Organisms and PopulationsNEET Botany · Class 12 · NCERT Chapter 4

Organisms and Populations is a Medium Weightage chapter with 2 to 4 questions in most NEET exams. Questions focus on: population growth equations (exponential vs logistic), carrying capacity (K), population interactions (mutualism, predation, competition, parasitism, commensalism, amensalism) and their effects on each species (+, -, 0), Gause's competitive exclusion principle, examples of each interaction type, abiotic factors, and adaptations (thermoregulation, osmoregulation). This chapter rewards students who memorise the + / - / 0 table for interactions and know the specific examples.

EXPONENTIAL (J-shaped) GROWTH: occurs when resources are unlimited. The population grows at a constant rate (r = intrinsic rate of natural increase). Equation: dN/dt = rN. The population size increases without limit (J-curve). Example: bacteria in a new rich medium, insects after entering a new territory. LOGISTIC (S-shaped) GROWTH: occurs when resources are limited. The growth rate decreases as the population approaches the carrying capacity (K). Equation: dN/dt = rN(K-N)/K. The (K-N)/K term is the "environmental resistance." At N = K: (K-K)/K = 0, so growth stops. At N = K/2: growth rate is maximum (inflection point of the S-curve). NEET key: when N is much less than K, logistic growth resembles exponential growth. The J-curve never reaches a plateau; the S-curve levels off at K.

Population interactions are classified by the effect on each species (+/+, +/-, etc.): (1) MUTUALISM (+/+): both species benefit. Examples: lichens (fungi + algae/cyanobacteria), mycorrhizae (fungi + plant roots), bees and flowers (pollination), clown fish and sea anemone, legume roots and Rhizobium. (2) COMPETITION (-/-): both species are harmed. Interspecific competition can lead to COMPETITIVE EXCLUSION (Gause's principle: two species with identical niches cannot coexist indefinitely). Examples: flamingoes and fish competing for zooplankton; Abingdon tortoise vs goat in Galapagos. (3) PREDATION (+/-): predator benefits; prey is harmed. Includes herbivory (plants as prey). Stabilises populations (prevents over-growth of prey). (4) PARASITISM (+/-): parasite benefits; host is harmed (but usually not killed rapidly). Examples: cuckoo in warbler nest (brood parasitism), Cuscuta (dodder) on plants, roundworms in gut. (5) COMMENSALISM (+/0): one species benefits; other is unaffected. Examples: orchid on a tree, barnacle on whale, cattle egret near cattle. (6) AMENSALISM (-/0): one is harmed; other is unaffected. Example: large trees shading out smaller plants; Penicillium producing penicillin that kills bacteria.

Gause's Competitive Exclusion Principle (1934) states that two closely related species competing for IDENTICAL resources (occupying the same ecological niche) CANNOT coexist stably — one will inevitably outcompete and exclude the other. In Gause's laboratory experiment with two Paramecium species (P. aurelia and P. caudatum) grown together, P. aurelia always won and P. caudatum was excluded when they competed for the same food. KEY DISTINCTION: if the two species occupy different niches (use different resources, forage at different times, different microhabitats), they CAN coexist — this is called RESOURCE PARTITIONING or niche differentiation. Real-world coexistence of similar species (e.g. multiple warblers in the same tree) is possible only through subtle niche differentiation.

Carrying capacity (K) is the maximum population size that a particular environment can sustainably support, given the available resources (food, space, water, light). In the logistic growth equation dN/dt = rN(K-N)/K: When N << K: (K-N)/K ≈ 1 → growth rate ≈ rN (exponential). When N = K/2: growth rate is at its MAXIMUM (fastest absolute increase in population size). When N = K: (K-N)/K = 0 → growth rate = 0 (population stable). When N > K: growth rate is negative (population decreases back toward K). K is not a fixed number for a species — it changes with environmental conditions (food availability, disease, predation, climate). Humans can increase K for domesticated species (livestock, crops) or decrease it through habitat destruction.

These are biogeographical rules about body adaptations to temperature: BERGMANN'S RULE: among endotherms (warm-blooded animals), body size tends to be LARGER in colder climates (higher latitudes). Larger body volume-to-surface area ratio means less heat loss per unit of tissue. Example: polar bears are larger than bears near the equator. ALLEN'S RULE: in endotherms, appendages (ears, limbs, tails) tend to be SHORTER and less protruding in colder climates. Reduces heat loss from extremities. Example: Arctic fox has small ears; desert fennec fox has very large ears. These rules are evolutionary adaptations (not individual plasticity). NEET also tests: thermoregulators (birds and mammals — maintain constant body temperature by generating heat internally or losing heat) vs thermoconformers (most invertebrates, fish, amphibians, reptiles — body temperature follows environmental temperature).