Organisms and Populations
Class 12 · Biology
4 ideas in Class 12 Biology, explained in plain words, with games and quizzes to practise.
Population Attributes
Part of: Count the Crowd
Populations carry per-capita birth and death rates, male-female ratios and age pyramids showing growth, stability or decline. Density may count numbers, biomass or cover, and tigers are censused by pug marks. Lotus birth rate 0.4 and fly death rate 0.1 show the arithmetic.
Why it matters. Rate arithmetic plus measure choice starts population questions.
Formulas
- Birth rate equals newborn divided by population
- Lotus rate is 0.4, fly rate is 0.1
- Density may use numbers, biomass or cover
Key words
- Population
- : Interbreeding group in one area.
- Birth rate
- : Per-capita newborn per time.
- Death rate
- : Per-capita deaths per time.
- Age pyramid
- : Age diagram showing growth status.
- Population density
- : Crowd size labelled N.
Try it in two minutes
- Divide newborn by population.
- Read pyramid shapes.
- Pick number versus biomass.
Try at home. Explain why one banyan outweighs 200 weeds.
Rise and Level
Part of: Count the Crowd
Density updates by births plus entries minus deaths plus exits. Unlimited resources grow J-curves by dN by dt equals rN, while limited habitats level into sigmoid curves capped at K. Chess doubling shows explosion power, and breed-once versus breed-many strategies tune fitness.
Why it matters. Equation reading plus curve choice is the mathematical heart.
Formulas
- Next density equals N plus B plus I minus D minus E
- Exponential rate is dN by dt equals rN
- Logistic rate is dN by dt equals rN times K minus N over K
Key words
- Natality
- : Births added per period.
- Mortality
- : Deaths lost per period.
- Intrinsic rate
- : Per-capita growth potential r.
- Carrying capacity
- : Maximum supported number K.
- Exponential growth
- : Unlimited J-shaped boom.
- Logistic growth
- : Limited sigmoid levelling.
Try it in two minutes
- Balance B, I, D and E.
- Plot J against sigmoid.
- Cap growth at K.
Try at home. Explain why Paramecium would overrun in 64 days.
Interaction Signs and Predation
Part of: Neighbours
Six interactions span mutualism plus-plus to competition minus-minus, with predation and parasitism at plus-minus. Predators move energy upward, check prey and guard diversity, as prickly-pear moths and Pisaster show. Prey answer with camouflage, poison and thorns.
Why it matters. Sign tables plus case reasoning answer interaction questions.
Key words
- Mutualism
- : Both partners gain.
- Competition
- : Both partners lose.
- Predation
- : Eater gains, eaten loses.
- Commensalism
- : One gains, other unaffected.
- Amensalism
- : One harmed, other unaffected.
- Camouflage
- : Colour hiding from hunters.
Try it in two minutes
- Fill six sign rows.
- Name three predator roles.
- List plant and animal shields.
Try at home. Explain why prudent predators spare their prey.
Competition, Parasites and Partners
Part of: Close Company
Gause exclusion yields to resource partitioning, as five warblers sharing one tree prove. Parasites trade organs for reproduction, from Cuscuta to cuckoos, while hosts fight back. Fig-wasp pairs and tricking orchids show mutualism sealed by co-evolution.
Why it matters. Coexistence mechanisms plus case sorting finish ecology questions.
Key words
- Exclusion principle
- : Inferior competitor exits.
- Resource partitioning
- : Sharing by different habits.
- Ectoparasite
- : Outer-surface feeder.
- Endoparasite
- : Inner-body resident.
- Brood parasitism
- : Eggs raised by hosts.
- Co-evolution
- : Partners evolving together.
Try it in two minutes
- State exclusion, then partition.
- Sort ecto from endo parasites.
- Pair each mutualist.
Try at home. Explain why the female mosquito is no parasite.
What your child will practise
- Competition, Parasites and Partners: Jungle Puzzle (Clue Puzzle)
- Competition, Parasites and Partners: Jungle Quick Fire (Quick Fire)
- Interaction Signs and Predation: Candy Puzzle (Clue Puzzle)
- Population Attributes: Jungle Quest (Flip and Match)
- Rise and Level: Space Quick Fire (Quick Fire)
- Quiz: Competition, Parasites and Partners (8 questions)
- Quiz: Interaction Signs and Predation (8 questions)
- Quiz: Population Attributes (8 questions)
- Quiz: Rise and Level (8 questions)