# Organisms and Populations (NEET Biology Class 12): Growth Models & Population Interactions Source: https://tracknation.in/story/organisms-and-populations-neet-biology-class-12-growth-models-population-interactions (TrackNation — free UPSC notes with PYQs) > NEET Class 12 Organisms and Populations, mined from NCERT: population attributes tabulated with worked birth/death rate examples, exponential growth (both equations, the J-curve, r values for each organism), logistic Verhulst-Pearl growth with carrying capacity, life history variation, the complete six-interaction +/− table (mutualism, competition, predation, parasitism, commensalism, amensalism), predation's ecological roles with the prickly pear and Pisaster case studies, prey and plant defenc ## Population Attributes & Growth Models 📌 **NEET priority: Important.** *2–4 questions a year. The **population interaction +/− table** and its named examples, plus the **exponential and logistic growth equations**, are almost the entire question set from this chapter.* **Ecology** studies interactions among organisms and between organisms and their physical (abiotic) environment, at **four levels**: **organisms, populations, communities and biomes**. ### Population attributes — what a population has that an individual doesn't | **Attribute** | **Definition** | **Worked example** | | Birth rate (natality) | Per capita births — change in numbers relative to population members | 20 lotus plants + 8 new = 28. Birth rate = 8/20 = 0.4 offspring per lotus per year | | Death rate (mortality) | Per capita deaths | 4 of 40 fruitflies died in a week. Death rate = 4/40 = 0.1 individuals per fruitfly per week | | Sex ratio | Proportion of males to females — an individual is simply male OR female | 60% females, 40% males | | Age pyramid | Plot of the per cent of individuals in each age group. Its SHAPE reveals whether the population is GROWING, STABLE or DECLINING | Human age pyramids show males and females separately | **Four factors change population density:** **natality and immigration INCREASE** it; **mortality and emigration DECREASE** it. In a newly colonised habitat, **immigration may contribute more** than natality. ### Exponential growth — unlimited resources - **dN/dt = (b − d) × N**, and letting **(b − d) = r**, this becomes **dN/dt = rN**, where **r is the INTRINSIC RATE OF NATURAL INCREASE** — the key parameter for assessing any biotic or abiotic impact on growth. - Integral form: **Nt = N0ert**, where **e = 2.71828** (base of natural logarithms). Plotting N against time gives a **J-SHAPED curve**. | **Organism** | **Value of r** | | Norway rat | 0.015 | | Flour beetle | 0.12 | | Human population in India (1981) | 0.0205 | ### Logistic growth — limited resources (the realistic model) - No habitat has unlimited resources, so individuals **compete**, and growth is capped at **nature's CARRYING CAPACITY (K)** for that species in that habitat. - The population shows a **lag phase → acceleration → deceleration → asymptote** at K, giving a **SIGMOID curve**. This is **Verhulst-Pearl Logistic Growth**: **dN/dt = rN[(K − N)/K]**. - Because resources for most animal populations are finite and become limiting sooner or later, **the logistic model is considered MORE REALISTIC**. ### Life history variation - Populations evolve to maximise **reproductive fitness — Darwinian fitness, i.e. a high r value**. Some breed **ONCE** in a lifetime (**Pacific salmon fish, bamboo**); others **many times** (most birds and mammals). Some produce **many small offspring** (**oysters, pelagic fishes**); others **few large offspring** (birds, mammals). ## Population Interactions & Predation ### Population interactions — the +/− table NEET asks from | **Interaction** | **Species A** | **Species B** | **Meaning** | | Mutualism | + | + | BOTH species benefit | | Competition | − | − | BOTH species lose | | Predation | + | − | One benefits, the other is harmed | | Parasitism | + | − | One benefits, the other is harmed | | Commensalism | + | 0 | One benefits, the other is NEITHER harmed nor benefited | | Amensalism | − | 0 | One is HARMED, the other is UNAFFECTED | **Predation, parasitism and commensalism share a common pattern** — one of the interacting species benefits. ### Predation — more than just killing - Predation is **nature's way of transferring energy** fixed by plants to higher trophic levels. A **sparrow eating a seed is no less a predator**; herbivores are, ecologically, not very different from predators. - **Predators keep prey populations under control** and prevent ecosystem instability. The **prickly pear cactus introduced into Australia in the early 1920s** spread over millions of hectares because it had **no natural predators** there — it was controlled only after a **cactus-feeding moth** from its natural habitat was introduced. - **Predators maintain species diversity** by reducing competition among prey. In the rocky intertidal communities of the American Pacific Coast, when the starfish ***Pisaster*** was experimentally removed, **more than 10 species of invertebrates became extinct within a year** due to inter-specific competition. - **Predators are 'PRUDENT'** — if a predator over-exploits its prey to extinction, the predator itself dies out for lack of food. ### Prey and plant defences - Insects and frogs may be **cryptically coloured (camouflaged)** or poisonous. The **Monarch butterfly** is highly distasteful to birds because of a special chemical — which it **acquires during its CATERPILLAR stage by feeding on a poisonous weed**, not by making it itself. - Nearly **25 per cent of all insects are phytophagous** (feeding on plant sap and parts). Plants cannot run, so they evolved defences: **thorns (Acacia, Cactus)** morphologically, and chemicals that sicken, inhibit feeding/digestion, disrupt reproduction or kill. ***Calotropis* produces highly poisonous CARDIAC GLYCOSIDES** — which is why cattle and goats never browse it. - **Nicotine, caffeine, quinine, strychnine and opium** — commercially extracted from plants — are actually **defences against grazers and browsers**. ## Competition, Parasitism, Commensalism & Mutualism ### Competition & resource partitioning - **Competition is best defined as a process in which the fitness of one species is significantly lower in the presence of another**. Importantly, **resources need NOT be limiting** for competition to occur — in **interference competition**, one species' feeding efficiency is simply reduced by another. - **Resource partitioning** lets competing species co-exist rather than one excluding the other — by choosing **different feeding times or foraging patterns**. **MacArthur** showed **five closely related warbler species living on the SAME tree** avoided competition through **behavioural differences in foraging**. ### Parasitism - Many parasites are **host-specific**, so host and parasite **CO-EVOLVE**. Parasitic adaptations: **loss of unnecessary sense organs, adhesive organs or suckers, loss of the digestive system, and high reproductive capacity**. - Life cycles are often complex: the **human liver fluke** (a trematode) needs **TWO intermediate hosts — a snail and a fish**; the malarial parasite needs a **vector (mosquito)**. | **Parasite type** | **Where it lives** | **Examples** | | Ectoparasite | On the EXTERNAL surface of the host | Lice on humans; ticks on dogs; ectoparasitic copepods on marine fish. CUSCUTA — a parasitic plant on hedge plants that has LOST its chlorophyll and leaves during evolution | | Endoparasite | INSIDE the host body (liver, kidney, lungs, RBCs) | Life cycles are MORE COMPLEX due to extreme specialisation; morphology is greatly SIMPLIFIED while reproductive potential is emphasised | | Brood parasitism | The parasitic BIRD lays eggs in the host's nest and lets the host incubate them | CUCKOO (koel) and CROW. The parasite's eggs have evolved to RESEMBLE the host's eggs in size and colour, so the host does not detect and eject them | ### Commensalism & mutualism — the named examples | **Interaction** | **Example** | **Who benefits** | | Commensalism | Orchid growing as an EPIPHYTE on a mango branch | Orchid benefits; mango tree unaffected | | Commensalism | Barnacles growing on the back of a whale | Barnacles benefit; whale unaffected | | Commensalism | Cattle egret and grazing cattle | Cattle stir up and flush out insects the egret then catches; cattle unaffected | | Commensalism | Sea anemone and clown fish | Clown fish gets protection from the stinging tentacles; anemone derives no apparent benefit | | Mutualism | LICHENS — fungus + photosynthesising algae or cyanobacteria | Both benefit | | Mutualism | MYCORRHIZAE — fungi + roots of higher plants | Fungi help absorb essential soil nutrients; plant provides energy-yielding carbohydrates | | Mutualism | FIG tree and its WASP — a tight ONE-TO-ONE relationship | The wasp pollinates the fig while searching for egg-laying sites; the fig offers some developing seeds as food for wasp larvae. A given fig species can be pollinated ONLY by its partner wasp species | Plants pay pollinators and seed dispersers in **pollen and nectar** and **juicy nutritious fruits**. The system must be safeguarded against **'cheaters'** — animals that steal nectar without pollinating. This is why plant-animal interactions often show **CO-EVOLUTION** of the mutualists. ## Why This Matters for NEET ### Why this matters for NEET - High-value one-liners: **dN/dt = rN** (exponential, **J-shaped**); **dN/dt = rN[(K−N)/K]** (logistic, **sigmoid**, = Verhulst-Pearl); **e = 2.71828**; **Pisaster** removal → 10+ invertebrate species extinct; prickly pear controlled by a **moth**; **MacArthur's five warblers**; **liver fluke needs 2 intermediate hosts**; **Cuscuta lost chlorophyll**; **cuckoo-crow** brood parasitism; **25%** of insects are phytophagous. - **Trap:** in **commensalism one species is UNAFFECTED (0)** while in **amensalism one is actively HARMED (−)** and the other unaffected — the difference is the sign on the second species. Also, the **Monarch butterfly does NOT manufacture** its distasteful chemical — it **acquires it as a caterpillar** from a poisonous weed. - **Second trap:** **competition does NOT require resources to be limiting** — interference competition happens even with abundant resources. And the **female mosquito is NOT considered a parasite** despite needing blood, because it does not live on or in the host. Finally, **logistic (not exponential) growth is the more realistic model**, since real resources are always finite. ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** Ecology is concerned with how many levels of biological organisation? - (a) Two - (b) Four - (c) Three - (d) Six **Answer: (b)** — Ecology deals with four levels: organisms, populations, communities and biomes. **Q2.** If 20 lotus plants produce 8 new plants in a year, the birth rate is: - (a) 2.5 per year - (b) 0.4 offspring per lotus per year - (c) 28 per year - (d) 8 per year **Answer: (b)** — Birth rate is per capita: 8/20 = 0.4 offspring per lotus per year. **Q3.** Which two factors INCREASE population density? - (a) Immigration and emigration - (b) Mortality and emigration - (c) Natality and immigration - (d) Natality and mortality **Answer: (c)** — Natality and immigration increase density; mortality and emigration decrease it. **Q4.** In the exponential growth equation dN/dt = rN, the term 'r' represents: - (a) Resource availability - (b) Population density - (c) Carrying capacity - (d) The intrinsic rate of natural increase **Answer: (d)** — r = (b − d) and is called the intrinsic rate of natural increase — a key parameter in population ecology. **Q5.** Exponential growth plotted against time gives: - (a) A J-shaped curve - (b) A sigmoid curve - (c) A bell curve - (d) A straight line **Answer: (a)** — Exponential or geometric growth results in a J-shaped curve; logistic growth gives a sigmoid curve. **Q6.** In the integral form Nt = N0 e^rt, the value of e is: - (a) 0.693 - (b) 3.14159 - (c) 2.71828 - (d) 1.618 **Answer: (c)** — e is the base of natural logarithms, 2.71828. **Q7.** The intrinsic rate of natural increase for the human population in India in 1981 was: - (a) 1.5 - (b) 0.0205 - (c) 0.12 - (d) 0.015 **Answer: (b)** — The r value for India's human population in 1981 was 0.0205; for the Norway rat 0.015 and flour beetle 0.12. **Q8.** Logistic growth is described by which equation? - (a) dN/dt = (b+d)N - (b) Nt = N0 e^rt - (c) dN/dt = rN - (d) dN/dt = rN[(K−N)/K] **Answer: (d)** — The Verhulst-Pearl logistic growth equation is dN/dt = rN[(K−N)/K], where K is the carrying capacity. **Q9.** The logistic growth model is considered more realistic because: - (a) It ignores carrying capacity - (b) Resources for most populations are finite and become limiting - (c) It is mathematically simpler - (d) It applies only to plants **Answer: (b)** — Since resources are finite and become limiting sooner or later, the logistic model better reflects reality. **Q10.** Which organisms breed only ONCE in their lifetime? - (a) Most birds and mammals - (b) Oysters - (c) All fishes - (d) Pacific salmon fish and bamboo **Answer: (d)** — Pacific salmon fish and bamboo breed only once in their lifetime; most birds and mammals breed many times. **Q11.** In amensalism, the two interacting species are affected as: - (a) − and − - (b) − and 0 - (c) + and + - (d) + and 0 **Answer: (b)** — In amensalism one species is harmed (−) while the other is unaffected (0); in commensalism one benefits (+) and the other is unaffected (0). **Q12.** The prickly pear cactus that invaded Australia in the 1920s was finally controlled by introducing: - (a) A fungus - (b) Grazing cattle - (c) A herbicide - (d) A cactus-feeding moth from its natural habitat **Answer: (d)** — The invasive cactus spread because Australia lacked its natural predators; a cactus-feeding moth from its home range controlled it. **Q13.** Removal of the starfish Pisaster from an intertidal community resulted in: - (a) More than 10 invertebrate species becoming extinct within a year - (b) No change - (c) An increase in species diversity - (d) Extinction of the starfish only **Answer: (a)** — Without the predator, inter-specific competition among prey caused over 10 invertebrate species to go extinct within a year. **Q14.** The Monarch butterfly is distasteful to predators because it: - (a) Acquires a chemical during its caterpillar stage by feeding on a poisonous weed - (b) Manufactures a toxin itself - (c) Has stinging hairs - (d) Is cryptically coloured **Answer: (a)** — The butterfly acquires the special chemical during its caterpillar stage by feeding on a poisonous weed. **Q15.** Calotropis is avoided by cattle and goats because it produces: - (a) Thorns - (b) Nicotine - (c) Caffeine - (d) Highly poisonous cardiac glycosides **Answer: (d)** — Calotropis produces highly poisonous cardiac glycosides, which is why no cattle or goats browse it. **Q16.** MacArthur showed that five closely related warbler species on the same tree co-existed by: - (a) Migrating away - (b) Behavioural differences in foraging activities — resource partitioning - (c) Eating each other - (d) Interbreeding **Answer: (b)** — Resource partitioning through behavioural differences in foraging allowed the warblers to avoid competition and co-exist. **Q17.** The human liver fluke requires how many intermediate hosts? - (a) One - (b) Two — a snail and a fish - (c) Three - (d) None **Answer: (b)** — The human liver fluke, a trematode, depends on two intermediate hosts — a snail and a fish — to complete its life cycle. **Q18.** Cuscuta, a parasitic plant, has lost its: - (a) Chlorophyll and leaves - (b) Roots only - (c) Stem - (d) Flowers **Answer: (a)** — Cuscuta has lost its chlorophyll and leaves during evolution and derives nutrition from its host plant. **Q19.** The interaction between the sea anemone and the clown fish is an example of: - (a) Commensalism - (b) Mutualism - (c) Predation - (d) Parasitism **Answer: (a)** — The clown fish gets protection from the stinging tentacles; the anemone derives no apparent benefit — commensalism. **Q20.** In the fig-wasp relationship: - (a) The fig gains nothing - (b) Any wasp can pollinate any fig - (c) A given fig species can be pollinated only by its partner wasp species - (d) The wasp harms the fig **Answer: (c)** — There is a tight one-to-one relationship: a given fig species can be pollinated only by its partner wasp species and no other. **Q21.** The curve marked A represents - (a) logistic growth - (b) population crash - (c) zero population growth - (d) exponential growth when resources are unlimited **Answer: (d)** — Curve A is the J-shaped exponential curve (dN/dt = rN), possible only when food and space are unlimited. Curve B is the S-shaped logistic curve. **Q22.** The dashed line marked C represents - (a) the biotic potential (r) - (b) the lag phase - (c) the carrying capacity (K) - (d) the death rate **Answer: (c)** — The population in curve B levels off at C, the carrying capacity — the maximum population the environment can sustain. **Q23.** The curve marked B is described by the equation - (a) dN/dt = rN - (b) dN/dt = rN (K − N)/K - (c) N(t) = N0 + rt - (d) dN/dt = (b − d)N **Answer: (b)** — The Verhulst-Pearl logistic equation dN/dt = rN(K−N)/K describes the S-shaped curve: growth slows as N approaches K. dN/dt = rN is the exponential (J) curve. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q24.** Lichens represent a mutualistic relationship between: (NEET PYQ) - (a) A fungus and plant roots - (b) A fungus and photosynthesising algae or cyanobacteria - (c) Two fungi - (d) Bacteria and algae **Answer: (b)** — Lichens are an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. **Q25.** In mycorrhizae, the plant provides the fungus with: (NEET PYQ) - (a) Energy-yielding carbohydrates - (b) Water only - (c) Nitrogen - (d) Phosphorus **Answer: (a)** — The fungi help the plant absorb essential soil nutrients, while the plant provides the fungi with energy-yielding carbohydrates. **Q26.** Brood parasitism is best exemplified by: (NEET PYQ) - (a) Cuscuta on hedge plants - (b) Ticks on dogs - (c) Lice on humans - (d) The cuckoo (koel) and the crow **Answer: (d)** — In brood parasitism the parasitic bird lays eggs in the host's nest; the cuckoo and crow is the classic example. **Q27.** Approximately what percentage of all insects are phytophagous? (NEET PYQ) - (a) 25 per cent - (b) 50 per cent - (c) 5 per cent - (d) 75 per cent **Answer: (a)** — Nearly 25 per cent of all insects are known to be phytophagous, feeding on plant sap and other plant parts. **Q28.** The carrying capacity K refers to: (NEET PYQ) - (a) The birth rate - (b) The maximum population size a habitat can support - (c) The initial population density - (d) The intrinsic rate of increase **Answer: (b)** — Carrying capacity is nature's limit — the maximum population of a species a given habitat can sustain. **Q29.** Which of the following pairs is an example of commensalism? (NEET PYQ) - (a) Cuscuta and hedge plant - (b) Lichen components - (c) Fig and wasp - (d) Cattle egret and grazing cattle **Answer: (d)** — The egret benefits from insects flushed out by the moving cattle, while the cattle are unaffected — commensalism. **Q30.** Competition can occur even when resources are NOT limiting, in the form of: (NEET PYQ) - (a) Amensalism - (b) Exploitation competition - (c) Interference competition - (d) Mutualism **Answer: (c)** — In interference competition the feeding efficiency of one species is reduced by another, even without resource scarcity. **Q31.** Adaptations of parasites include: (NEET PYQ) - (a) Enhanced sense organs and digestive system - (b) Increased locomotion - (c) Photosynthetic ability - (d) Loss of unnecessary sense organs, adhesive organs or suckers, loss of digestive system and high reproductive capacity **Answer: (d)** — Parasites evolved loss of unnecessary sense organs, adhesive organs/suckers, loss of the digestive system and a high reproductive capacity. **Q32.** The shape of an age pyramid reflects: (NEET PYQ) - (a) Whether the population is growing, stable or declining - (b) The sex ratio only - (c) The intrinsic rate of increase - (d) The carrying capacity **Answer: (a)** — The shape of the age pyramid shows the growth status of the population — growing, stable or declining. **Q33.** Nicotine, caffeine, quinine and opium are produced by plants primarily as: (NEET PYQ) - (a) Waste products - (b) Pollinator attractants - (c) Defences against grazers and browsers - (d) Growth hormones **Answer: (c)** — These chemicals, extracted commercially, are actually produced by plants as defences against grazers and browsers. ### Active Recall Prompt Write everything you can recall about **Organisms and Populations**, naming each part first: the **four levels** of ecology; **population attributes** (birth and death rate with the worked examples, sex ratio, age pyramid, and the four factors changing density); **exponential growth** (both equations, the J-curve, r values for each organism); **logistic growth** (carrying capacity, the equation, the sigmoid curve, Verhulst-Pearl, why it is more realistic); **life history variation**; the **six population interactions** with their +/− signs; **predation** (its roles, the prickly pear and Pisaster cases, prey and plant defences); **competition** and resource partitioning; **parasitism** (adaptations, ecto vs endo, brood parasitism); and **commensalism and mutualism** with all their named examples. Begin each fact with its topic and end it with a full stop.