Chapter 13 — Organisms and Populations — introduces ecology concepts: organism responses to environment, populations and their attributes. Carries 4-6 marks.
Key Concepts
Organism Responses to Environment
| Response | Description | Example |
|---|---|---|
| Regulate | Maintain constant internal environment | Mammals regulate body temperature (homeotherms) |
| Conform | Body changes with environment | Fish body temp changes with water (poikilotherms) |
| Migrate | Move to favourable habitat | Siberian cranes to India in winter |
| Suspend | Dormancy during stress | Hibernation (bear), Aestivation (snail), Diapause (insects) |
Adaptations
- Allen’s rule: Animals in cold regions have shorter ears/limbs (reduce heat loss)
- Bergmann’s rule: Animals in cold regions are larger (reduce SA/volume ratio)
- Desert plants: Thick cuticle, sunken stomata, CAM pathway (fix CO₂ at night)
- Kangaroo rat: Survives without drinking water (metabolic water)
Population Attributes
Population density (N): Number of individuals per unit area/volume
Birth rate (b): Births per capita per unit time
Death rate (d): Deaths per capita per unit time
Population growth: Nt = N0 + (B + I) − (D + E)
Where B = births, I = immigration, D = deaths, E = emigration
Birth rate (b): Births per capita per unit time
Death rate (d): Deaths per capita per unit time
Population growth: Nt = N0 + (B + I) − (D + E)
Where B = births, I = immigration, D = deaths, E = emigration
Growth Models
Exponential (J-shaped): dN/dt = rN (unlimited resources)
N(t) = N₀e^(rt) where r = intrinsic rate of natural increase
Logistic (S-shaped/Sigmoid): dN/dt = rN(K−N)/K
K = carrying capacity (maximum population environment can support)
When N = K, growth rate = 0
N(t) = N₀e^(rt) where r = intrinsic rate of natural increase
Logistic (S-shaped/Sigmoid): dN/dt = rN(K−N)/K
K = carrying capacity (maximum population environment can support)
When N = K, growth rate = 0
Population Interactions
| Interaction | Species 1 | Species 2 | Example |
|---|---|---|---|
| Mutualism (+/+) | Benefits | Benefits | Lichen (algae + fungus), Mycorrhiza |
| Competition (−/−) | Harmed | Harmed | Flamingoes vs fish for zooplankton |
| Predation (+/−) | Benefits (predator) | Harmed (prey) | Tiger and deer |
| Parasitism (+/−) | Benefits (parasite) | Harmed (host) | Cuscuta on host plant, tapeworm |
| Commensalism (+/0) | Benefits | Unaffected | Orchid on mango tree (epiphyte) |
| Amensalism (−/0) | Harmed | Unaffected | Penicillium inhibits bacteria |
Gause’s Competitive Exclusion Principle: Two species competing for the same resource cannot co-exist indefinitely — one will eliminate the other.
Quick Revision Points
- Regulators (maintain constancy) vs Conformers (change with environment)
- Hibernation (winter sleep), Aestivation (summer sleep), Diapause (insects, suspended development)
- Exponential growth: J-shaped, dN/dt = rN; Logistic: S-shaped, dN/dt = rN(K-N)/K
- K = carrying capacity; at K, growth = 0
- Mutualism (+/+), Predation (+/−), Parasitism (+/−), Competition (−/−), Commensalism (+/0)
- Gause’s principle: competitive exclusion
Chapter Navigation
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Next: Ecosystem Class 12 Notes
Related Chapters in Class 12 Biology
- Ecosystem Class 12 Notes
- Biodiversity and Conservation Class 12 Notes
- Environmental Issues Class 12 Notes
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