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Organisms and Populations

Biology · Class 12NCERT pp. 188–2022 sets × 25 questions

Key facts and formulas for Organisms and Populations, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.

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Key facts for this chapter

IdeaRule or fact
Population changeN(t+1) = N(t) + (B + I) − (D + E); birth rate = births ÷ initial number
Exponential growthdN/dt = rN; N t = N0 e^(rt); J-shaped; doubling time = 0.693 ÷ r
Logistic growthdN/dt = rN(1 − N/K); S-shaped; zero at N = K; maximum at N = K/2 (rK/4)
Logistic phasesLag, acceleration, deceleration, asymptote
Species interaction signsMutualism ++, commensalism +0, amensalism −0, parasitism +−, predation +−, competition −−
Predators and PisasterPredators transfer energy, keep prey in check, reduce prey competition; Pisaster is a keystone species
CompetitionGause: competitive exclusion in Paramecium; Connell: Balanus and Chathamalus, competitive release; interference competition
ParasitesHost-specific, loss of sense organs, adhesive organs, high reproduction; Cuscuta ectoparasite, liver fluke endoparasite, cuckoo brood parasite
Life historyr-selected: many small young; K-selected: few large young near K; salmon breeds once; Type III survivorship in oysters
AdaptationsAllen: shorter extremities in cold; Bergmann: larger body in cold; regulators are birds and mammals; kangaroo rat uses metabolic water

Three sample questions

Q1. A pond population of 50 lotus plants added 15 new plants in a year. The birth rate is
  1. 0.23 per plant per year
  2. 0.3 per plant per year
  3. 3.3 per plant per year
  4. 15 per plant per year
Hint and solution

Hint: Birth rate is new individuals divided by the number present at the start, not at the end.

  1. birth rate = births ÷ initial number — Birth rate per capita is the number of new individuals divided by the starting population.
  2. births = 15, initial N = 50 — Read the values: 15 new plants, 50 plants at the start.
  3. 15 ÷ 50 = 0.3 — Divide 15 by 50 to get 0.3 per plant per year.
  4. Answer: 0.3 per plant per year — Do not use 65, because the birth happened to the original 50. Option B.
Answer: B. Birth rate = 15 divided by the starting 50 = 0.3. Dividing by 65 would be wrong.
Q2. In a barn natality was 120, mortality 100, immigration 10 and emigration 40. The population
  1. increased by 10
  2. decreased by 30
  3. stayed the same
  4. decreased by 10
Hint and solution

Hint: Add the two things that increase the population, subtract the two that decrease it.

  1. gain = natality + immigration — Individuals added are births plus immigrants: 120 + 10 = 130.
  2. loss = mortality + emigration — Individuals removed are deaths plus emigrants: 100 + 40 = 140.
  3. net = 130 − 140 = −10 — Subtract loss from gain. The result is negative.
  4. Population falls by 10 — A negative change means the population decreased by 10. Option D.
Answer: D. Net change = (120 + 10) − (100 + 40) = 130 − 140 = −10.
Q3. If N(t) = 500, B = 60, I = 15, D = 40 and E = 25, then N(t+1) is
  1. 510
  2. 490
  3. 540
  4. 575
Hint and solution

Hint: Use N(t+1) = N(t) + (B + I) − (D + E).

  1. N(t+1) = N(t) + (B + I) − (D + E) — The population next time is the present number plus entries minus exits.
  2. B + I = 60 + 15 = 75 — Births and immigrants add 75 individuals.
  3. D + E = 40 + 25 = 65 — Deaths and emigrants remove 65 individuals.
  4. 500 + 75 − 65 = 510 — The net gain is 10, so the population becomes 510. Option A.
Answer: A. N(t+1) = 500 + (60 + 15) − (40 + 25) = 500 + 75 − 65 = 510.

How to use this chapter

Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.

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