Organisms, Populations & Ecosystem

Ecology, spanning organisms to ecosystems, is among the most reliably tested areas in NEET. Understanding population growth equations, energy flow rules, and ecological pyramid behaviour is non-negotiable.

Abiotic factors govern organism distribution. Temperature is the most ecologically relevant — it affects enzyme kinetics and metabolic rates. Water limits productivity in deserts. Light influences photoperiod, triggering flowering, migration, and reproduction. Soil properties (pH, minerals, grain size) determine plant communities. Organisms respond to abiotic stress through four strategies: regulators maintain homeostasis (mammals regulate body temperature), conformers allow internal conditions to fluctuate with the environment (fish, reptiles), migrants move to favourable habitats (migratory birds), and species that suspend activity enter dormancy — hibernation (winter), aestivation (summer), or diapause (zooplankton developmental arrest).

Population attributes include density (N, measured by quadrats or mark-recapture), natality (birth rate, b), mortality (death rate, d), sex ratio, and age distribution. Age pyramids classify populations as expanding (broad base — high birth rate), stable (uniform shape — zero growth), or declining (narrow base — negative growth).

Population growth follows two models. Exponential growth (J-shaped curve) occurs with unlimited resources: dN/dt = rN, where r is the intrinsic rate of natural increase (r = b - d), giving Nt = N₀e^(rt). Logistic growth (S-shaped/sigmoid curve) incorporates carrying capacity K: dN/dt = rN(K-N)/K. When population N is far below K, growth approximates exponential. As N approaches K, the (K-N)/K term approaches zero, decelerating growth. The maximum growth rate occurs at the inflection point, where N = K/2.

Six population interactions define species relationships. Mutualism (+/+): both benefit (Rhizobium-legume, mycorrhiza, lichen). Competition (-/-): both harmed; Gause's competitive exclusion principle states two species occupying the same niche cannot coexist indefinitely — resource partitioning enables coexistence. Predation (+/-): predators regulate prey populations; prey defenses include cryptic coloration, mimicry (Batesian — palatable mimics unpalatable; Mullerian — multiple unpalatable species resemble each other), and chemical defenses (cardiac glycosides in Calotropis). Parasitism (+/-): ectoparasites (lice, ticks) and endoparasites (Plasmodium, tapeworm); brood parasitism involves cuckoo laying eggs in crow's nests. Commensalism (+/0): one benefits without affecting the other (orchid epiphytes on mango trees, cattle egret with grazing cattle). Amensalism (-/0): one is harmed while the other is unaffected (Penicillium inhibiting Staphylococcus).

Ecosystem productivity distinguishes GPP (Gross Primary Productivity — total energy fixed by photosynthesis per unit area per unit time) from NPP (Net Primary Productivity = GPP - Respiration, representing energy available to herbivores). The most productive ecosystems are tropical rainforests, coral reefs, and estuaries.

Decomposition proceeds through five steps: fragmentation (detritivores like earthworms break down detritus), leaching (water-soluble nutrients percolate into soil), catabolism (enzymatic degradation by bacteria and fungi), humification (formation of dark, resistant humus), and mineralization (humus releases inorganic nutrients into soil). Warm, moist conditions with high nitrogen content accelerate decomposition; high lignin content retards it.

Energy flow is unidirectional — energy cannot be recycled. Lindeman's 10% law states that only approximately 10% of energy transfers from one trophic level to the next; the remaining 90% is lost as heat. The grazing food chain begins with living plants, while the detritus food chain starts with dead organic matter. Food webs (interconnected food chains) provide ecosystem stability.

Ecological pyramids vary by type. The pyramid of numbers is upright in grasslands (grass > insects > frogs > snakes > hawks) but inverted in tree ecosystems (one tree supports many insects). The pyramid of biomass is upright in terrestrial ecosystems but inverted in aquatic systems (phytoplankton biomass is less than zooplankton biomass at any given time). The pyramid of energy is always upright — this is the most fundamental rule, a direct consequence of the 10% law.

Nutrient cycling operates as biogeochemical cycles. The carbon cycle involves photosynthesis (CO₂ to organic carbon), respiration and decomposition (organic carbon back to CO₂), fossil fuel combustion, and ocean reservoirs (holding 71% of Earth's CO₂). The phosphorus cycle is unique — it is a sedimentary cycle with no gaseous phase, unlike carbon and nitrogen. Phosphorus moves from rock weathering to soil, through organisms, and back to soil and rock.

Ecological succession is the sequential replacement of communities over time. Primary succession begins on bare lifeless substrates (volcanic lava, bare rock) with pioneer species — crustose lichens in xerosere — and progresses through mosses, herbs, shrubs, to a climax forest community, potentially taking over a thousand years. Secondary succession occurs on disturbed but previously vegetated areas (after fire, flood, deforestation) and proceeds faster because soil and seed banks persist. Hydrosere describes succession in water bodies (free-floating plants to rooted submerged to reed swamp to woodland to climax forest). The climax community is the stable, self-perpetuating end-state in equilibrium with the environment.

The key testable concept is that the pyramid of energy is always upright (no exceptions, due to the 10% law), and the logistic growth equation dN/dt = rN(K-N)/K with maximum growth rate at N = K/2.