Key Points

An ecosystem is a functional unit of nature where living organisms (biotic) interact with each other and their physical environment (abiotic). The main components are productivity, decomposition, energy flow, and nutrient cycling.

The vertical distribution of different species occupying different levels is called stratification. For example, in a forest, trees occupy the top strata, shrubs the middle, and herbs the bottom.

Primary production is the amount of biomass produced by plants via photosynthesis per unit area over time. The rate of biomass production is called productivity, expressed in units like $\text{g m}^{-2} \text{yr}^{-1}$ or $(\text{kcal m}^{-2}) \text{yr}^{-1}$.

Gross Primary Productivity (GPP) is the total rate of organic matter production during photosynthesis. Net Primary Productivity (NPP) is the available biomass for heterotrophs after accounting for plant respiration (R), given by the equation: $\text{GPP} - \text{R} = \text{NPP}$.

Secondary productivity is the rate of formation of new organic matter by consumers. It represents the energy stored at the heterotrophic levels of the ecosystem.

Decomposition is the breakdown of complex organic matter (detritus) into simpler inorganic substances like carbon dioxide ($\text{CO}_2$), water, and nutrients by decomposers. Detritus includes dead plant and animal remains and fecal matter.

The key steps in decomposition are fragmentation (breakdown by detritivores), leaching (seepage of water-soluble nutrients), and catabolism (enzymatic degradation by bacteria and fungi). This is followed by humification (humus formation) and mineralisation (release of inorganic nutrients).

Decomposition is an oxygen-requiring process, faster in warm and moist conditions. The rate is quicker for detritus rich in nitrogen and sugars, and slower for detritus rich in lignin and chitin.

Energy flow in an ecosystem is unidirectional, from the sun to producers and then to consumers. Less than 50% of incident solar radiation is Photosynthetically Active Radiation (PAR), and plants capture only 2-10% of this PAR.

Organisms occupy a specific trophic level in a food chain based on their food source. Producers are at the first level, primary consumers (herbivores) at the second, and secondary consumers (carnivores) at the third.

The Grazing Food Chain (GFC) starts with producers, while the Detritus Food Chain (DFC) begins with dead organic matter. GFC is the major energy pathway in aquatic ecosystems, whereas DFC is dominant in terrestrial ecosystems.

Only about 10 percent of the energy from a lower trophic level is transferred to the next higher trophic level. The remaining energy is lost as heat, which limits the number of trophic levels in a food chain.

Standing crop is the total mass of living organisms (biomass) in a particular trophic level at a given time. It is measured as fresh or dry weight, with dry weight being more accurate.

An ecological pyramid is a graphical representation of the relationship between organisms at different trophic levels, expressed in terms of number, biomass, or energy. The base represents producers and the apex represents top consumers.

This pyramid represents the number of individuals at each trophic level. It is typically upright (e.g., grassland ecosystem) but can be inverted (e.g., a single large tree supporting many insects).

This pyramid shows the total dry weight of organisms at each trophic level. It is usually upright, but can be inverted in aquatic ecosystems where the biomass of zooplankton exceeds that of phytoplankton.

The pyramid of energy is always upright and can never be inverted. This is because energy is always lost as heat when it is transferred from one trophic level to the next, in accordance with the second law of thermodynamics.

Nutrient cycling is the movement of nutrient elements through the various biotic and abiotic components of an ecosystem. It can be gaseous (e.g., carbon cycle with an atmospheric reservoir) or sedimentary (e.g., phosphorus cycle with Earth's crust as the reservoir).