CBSE Class 10 Science 2020 · Paper 31/5/3 · Question 18(b)

(a) All the interacting organisms in an area together with the non-living constituents of their environment form an ecosystem; it has biotic (living) and abiotic (non-living) components. (b) A forest and a pond (a lake, a grassland, a desert or the sea are other natural ecosystems; any two correct examples earn credit). (c) Ponds and lakes are natural ecosystems in which decomposers (bacteria and fungi) break down dead remains and wastes and return the nutrients to the water, so wastes do not collect. An aquarium is an artificial ecosystem in which decomposers are absent (or too few), so wastes and dead matter pile up and pollute the water; hence it must be cleaned regularly.