An ecosystem is a functional unit of nature where living organisms interact among themselves and with the surrounding physical environment. Ecosystems vary greatly in size, ranging from a small pond to a large forest or a sea, and can be categorized as terrestrial (forest, grassland, desert) or aquatic (pond, lake, wetland, river, estuary).

Structural Components: The interaction of biotic and abiotic components results in a physical structure characteristic of each ecosystem. Two important structural features are: (i) Species composition – identification and enumeration of plant and animal species of an ecosystem, and (ii) Stratification – vertical distribution of different species occupying different levels. For example, in a forest, trees occupy the top vertical strata, shrubs the second layer, and herbs and grasses occupy the bottom layers.

Functional Components: The components of an ecosystem function as a unit through four key aspects: (i) Productivity, (ii) Decomposition, (iii) Energy flow, and (iv) Nutrient cycling.

Example – Pond Ecosystem: A pond illustrates all ecosystem components clearly. The abiotic component includes water with dissolved inorganic and organic substances, soil deposit at the bottom, solar input, and climatic conditions. The autotrophic components include phytoplankton, algae, and floating, submerged and marginal plants. The consumers are represented by zooplankton, free swimming and bottom dwelling forms. The decomposers include fungi, bacteria and flagellates, especially abundant at the bottom of the pond.