CBSE/NCERT/Notes/Class_12_Biology
Organisms and Populations
Note on Organisms and Populations (Chapter 11)
Organisms and Populations
Key Concepts
Levels of Biological Organisation
Ecology is concerned with four levels: Organisms, Populations, Communities, and Biomes.
Population Attributes
A population is a group of individuals of the same species in a defined area, sharing resources and interbreeding.
- Birth Rate & Death Rate: Expressed as per capita changes (e.g., 0.4 offspring per lotus per year).
- Sex Ratio: Ratio of males to females in a population.
- Age Pyramid: Graphic representation of age distribution. Shapes indicate:
- Growing: Broad base (more young individuals).
- Stable: Uniform distribution.
- Declining: Narrow base.
- Population Density (N): Measured by number, biomass, or per cent cover. Indirect methods include pug marks and fecal pellets for tiger census.
Population Growth
Fluctuates based on:
- Natality (B): Number of births.
- Mortality (D): Number of deaths.
- Immigration (I): Number of individuals entering.
- Emigration (E): Number of individuals leaving.
- Equation: Nt+1 = Nt + [(B + I) – (D + E)]
Growth Models
- Exponential Growth: Occurs when resources are unlimited. Results in a J-shaped curve.
- Equation: dN/dt = rN (where r is the intrinsic rate of natural increase).
- Integral form: Nt = N0 ert.
- Logistic Growth: More realistic; resources are limited. Results in a Sigmoid (S-shaped) curve.
- Carrying Capacity (K): Maximum number a habitat can support.
- Equation (Verhulst-Pearl): dN/dt = rN [(K-N)/K]
Population Interactions
| Interaction | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | + | + | Lichens (Algae + Fungus); Mycorrhiza |
| Competition | - | - | Flamingoes and Fish for zooplankton |
| Predation | + | - | Tiger and Deer; Sparrow and Seed |
| Parasitism | + | - | Ticks on dogs; Cuscuta on hedges |
| Commensalism | + | 0 | Orchid on mango branch; Cattle egret |
| Amensalism | - | 0 | Penicillium inhibiting bacteria |
- Predation: Predators act as conduits for energy transfer and keep prey populations under control. Prey defenses include camouflage, poisons (Monarch butterfly), and thorns (Cactus).
- Competition:
- Gause’s Competitive Exclusion Principle: Two closely related species competing for the same limited resources cannot co-exist.
- Resource Partitioning: Avoiding competition by different feeding times or foraging patterns (e.g., MacArthur's warblers).
- Parasitism:
- Ectoparasites: Live on the surface (Lice, Ticks).
- Endoparasites: Live inside the body (Liver fluke, Plasmodium).
- Brood Parasitism: Cuckoo laying eggs in a crow's nest.
- Mutualism: Co-evolution is common (e.g., Fig tree and Wasp). Sexual Deceit is used by the Mediterranean orchid Ophrys to attract bee pollinators.
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