Organisms and Populations Class 12 Notes

• Abiotic factors — temperature, water, light and soil.
• Biotic factors — other organisms: competitors, predators, pathogens and parasites.
• The range over which an organism tolerates a factor is its tolerance range; the optimum range gives best growth.
• Organisms respond to their environment via regulators, conformers, partial regulators or migrators.

• Water — critical for life; salinity, availability and moisture govern where organisms live.
• Light — needed for photosynthesis and photoperiodism that times flowering, breeding and migrations.
• Soil — key edaphic factor; its composition, pH and mineral content influence rooted plants, burrowing animals and microbes.
• Example of water stress: the camel tolerates large water loss and high body-temperature change, and conserves water.

• Kangaroo rat — obtains water from the oxidation of fats and produces concentrated urine, so it never drinks water.
• Camel — tolerates high body temperature and large water loss, conserves water by reducing its fat to hump tissue and by concentrated urine.
• Opuntia (prickly pear cactus) — leaves reduced to spines with thick, waxy cuticle to reduce water loss; photosynthesis occurs in stems.
• Allen's rule — mammals in colder climates have shorter ears and limbs to reduce heat loss (e.g. arctic fox vs tropical fox).
• Bergmann's rule — animals in colder regions tend to be larger-bodied to conserve heat (larger surface-area-to-volume ratio in the reverse sense).

• Population size/density — often measured by number per unit area or volume; sometimes estimated by indirect indicators like the number of faecal pellets.
• Natality (birth rate) — births per individual per unit time.
• Mortality (death rate) — deaths per individual per unit time.
• Immigration — individuals moving into a population from elsewhere; increases population.
• Emigration — individuals leaving a population; decreases population.
• Age structure — the proportion of individuals in different age groups; shown by age pyramids (expanding, stable or declining).

Age pyramids plot the proportion of pre-reproductive, reproductive and post-reproductive individuals. A broad base indicates an expanding population; a narrow base indicates a declining one; a balanced profile indicates a stable population.

Life history traits — such as the age of first reproduction, number of offspring and parental care — vary by species and habitat. Species either invest in many small offspring with little care (like most fishes) or few large offspring with heavy care (like mammals and birds).

• Mutualism (++) — both species benefit, e.g. lichens (fungus + alga), mycorrhizae, and pollination by insects.
• Competition (−−) — both suffer; Gause's competitive exclusion principle states two species can never coexist in the same niche; one competitively excludes the other.
• Predation (+−) — predator benefits, prey suffers; it naturally regulates prey populations.
• Parasitism (+−) — parasite benefits at the host's expense; e.g. Plasmodium causes malaria in humans.
• Commensalism (+0) — one benefits, the other is unaffected; e.g. barnacles on whales, orchids on trees.
• Amensalism (−0) — one is harmed, the other unaffected; e.g. the antibiotic penicillin (from Penicillium) killing bacteria.

Example: A population growing exponentially has an intrinsic rate of increase r = 0.2 per year and a current size N = 1000. What does the exponential model dN/dt = rN predict for the instantaneous rate of change?

Solution: dN/dt = rN = 0.2 × 1000 = 200 individuals per year. This is the instantaneous rate of increase; because growth is exponential, this rate itself rises as N grows, producing the characteristic J-shaped curve.

Example: Distinguish between commensalism and amensalism with one example each.

Solution: Commensalism is a +0 interaction where one species benefits and the other is unaffected — e.g. barnacles attached to a whale (they get a moving base; the whale is unaffected). Amensalism is a −0 interaction where one species is harmed and the other unaffected — e.g. Penicillium secreting penicillin that kills neighbouring bacteria. The key difference is whether one partner benefits (+) or is only harmed (−).

Exercises (NCERT)

1. 1
   What is an ecological niche?
2. 2
   Name the major abiotic factors that affect organisms.
3. 3
   What is homeostasis? How do organisms maintain it?
4. 4
   Differentiate between conformers and regulators.
5. 5
   What is hibernation? Give an example.
6. 6
   What is aestivation? Give an example.
7. 7
   What is diapause?
8. 8
   What is a population?
9. 9
   Define population density.
10. 10
    What are the attributes of a population?
11. 11
    What is the exponential growth equation?
12. 12
    What is the logistic growth equation?
13. 13
    What is 'K' in the logistic growth model?
14. 14
    What is natality? What is mortality?
15. 15
    What is life history variation?
16. 16
    Define the terms mutualism and commensalism.
17. 17
    What is competition? How do organisms avoid it?
18. 18
    What is parasitism? Give an example.
19. 19
    What is predation? Give an example.
20. 20
    What is Gause's competitive exclusion principle?

Answers at a glance

1. 1
   An ecological niche is the functional role and position of a species in its ecosystem.
2. 2
   Temperature, water, light and soil are the major abiotic factors.
3. 3
   Homeostasis is the maintenance of a steady internal state; organisms regulate it physiologically, behaviourally or through migration.
4. 4
   Conformers change with the environment; regulators maintain a constant internal environment.
5. 5
   Hibernation is winter dormancy; e.g. bears and frogs.
6. 6
   Aestivation is summer dormancy; e.g. snails and some fish.
7. 7
   Diapause is a period of suspended development; e.g. some zooplankton.
8. 8
   A population is a group of individuals of the same species in a given area.
9. 9
   Population density is the number of individuals per unit area or volume.
10. 10
    Population attributes include density, birth rate, death rate, age distribution and sex ratio.
11. 11
    Exponential growth: dN/dt = rN, giving N(t) = N0·e^(rt).
12. 12
    Logistic growth: dN/dt = rN[(K − N)/K].
13. 13
    K is the carrying capacity — the maximum population the environment can support.
14. 14
    Natality is the birth rate; mortality is the death rate.
15. 15
    Life history variation describes how organisms trade off resources between growth, reproduction and survival.
16. 16
    Mutualism benefits both species (+/+); commensalism benefits one and does not affect the other (+/0).
17. 17
    Competition is a −/− interaction; organisms avoid it by resource partitioning.
18. 18
    Parasitism is a +/− interaction where the parasite benefits at the host's expense; e.g. Cuscuta on trees.
19. 19
    Predation is a +/− interaction where the predator kills and eats the prey; e.g. tiger and deer.
20. 20
    Gause's principle states that two species competing for the same resource cannot coexist indefinitely; the inferior one is eliminated.

MCQ Questions

2 Mark Questions

1. 1
   Differentiate between conformers and regulators.
2. 2
   What is hibernation? Give an example.
3. 3
   What is population density?
4. 4
   Define mutualism and commensalism.
5. 5
   What is Gause's competitive exclusion principle?

3 Mark Questions

1. 1
   Describe the major abiotic factors that affect organisms.
2. 2
   Describe the responses of organisms to abiotic stress.
3. 3
   Describe the attributes of a population.
4. 4
   Explain the exponential and logistic growth models.
5. 5
   Describe the major population interactions with examples.

5 Mark Questions

1. 1
   Describe the adaptations of organisms to their environment.
2. 2
   Describe population growth models and population interactions.
3. 3
   Explain the major population interactions with suitable examples.