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The ecology chapters hold some of the few calculations in NEET biology, and they are easy marks for students who have practised them. Organisms and Populations asks you to work out birth and death rates, use two growth equations, and sort species interactions by who benefits. This post works through each, with numbers, then lists the traps and a practice plan. For food chains and ecological pyramids, see our separate guide to ecology questions for NEET.
What does the syllabus include?
The NEET (UG) 2026 syllabus lists, under organisms and environment: population interactions (mutualism, competition, predation, parasitism) and population attributes (growth, birth rate and death rate, age distribution). NCERT Class 12 Chapter 11, Organisms and Populations covers these and develops the exponential and logistic growth models in detail.
How do you calculate birth and death rates?
In a population, birth and death rates are per capita: change in number divided by the number of members. NCERT's examples:
A pond had 20 lotus plants last year, and 8 new plants were added. Birth rate = 8/20 = 0.4 offspring per lotus per year.
In a lab population of 40 fruit flies, 4 died in a week. Death rate = 4/40 = 0.1 individuals per fly per week.
Population density changes through four processes: natality (B) and immigration (I) add individuals; mortality (D) and emigration (E) remove them. So Nₜ₊₁ = Nₜ + [(B + I) − (D + E)]. If 50 deer are born, 10 arrive, 30 die and 5 leave, a herd of 400 becomes 400 + (60 − 35) = 425.
What is exponential growth?
When food and space are unlimited, a population grows in proportion to its size: dN/dt = rN, where r = b − d is the intrinsic rate of natural increase. Integrated, this gives Nₜ = N₀eʳᵗ, a J-shaped curve. NCERT quotes r values of 0.015 for the Norway rat and 0.12 for the flour beetle, and 0.0205 for India's human population in 1981.
Worked example. NCERT asks: if a population growing exponentially doubles in 3 years, what is r?
Doubling means Nₜ = 2N₀ at t = 3 years.
So 2 = e³ʳ, and 3r = ln 2 = 0.693.
r = 0.693 / 3 = 0.231 per year.
A useful shortcut: doubling time = 0.693 / r. For the flour beetle with r = 0.12, that is about 5.8 time units.
What is logistic growth?
No habitat has unlimited resources. Every habitat has a carrying capacity, K, beyond which the population cannot grow. Growth then follows dN/dt = rN(K − N)/K, a sigmoid curve with a lag phase, acceleration, deceleration and an asymptote at K. NCERT calls it Verhulst-Pearl logistic growth. Raymond Pearl and Lowell Reed applied this kind of curve to the census population of the United States in a 1920 paper, and NCERT considers the logistic model the more realistic of the two.
Worked example. A population has r = 0.5 per year and K = 1000.
At N = 200: dN/dt = 0.5 × 200 × (1000 − 200)/1000 = 0.5 × 200 × 0.8 = 80 per year.
At N = 500: dN/dt = 0.5 × 500 × 0.5 = 125 per year.
At N = 900: dN/dt = 0.5 × 900 × 0.1 = 45 per year.
Growth is fastest at N = K/2 and slows to zero as N approaches K. Questions often ask exactly where on the curve the rate is highest.
How are species interactions classified?
NCERT sorts them by the effect on each species: + for benefit, − for harm, 0 for no effect.
Mutualism (+, +): fig trees and the wasps that pollinate them; orchids and their bee pollinators.
Competition (−, −): NCERT's field evidence includes Connell's experiments on the coast of Scotland, where the larger barnacle *Balanus* excluded the smaller *Chthamalus* from the intertidal zone.
Predation (+, −): NCERT notes that herbivores are, in a broad sense, not very different from predators. Its example of predators controlling a population is the cactus-feeding moth that brought the invasive prickly pear under control in Australia.
Parasitism (+, −): the cuckoo laying eggs in a crow's nest is brood parasitism.
Commensalism (+, 0): an orchid growing on a mango branch; the cattle egret feeding near grazing cattle.
Amensalism (−, 0): one species is harmed, the other unaffected.
Gause's competitive exclusion principle says two closely related species competing for the same limiting resource cannot coexist indefinitely. NCERT adds that species can avoid this by resource partitioning, as in MacArthur's five warbler species feeding on the same tree.
Common mistakes and how to avoid them
Dividing by the new population. The lotus birth rate is 8/20, not 8/28.
Using log base 10. NCERT's equation uses e, so use ln 2 = 0.693, not log 2 = 0.301.
Putting maximum growth at K. It is at K/2; at K the growth rate is zero.
Calling an epiphytic orchid a parasite. It is commensal; the mango tree gets no apparent harm or benefit.
A practice plan
Redo NCERT's lotus and fruit fly calculations, then write five of your own with different numbers.
Sketch the J and S curves from memory and mark K and K/2 on the S curve.
Calculate dN/dt in the logistic equation at N = K/10, K/2 and 9K/10 for any r and K you choose.
Make a list of the six interactions with their signs and NCERT examples, cover it, and recall it two days later.
If the growth equations still feel like symbols rather than populations, the Fermi Concept Library covers every concept in biology and mathematics with its own lesson, so you can revisit exponential functions or carrying capacity one idea at a time. For the conservation side of ecology, read our biodiversity and conservation guide.
This week, solve the doubling question again for a population that triples in 4 years, and check that you get r = ln 3 / 4, about 0.27 per year.
