Organisms and Populations (NEET Biology Class 12): Growth Models & Population Interactions
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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]
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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
Show answer
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.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q21. 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
Show answer
Answer: (b) — Lichens are an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.
Q22. In mycorrhizae, the plant provides the fungus with: (NEET PYQ)
(a) Energy-yielding carbohydrates
(b) Water only
(c) Nitrogen
(d) Phosphorus
Show answer
Answer: (a) — The fungi help the plant absorb essential soil nutrients, while the plant provides the fungi with energy-yielding carbohydrates.
Q23. 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
Show answer
Answer: (d) — In brood parasitism the parasitic bird lays eggs in the host's nest; the cuckoo and crow is the classic example.
Q24. 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
Show answer
Answer: (a) — Nearly 25 per cent of all insects are known to be phytophagous, feeding on plant sap and other plant parts.
Q25. 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
Show answer
Answer: (b) — Carrying capacity is nature's limit — the maximum population of a species a given habitat can sustain.
Q26. 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
Show answer
Answer: (d) — The egret benefits from insects flushed out by the moving cattle, while the cattle are unaffected — commensalism.
Q27. 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
Show answer
Answer: (c) — In interference competition the feeding efficiency of one species is reduced by another, even without resource scarcity.
Q28. 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
Show answer
Answer: (d) — Parasites evolved loss of unnecessary sense organs, adhesive organs/suckers, loss of the digestive system and a high reproductive capacity.
Q29. 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
Show answer
Answer: (a) — The shape of the age pyramid shows the growth status of the population — growing, stable or declining.
Q30. 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
Show answer
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.