the interaction between organisms and their environment is called an ecosystem.

(a) Producers: Organisms, which can synthesize their own food are included under producers, e.g., Volvox, Pandorina, Oedogonium, Saggitaria, Utricularia, Azolla, Trapa, Lemna, Typha, Nymphaea etc. form the producer class of the pond ecosystem.

(b) Consumers:

(i) Primary consumer: Animals, which feed on producers are included in this category e.g., Daphnia, Cyclops, Paramoecium, Amoeba, and small fishes.

(ii) Secondary consumers: Primary consumers also serve as food for water snakes, a few tortoises, few types of fish, etc. hence, these are carnivores.

(iii) Tertiary consumers: Secondary consumers also serve as food for aquatic birds like kingfishers, cranes, big fish and these together form a top-class carnivorous group and called tertiary consumers.

(c) Decomposers: All producers and consumers die and accumulate on the floor of the pond. Even the waste material and feces of these animals get accumulated on the floor of the pond. Similarly, the floor of the pond is also occupied by decomposers, which include bacteria and fungi. These decomposers decompose complex organic compounds of then- bodies into simpler forms which are finally mixed with the soil of the floor of ponds. These are again absorbed by the roots of producer plants and thus matter is recycled.