Ecology and ecosystems

In short

Ecology is the study of how organisms interact with one another and with their physical surroundings; an ecosystem is a functional unit where a biotic community and its abiotic environment exchange energy and matter. The subject explains how sunlight becomes plant biomass, how that biomass feeds every other trophic level, how nutrients circulate and how communities change over time. It carries heavy weight in B.Sc Botany papers (Ecology and Environment), in CSIR-UGC NET Life Sciences, IIT-JAM, GATE Ecology and Evolution, ICAR and forest-service exams. Working applications are in environmental consultancy (EIA and biodiversity studies under the Environment (Protection) Act, 1986), State Forest Departments, ICFRE and ICAR institutes, wetland and watershed projects, and ESG/carbon-accounting teams.

Explained

Scope, levels of organisation and ecological factors

• Levels: individual → population (same species, one area) → community (all populations) → ecosystem (community + abiotic environment) → landscape → biome → biosphere.
• Branches: autecology (one species), synecology (communities), population ecology, ecosystem ecology, production ecology, systems ecology, palaeoecology, chemical ecology.
• Habitat is the physical address, niche the functional profession. Fundamental niche is the full range tolerated; realised niche is the narrower part actually occupied after competition.
• Abiotic factors: climatic (light, temperature, water, wind, humidity), edaphic (soil texture, pH, nutrients, soil water, soil air, soil organisms), topographic (altitude, slope, aspect) and physiographic.
• Soil water fractions: gravitational (drains away), capillary water (the only fraction largely available to roots), hygroscopic (held too tightly). Available water = field capacity − permanent wilting point.
• Plant classes by water relation: hydrophytes, mesophytes, xerophytes (succulents, non-succulent perennials, ephemerals), halophytes (salt; vivipary and pneumatophores in mangroves).
• Light: heliophytes (sun plants) versus sciophytes (shade plants); photoperiod controls flowering (short-day, long-day, day-neutral).
• Liebig's Law of the Minimum: growth is limited by the resource in shortest supply. Shelford's Law of Tolerance: each species has a minimum, optimum and maximum for every factor; eurytopic species tolerate wide ranges, stenotopic narrow ones.
• Ecotone is the transition zone between two communities; the edge effect is the greater species richness often found there.

Ecosystem structure and components

• Abiotic: inorganic nutrients (C, N, P, K, Ca), organic matter (humus), climatic regime.
• Biotic: producers (autotrophs - green plants, algae, cyanobacteria, chemosynthetic bacteria), consumers (herbivores = primary; carnivores = secondary, tertiary; omnivores), decomposers/reducers (saprotrophic bacteria and fungi) and transformers.
• Trophic level: the position in the energy chain - T1 producers, T2 herbivores, T3 primary carnivores, T4 top carnivores. A species may occupy more than one level.
• Types: natural terrestrial (forest, grassland, desert), natural aquatic (freshwater lentic - pond, lake; lotic - river, stream; marine - ocean, estuary, mangrove) and artificial/man-made (cropland, orchard, aquarium, reservoir).
• Pond zonation: littoral (rooted plants), limnetic (open sunlit water, phytoplankton), profundal (below light penetration), benthic (bottom sediment).
• Forest stratification: emergent, canopy, understorey, shrub layer, herb layer, forest floor - each with its own microclimate.
• Homeostasis: negative feedback keeps an ecosystem near a steady state; resilience is the capacity to return after disturbance, resistance the capacity to avoid change.

Energy flow and productivity

• One-way flow: energy enters as sunlight, is fixed by photosynthesis, and is lost as heat at every transfer - it never cycles. Nutrients, in contrast, circulate.
• Thermodynamics: the first law - energy is transformed, not created; the second law - every transfer loses usable energy as respiratory heat, which is why food chains are short (3-5 links).
• Only about 1-2% of incident photosynthetically active radiation is fixed by vegetation, though crop canopies under ideal conditions can reach 3-5%.
• GPP (gross primary productivity) is total organic matter fixed; NPP = GPP − R (plant respiration) is what is available to consumers; NEP = NPP − respiration of heterotrophs. Secondary productivity is energy stored by consumers. Units: g dry matter m⁻² yr⁻¹, kcal m⁻² yr⁻¹ or t ha⁻¹ yr⁻¹.
• Standing crop (biomass) is the material present at an instant - a stock; productivity is a rate. Standing crop is measured as fresh weight, dry weight or energy content.
• Ten per cent law (Lindeman, 1942): roughly 10% of energy at one trophic level passes to the next; values in nature range from about 2% to 20%.
• Ecological efficiencies: consumption, assimilation, production and trophic-level transfer efficiency.
• Food chains: grazing chain (producer → herbivore → carnivore) and detritus chain (detritus → detritivore → predator). In most terrestrial and especially forest ecosystems, the greater share of NPP flows through the detritus chain. Food web: interconnected chains, which give stability because an alternative pathway exists if one link fails.
• Ecological pyramids (Elton): pyramid of numbers (can be inverted, e.g. one large tree supporting many insects), pyramid of biomass (inverted in open oceans, where small phytoplankton biomass supports larger zooplankton and fish biomass) and pyramid of energy (always upright, since energy is lost at each step).
• Decomposition steps: fragmentation → leaching → catabolism → humification (stable dark humus) → mineralisation (release of inorganic nutrients). Rate rises with warmth, moisture, nitrogen-rich and sugar-rich detritus; slowed by lignin, chitin, cold, anaerobic and acidic conditions.

Biogeochemical cycles

• Types: gaseous cycles with atmospheric reservoirs (carbon, nitrogen, oxygen, water) are relatively quick and self-regulating; sedimentary cycles with rock reservoirs (phosphorus, sulphur, calcium) are slow and easily disrupted.
• Carbon cycle: CO₂ fixed by photosynthesis; returned by respiration, decomposition, fire and fossil-fuel burning. Oceans are the largest active sink. Atmospheric CO₂ has risen from about 280 ppm pre-industrial to roughly 420 ppm and keeps rising - quote current figures as approximate.
• Nitrogen cycle steps: (1) fixation - biological (*Rhizobium* in legume nodules, *Frankia* in *Casuarina*, free-living *Azotobacter*, *Clostridium*, *Azospirillum*, cyanobacterial *Anabaena* in *Azolla* used in paddy, *Nostoc*), atmospheric (lightning) and industrial (Haber-Bosch); (2) ammonification - organic N to NH₄⁺ by decomposers; (3) nitrification - NH₄⁺ → NO₂⁻ (*Nitrosomonas*) → NO₃⁻ (*Nitrobacter*); (4) assimilation by plants, mostly as NO₃⁻; (5) denitrification - NO₃⁻ → N₂ by *Pseudomonas*, *Thiobacillus* in waterlogged soils. Nitrogenase is oxygen-sensitive; leghaemoglobin protects it in nodules.
• Phosphorus cycle: weathering of apatite rock → soil solution → plants → consumers → decomposers; no significant gaseous phase, so losses to deep ocean sediment are near-permanent. Phosphorus is the usual limiting nutrient in freshwater; its availability is best near pH 6.5 and is fixed by Fe/Al at low pH and Ca at high pH. Mycorrhizae greatly aid uptake.
• Sulphur cycle: has rock, soil and atmospheric pools; SO₂ emissions cause acid deposition.
• Hydrological cycle: evaporation, transpiration (together evapotranspiration), condensation, precipitation, infiltration, runoff.
• Eutrophication: nutrient enrichment (mainly N and P from fertiliser runoff and sewage) → algal bloom → shading and die-off → decomposition consumes dissolved oxygen → fish kills. Natural eutrophication is slow; cultural eutrophication is rapid.
• Biomagnification: fat-soluble, non-degradable substances (DDT, mercury) increase in concentration at successive trophic levels; bioaccumulation is build-up within one organism's lifetime.

Population and community ecology

• Population attributes: density, natality, mortality, immigration, emigration, sex ratio, age structure (age pyramids - expanding, stable, declining), dispersion (random, uniform, clumped - clumped is commonest).
• Growth forms: exponential (J-shaped) under unlimited resources; logistic (S-shaped) with lag, log, deceleration and stationary phases, levelling at carrying capacity (K).
• Life-history strategies: r-selected - small size, short life, many offspring, fluctuating populations (weeds, annuals); K-selected - large, long-lived, few offspring, competitive, near K (forest trees).
• Survivorship curves: Type I (convex, low juvenile mortality), Type II (diagonal, constant risk), Type III (concave, heavy early loss - most plants).
• Species interactions: mutualism (+/+) - mycorrhizae, legume-*Rhizobium*, pollination; commensalism (+/0) - epiphytic orchids; competition (−/−) - interspecific and intraspecific, Gause's competitive exclusion principle states two species with identical niches cannot coexist indefinitely; predation (+/−) - also herbivory; parasitism (+/−) - *Cuscuta* (total stem parasite), *Striga*, *Orobanche*, *Viscum* (partial); amensalism (−/0) - allelopathy, as in *Juglans* (juglone), *Eucalyptus*, *Parthenium hysterophorus*.
• Community characters: analytic - frequency, density, abundance, cover, basal area, physiognomy, stratification, life forms (Raunkiaer's phanerophytes, chamaephytes, hemicryptophytes, cryptophytes, therophytes); synthetic - presence, constancy, fidelity, Importance Value Index (IVI) = relative frequency + relative density + relative dominance.
• Succession: orderly, directional change in community composition at a site. Primary succession starts on bare substrate (nudation) - xerarch (lithosere on rock: crustose lichens → foliose lichens → mosses → annual herbs → perennial herbs/grasses → shrubs → climax forest; psammosere on sand; halosere on salt) and hydrarch (hydrosere in a pond: phytoplankton → submerged → floating → reed-swamp → sedge-meadow → woodland → climax forest). Secondary succession follows disturbance on soil already formed, and is faster.
• Process: nudation → migration (dispersal) → ecesis (establishment) → aggregation → competition and coaction → reaction (the community modifies the habitat) → stabilisation (climax). Autogenic succession is driven by the community itself, allogenic by external change.
• Climax theories: monoclimax (Clements - climate alone determines one climax), polyclimax (many local climaxes from soil, fire, grazing) and climax-pattern (Whittaker). Edaphic, fire and grazing climaxes are subclimaxes held below the climatic climax; disclimax is disturbance-maintained.
• Trends during succession: biomass, species diversity, stratification, niche specialisation, humus and nutrient conservation rise; NPP/biomass ratio, net community production and the share of r-strategists fall; GPP approaches total respiration (P/R → 1 at climax).

Indian ecosystems, services and legal framework

• Biogeographic zones of India (Rodgers and Panwar, 10 zones): Trans-Himalaya, Himalaya, Desert, Semi-arid, Western Ghats, Deccan Peninsula, Gangetic Plain, North-East, Coasts, Islands.
• Champion and Seth forest types (1968): tropical wet evergreen, semi-evergreen, moist deciduous (sal, teak), dry deciduous, thorn, dry evergreen, montane subtropical, montane temperate, sub-alpine, alpine - 16 type groups in all.
• Ecosystem services (Millennium Ecosystem Assessment): provisioning (food, timber, fuel, fibre, medicine), regulating (climate, flood, disease, water purification, pollination), supporting/habitat (nutrient cycling, soil formation, primary production) and cultural (recreation, sacred groves, ecotourism).
• Key Indian law and practice: Indian Forest Act, 1927; Wildlife (Protection) Act, 1972; Water (Prevention and Control of Pollution) Act, 1974; Air Act, 1981; Environment (Protection) Act, 1986 - the parent Act under which the EIA Notification, 2006 (amended since) requires environmental clearance with baseline ecology, flora-fauna inventory and an environmental management plan; Forest (Conservation) Act, 1980, renamed the Van (Sanrakshan Evam Samvardhan) Adhiniyam, 1980 by the 2023 amendment; Biological Diversity Act, 2002 (amended 2023); Coastal Regulation Zone Notification, 2019; Wetlands (Conservation and Management) Rules, 2017. Regulators: MoEFCC, CPCB and State Pollution Control Boards, NGT, with monitoring by FSI, ZSI, BSI and ICFRE. A note on currency: EIA notification schedules and threshold limits are amended frequently - verify the latest text before applying a limit.
• Programmes: National Afforestation Programme, Green India Mission under NAPCC, CAMPA compensatory afforestation, National Mission for Clean Ganga, Project Tiger and Project Elephant, Ramsar site designation (India has over 85 Ramsar wetlands, the count rising), MAB biosphere reserves.

Key points

• Ecosystem - a self-regulating unit of a biotic community plus its abiotic environment, linked by energy flow and nutrient cycling; term coined by A. G. Tansley (1935).
• Energy flow - unidirectional and non-cyclic; nutrients cycle, energy does not.
• GPP vs NPP - NPP = GPP − plant respiration; only NPP is available to herbivores.
• Ten per cent law - about 10% of energy transfers between trophic levels, limiting food chains to 3-5 links.
• Pyramid of energy - always upright; number and biomass pyramids can be inverted.
• Detritus chain - carries most of the energy in forest ecosystems, not the grazing chain.
• Liebig's Law of the Minimum - the scarcest essential factor caps growth.
• Shelford's Law of Tolerance - each species has minimum, optimum and maximum limits for every factor.
• Niche - functional role; fundamental niche is reduced to a realised niche by competition.
• Competitive exclusion (Gause) - complete competitors cannot coexist; resource partitioning allows coexistence.
• Carrying capacity (K) - the maximum population the environment can sustain; logistic growth levels at K.
• r vs K strategists - many small short-lived offspring versus few large competitive ones.
• Nitrification - *Nitrosomonas* NH₄⁺→NO₂⁻, *Nitrobacter* NO₂⁻→NO₃⁻; denitrification returns N₂ to air.
• Gaseous vs sedimentary cycles - C and N are fast and self-correcting; P and S are slow and leak to sediment.
• Eutrophication - N and P enrichment leading to blooms and oxygen depletion; P usually limits fresh water.
• Succession - nudation, migration, ecesis, aggregation, competition, reaction, stabilisation; hydrosere and xerosere both converge on a mesic climax.
• Climax indicators - high biomass and diversity, tight nutrient cycling, P/R ratio approaching 1.
• IVI - relative frequency + relative density + relative dominance, used to rank a species' importance in a stand.

Formulas and examples

Example 1 - Energy flow through a food chain. A grassland fixes GPP = 20,000 kJ m⁻² yr⁻¹; plant respiration uses 40% of it. Transfer efficiency is 10% at each step.

• NPP = 20,000 − (0.40 × 20,000) = 20,000 − 8,000 = 12,000 kJ m⁻² yr⁻¹.
• Herbivore production = 10% of 12,000 = 1,200 kJ m⁻² yr⁻¹.
• Primary carnivore = 10% of 1,200 = 120 kJ m⁻² yr⁻¹.
• Top carnivore = 12 kJ m⁻² yr⁻¹ - 0.06% of GPP, which is why a fifth link is rarely supportable.

Example 2 - Photosynthetic efficiency. Incident PAR on a field is 2,000 kJ m⁻² day⁻¹; the crop fixes 36 kJ m⁻² day⁻¹ as GPP.

• Efficiency = (36 / 2,000) × 100 = 1.8% - typical of a field crop.
• Over 120 days, GPP = 36 × 120 = 4,320 kJ m⁻². At 17 kJ per g dry matter, that is 4,320/17 ≈ 254 g dry matter m⁻², i.e. about 2.54 t ha⁻¹ of gross dry matter (10,000 m² per ha, 1 t = 10⁶ g).

Example 3 - Quadrat analysis and IVI. Ten 10 m × 10 m quadrats (total 1,000 m² = 0.1 ha) are laid in a dry deciduous forest. *Tectona grandis* occurs in 7 quadrats with 35 individuals in all; total individuals of all species = 140; total basal area of all species = 2.8 m²; teak's basal area = 1.12 m².

• Frequency = (7/10) × 100 = 70%.
• Density = 35/1,000 m² = 0.035 m⁻² = 350 stems ha⁻¹.
• Abundance = 35/7 = 5 individuals per occupied quadrat.
• Relative density = (35/140) × 100 = 25%.
• Relative dominance = (1.12/2.8) × 100 = 40%.
• If the sum of all species' frequencies is 350%, relative frequency = (70/350) × 100 = 20%.
• IVI = 20 + 25 + 40 = 85 out of 300 - teak is the leading species by dominance here.

Example 4 - Logistic growth. A weed population has N = 200, K = 1,000 and r = 0.5 yr⁻¹.

• dN/dt = 0.5 × 200 × (1,000 − 200)/1,000 = 0.5 × 200 × 0.8 = 80 individuals yr⁻¹.
• At N = 500 (= K/2): dN/dt = 0.5 × 500 × 0.5 = 125 yr⁻¹ - the maximum growth rate, the inflection of the S-curve.
• At N = 900: dN/dt = 0.5 × 900 × 0.1 = 45 yr⁻¹ - growth slows as N approaches K.

Example 5 - Shannon-Wiener diversity. A plot holds 4 species with 50, 30, 15 and 5 individuals (N = 100), so pᵢ = 0.50, 0.30, 0.15, 0.05.

• −Σ pᵢ ln pᵢ: 0.50 × 0.6931 = 0.3466; 0.30 × 1.2040 = 0.3612; 0.15 × 1.8971 = 0.2846; 0.05 × 2.9957 = 0.1498.
• H' = 0.3466 + 0.3612 + 0.2846 + 0.1498 = 1.142 (natural-log units).
• Simpson's D = 0.25 + 0.09 + 0.0225 + 0.0025 = 0.365, so 1 − D = 0.635. Maximum possible H' = ln 4 = 1.386, so evenness = 1.142/1.386 = 0.824 - a fairly even community.

Where it is used

• Environmental consultancy and EIA: baseline flora surveys, quadrat-based IVI and diversity indices, tree enumeration and environmental management plans for clearance under the Environment (Protection) Act, 1986 - roles at WAPCOS, TCE, Ramboll, SGS and state-accredited NABET consultants.
• Forest management: working plans, growing-stock and NPP estimates, plantation and regeneration monitoring with State Forest Departments, FSI's biennial India State of Forest Report, ICFRE institutes (FRI Dehradun, IWST Bengaluru).
• Agriculture and restoration: biofertilisers (*Rhizobium*, *Azotobacter*, *Azolla*-*Anabaena* in paddy, mycorrhizal inoculants), nutrient budgeting, watershed treatment, mine-spoil and mangrove restoration, agroforestry under ICAR and state agricultural universities.
• Wetland and water-quality work: eutrophication and trophic-status assessment, phytoremediation and constructed wetlands, lake-restoration projects (Loktak, Chilika, Dal, Bellandur) with CPCB and State Pollution Control Boards.
• Climate and carbon work: biomass and carbon-stock inventories, REDD+ and corporate ESG/carbon-accounting teams, green-belt and carbon-sequestration planning for industry.
• Research and teaching: NET/JRF-funded ecology posts at BSI, ZSI, NBRI, CSIR labs, ATREE, WII, and university departments; roles as ecologist, biodiversity analyst, environmental officer and GIS-ecology analyst.

Interview questions

• Q: Why is the pyramid of energy always upright? A: Energy is lost as respiratory heat at every transfer, so each higher trophic level must contain less energy than the one below it; no accounting arrangement can invert it.
• Q: How can a pyramid of biomass be inverted? A: In open oceans, phytoplankton have a small standing biomass but a very high turnover rate, so they support a larger standing biomass of zooplankton and fish.
• Q: Difference between standing crop and productivity? A: Standing crop is the biomass present at one moment (a stock, g m⁻²); productivity is the rate of its formation (g m⁻² yr⁻¹).
• Q: Why are food chains limited to 3-5 links? A: Only about 10% of energy passes to each successive level, so after four or five transfers too little energy remains to support a viable population.
• Q: Which nutrient usually limits freshwater productivity, and why? A: Phosphorus, because it has no gaseous phase, is derived slowly from rock weathering and is readily fixed by iron, aluminium and calcium in sediment.
• Q: State and distinguish Liebig's and Shelford's laws. A: Liebig's Law of the Minimum says the scarcest essential factor limits growth; Shelford's Law of Tolerance adds that both too little and too much of a factor limit it, each species having its own range.
• Q: Why is nitrogen fixation energetically expensive? A: Breaking the N≡N triple bond requires about 16 ATP per N₂ reduced by nitrogenase, and the enzyme must additionally be shielded from oxygen - by leghaemoglobin in nodules and heterocysts in cyanobacteria.
• Q: What happens to the P/R ratio during succession? A: It is well above 1 in early stages (production exceeds respiration, so biomass accumulates) and approaches 1 at climax, when gross production is balanced by total community respiration.
• Q: Hydrarch and xerarch succession - do they end differently? A: No; a pond fills and dries while bare rock accumulates soil and moisture, so both converge on a mesic climax community determined by the regional climate.
• Q: Can two species occupy exactly the same niche? A: Not indefinitely - by Gause's competitive exclusion principle one outcompetes the other, unless resource partitioning or niche differentiation separates them.
• Q: Why is DDT more concentrated in a fish-eating bird than in pond water? A: It is fat-soluble and metabolically stable, so it is retained and passed up each trophic level - biomagnification, which can reach thousands of times the ambient concentration.
• Q: Which Indian instrument requires an ecological baseline before a project is built? A: The EIA Notification, 2006 issued under the Environment (Protection) Act, 1986, which mandates flora-fauna inventory and an environmental management plan for environmental clearance by MoEFCC or the SEIAA.
• Q: Why does decomposition slow in a waterlogged, acidic soil? A: Anaerobic and acidic conditions suppress the aerobic bacteria and fungi that carry out catabolism, so organic matter accumulates as peat and nutrients are not mineralised.

Common mistakes

• Writing that energy, like nutrients, cycles through an ecosystem - energy flow is strictly one-way.
• Confusing GPP with NPP, or subtracting decomposer respiration from GPP to get NPP (that gives NEP).
• Treating the 10% law as exact; stating transfer efficiency without noting that real values range from about 2% to 20%.
• Claiming all ecological pyramids are upright - only the energy pyramid must be.
• Mixing up nitrification with nitrogen fixation, or swapping *Nitrosomonas* and *Nitrobacter*.
• Saying the grazing food chain carries most energy in a forest; it is the detritus chain.
• In quadrat work, dividing by total quadrats instead of occupied quadrats when computing abundance, and forgetting that frequency uses presence while density uses counts.
• Calling secondary succession a repeat of primary succession - soil is already formed, there is a seed bank, and no pioneer soil-building stage is needed.

Related topics

• Plant physiology: photosynthesis and respiration · Soil science and plant nutrition · Environmental pollution and toxicology · Phytogeography and vegetation of India · Microbiology: nitrogen-fixing and decomposer organisms · Biostatistics and ecological sampling methods