11Class 12 Biology
Organisms and Populations short notes
Short notes on organisms and their environment, population growth and how species interact. The growth equations are often tested.
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Organisms and the environment
- Regulators keep their internal state constant, while conformers don't.
- Allen's rule: colder climates favour shorter ears and limbs.
- Adaptations include CAM photosynthesis in desert plants, and kangaroo rats, which get water from their food.
Populations
- Density changes through natality, mortality, immigration and emigration.
- Age pyramids can be expanding, stable or declining.
- Exponential growth: dN/dt = rN, a J-shaped curve.
- Logistic growth: dN/dt = rN(K − N)/K, an S-shaped curve. K is carrying capacity.
Interactions
- Mutualism: both benefit (+, +), as in lichens and mycorrhiza.
- Competition: both are harmed (−, −).
- Predation and parasitism: one gains and the other is harmed (+, −).
- Commensalism: one gains, and the other isn't affected (+, 0), as with barnacles on whales.
- Amensalism: one is harmed, and the other isn't affected (−, 0).
- Gause's principle: two closely related species competing for the same resources can't coexist indefinitely.
- Mutualistic pairs often co-evolve, like fig and wasp.
Where students slip up
- Mixing up the J and S curves. Logistic growth is the S-shaped one, limited by K.
- Mixing up commensalism and amensalism.
- Thinking cuckoos aren't parasites. Brood parasitism is parasitism.
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