Ecology & ecosystem basics

Defining the Discipline

Ecology is the scientific study of interactions among organisms and between organisms and their physical environment. The term was coined by the German zoologist Ernst Haeckel in 1866 from the Greek oikos (household) and logos (study). Arthur Tansley introduced the term 'ecosystem' in 1935, defining it as the integrated unit of biotic community and abiotic environment functioning together.

Ecology is studied at hierarchical levels of organization: organism → population → community → ecosystem → biome → biosphere. A population is a group of interbreeding individuals of one species in a defined area; a community is all interacting populations in that area; an ecosystem adds the abiotic environment; a biome is a large climatically determined assemblage (e.g., tropical rainforest, tundra, grassland); and the biosphere is the global sum of all ecosystems.

Biotic and Abiotic Components

Every ecosystem has two structural components. Abiotic components include climatic factors (light, temperature, water, humidity) and edaphic factors (soil, minerals, topography). Biotic components are classified by trophic function:

• Producers (autotrophs): green plants, algae and cyanobacteria that fix solar energy via photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂). Chemosynthetic bacteria at hydrothermal vents are an exception, using inorganic chemical energy.
• Consumers (heterotrophs): herbivores (primary consumers), carnivores (secondary and tertiary consumers), and omnivores.
• Decomposers (saprotrophs): fungi and bacteria that break down dead organic matter, returning nutrients to the abiotic pool.

Productivity

Gross Primary Productivity (GPP) is the total rate of photosynthetic energy capture. Net Primary Productivity (NPP) = GPP minus respiration losses (R); NPP is the biomass available to consumers. Globally, oceans cover most of the surface but contribute roughly half of net primary production, while tropical rainforests and coral reefs and estuaries rank among the most productive ecosystems per unit area. Secondary productivity is the rate of biomass formation by consumers.

These definitions are not academic trivia for UPSC — the distinction between GPP and NPP, the precise role of decomposers, and the trophic classification of organisms have all appeared in Prelims framing. Mastering the vocabulary here is the foundation for every later lesson on biodiversity, climate and conservation.

Energy Flow and the Ten Percent Law

Energy flow in ecosystems is unidirectional — it enters as sunlight, is captured by producers, and dissipates as heat at each transfer, obeying the laws of thermodynamics. Energy is never recycled. This contrasts sharply with nutrients, which cycle.

Lindeman's Ten Percent Law (Raymond Lindeman, 1942) states that only about 10% of energy at one trophic level is transferred to the next; the remaining ~90% is lost in respiration, metabolic heat and undigested matter. Consequently food chains rarely exceed four or five trophic links — there is insufficient energy to sustain higher levels.

A food chain is a linear sequence of energy transfer (grass → grasshopper → frog → snake → hawk). Two principal types exist: the grazing food chain (GFC), beginning with living plants, and the detritus food chain (DFC), beginning with dead organic matter. In most terrestrial and shallow-water ecosystems the DFC carries the larger share of energy flow. Interlinked food chains form a food web, which confers stability — the more cross-links, the greater the resilience to loss of any single species.

Ecological Pyramids

Charles Elton (1927) developed the concept of ecological pyramids — graphical representations of trophic structure. Three types exist:

• Pyramid of numbers: can be upright (grassland) or inverted (a single tree supporting many insects).
• Pyramid of biomass: usually upright on land but inverted in oceans, where small phytoplankton with rapid turnover support a larger standing biomass of zooplankton at any instant.
• Pyramid of energy: always upright, because energy obeys the ten percent law and cannot increase up the chain. This is the most reliable pyramid.

Biogeochemical Cycles

Unlike energy, nutrients cycle between biotic and abiotic reservoirs. Gaseous cycles (carbon, nitrogen, oxygen) have their main reservoir in the atmosphere or hydrosphere and are relatively rapid. Sedimentary cycles (phosphorus, sulphur) have reservoirs in the Earth's crust and are slower; phosphorus has no significant gaseous phase, which is why it often limits productivity.

The nitrogen cycle involves fixation (by Rhizobium, Azotobacter, cyanobacteria such as Anabaena, and lightning), nitrification, assimilation, ammonification and denitrification. The carbon cycle links photosynthesis, respiration, decomposition and combustion of fossil fuels — the disruption of which drives anthropogenic climate change, the subject of a later lesson.

Ecological succession — the orderly change in community composition over time toward a stable climax community — completes the picture. Primary succession begins on bare substrate (e.g., a new volcanic island); secondary succession follows a disturbance where soil survives (e.g., abandoned farmland or post-fire forest).