Ecology is the study of interactions between organisms and their environment, spanning four levels of biological organisation: organism, population, community, and ecosystem. NEET consistently draws 3–4 questions per year from this chapter, making it one of the highest-yield topics in Biology.

Abiotic Factors and Organism Responses

The physical environment is described by abiotic factors. Temperature is the most ecologically significant — it governs enzyme kinetics, metabolic rates, and the geographic distribution of species. Water limits productivity in arid systems; light (photoperiod) regulates flowering, migration, and reproductive cycles. Soil properties (pH, mineral content, grain size) dictate which plant communities can establish.

Organisms respond to abiotic stress through four strategies. Regulators maintain internal homeostasis through physiological mechanisms (mammals thermoregulate using sweat and metabolic heat). Conformers allow their internal state to vary with external conditions (fish, reptiles). Migrants leave unfavourable habitats seasonally (migratory birds). Suspenders enter dormancy: hibernation (winter), aestivation (summer heat and drought), or diapause (developmental arrest in zooplankton and insects under unfavourable conditions).

Population Attributes and Growth

A population is a group of individuals of the same species in a defined area at a given time. Key attributes are: density (N — measured by quadrats for plants or mark-recapture for mobile animals), natality (b), mortality (d), sex ratio, and age distribution. Age pyramids are triangular (expanding — high birth rate), bell-shaped (stable — zero growth), or urn-shaped (declining — negative growth).

Population growth is modelled by two equations. Exponential growth (J-shaped curve): dN/dt = rN, where r = b − d is the intrinsic rate of natural increase. The integrated form is Nt = $N_{0}$e^(rt). This applies when resources are unlimited (laboratory cultures, early colonisation). Logistic growth (S-shaped/sigmoid curve): dN/dt = rN(K−N)/K, where K is carrying capacity. The (K−N)/K term is the environmental resistance factor; as N approaches K it approaches zero, decelerating growth. Maximum growth rate occurs at the inflection point where N = K/2.

Population Interactions

Six interaction types define how species pairs affect each other. Mutualism (+/+): both species benefit — Rhizobium fixes nitrogen in legume root nodules; mycorrhizal fungi enhance plant nutrient uptake; lichen is a mutualism between algae and fungus. Competition (−/−): both harmed — Gause's competitive exclusion principle states that two species occupying the same ecological niche cannot coexist indefinitely (demonstrated with Paramecium aurelia and P. caudatum). Resource partitioning (differential use of niche dimensions) allows coexistence. Predation (+/−): predators control prey populations; prey defences include cryptic coloration (camouflage), Batesian mimicry (palatable species mimics unpalatable model), Müllerian mimicry (multiple unpalatable species converge on a common warning pattern), and chemical defences (Calotropis produces cardiac glycosides toxic to herbivores). Parasitism (+/−): parasites derive nutrition at the host's expense — ectoparasites (lice, ticks, mites) live on the host surface; endoparasites (Plasmodium, Ascaris, tapeworm) live inside; brood parasitism (cuckoo laying eggs in crow nests) is a behavioural form. Commensalism (+/0): one benefits, other unaffected — orchid epiphytes on mango trees (orchid gains light access; mango unaffected); cattle egrets foraging near grazing cattle (flush up insects). Amensalism (−/0): one harmed, other unaffected — Penicillium secretes penicillin that inhibits Staphylococcus growth.

Ecosystem Productivity and Decomposition

Gross Primary Productivity (GPP) is the total rate of photosynthetic energy fixation per unit area per unit time. Net Primary Productivity (NPP) = GPP − Respiration, representing energy available for herbivores. The most productive ecosystems are tropical rainforests, coral reefs, and estuaries.

Decomposition breaks down detritus (dead organic matter) through five sequential steps: Fragmentation (detritivores — earthworms, dung beetles — physically break detritus into smaller pieces, vastly increasing surface area); Leaching (water-soluble nutrients percolate into soil); Catabolism (bacteria and fungi secrete extracellular enzymes that chemically degrade organic compounds); Humification (formation of dark, recalcitrant humus that improves soil structure); Mineralization (microbial action releases inorganic ions — phosphate, nitrate, sulphate — back into soil solution). Warm, moist, nitrogen-rich conditions with low lignin content accelerate decomposition; low temperature, waterlogging (anaerobic), and high lignin slow it.

Energy Flow

Energy flows through ecosystems in one direction — it cannot be recycled (unlike matter). Lindeman's 10% law (Lindeman's efficiency) states that approximately 10% of energy is transferred from one trophic level to the next; 90% is dissipated as heat through cellular respiration, incomplete assimilation, and metabolic work. Grazing food chains begin with living plants (green plant → herbivore → carnivore). Detritus food chains begin with dead organic matter. Food webs (multiple interlocking food chains) confer ecosystem stability.

Ecological Pyramids

Three pyramid types summarise trophic structure. Pyramid of numbers (individuals per trophic level): upright in grasslands; inverted in tree ecosystems (one large tree supports thousands of insects). Pyramid of biomass (dry weight per trophic level): upright in terrestrial ecosystems; inverted in aquatic ecosystems (low phytoplankton standing crop biomass despite high turnover rate). Pyramid of energy: always upright — a direct, inescapable consequence of the 10% law. No exception exists. This is the most reliably tested ecological pyramid rule in NEET.

Nutrient Cycling

Carbon cycle: photosynthesis incorporates atmospheric $CO_{2}$ into organic compounds; respiration and decomposition return $CO_{2}$; fossil fuel combustion adds a geological flux; oceans hold 71% of Earth's carbon as dissolved $CO_{2}$. Phosphorus cycle: unique among major biogeochemical cycles — it has no gaseous phase. Phosphorus moves from rock (weathering) → soil → plants and animals → detritus → soil and sediment. There is no atmospheric reservoir, making it a purely sedimentary cycle.

Ecological Succession

Succession is the sequential, directional replacement of communities over time toward a climax community. Primary succession starts on bare, lifeless substrate (volcanic lava, bare rock, newly exposed glacial till) with pioneer species — crustose lichens in xerosere — and proceeds through foliose lichens → mosses → herbs → shrubs → trees (climax forest), taking hundreds to thousands of years. Secondary succession occurs on disturbed but previously vegetated areas (post-fire, post-flood, abandoned farmland) where soil and seed banks remain, and proceeds much faster (decades to centuries). Hydrosere is succession in an aquatic body: free-floating algae → rooted submerged plants → reed swamp → woodland → climax forest. The climax community is the stable, self-perpetuating end state in equilibrium with its environment.