Organisms and Populations — Short Notes
Ecology studies interactions among organisms and their environment at four levels: organism → population → community → ecosystem.
Habitat and Niche
- Habitat — physical place where an organism lives.
- Niche — the "role" of the organism (all factors it exploits).
- Two species can share a habitat but not the same niche (competitive exclusion).
Major Abiotic Factors
- Temperature — most ecologically relevant; affects enzymes, metabolism, distribution.
- Water — availability governs terrestrial life; salinity governs aquatic.
- Light — for photosynthesis; photoperiod controls reproduction, migration, flowering.
- Soil — texture, pH, minerals → vegetation type.
Responses to Abiotic Factors
- Regulate — maintain constant internal environment (mammals, birds — homeostasis).
- Conform — internal state changes with external (most animals & all plants).
- Migrate — move to a more favourable location (Siberian cranes to Bharatpur).
- Suspend — dormancy in unfavourable conditions:
- Hibernation — winter dormancy (bears).
- Aestivation — summer dormancy (snails, fish).
- Diapause — halted development in zooplankton.
Adaptations
- Kangaroo rat — no drinking water; concentrates urine.
- Desert plants — thick cuticle, sunken stomata, CAM photosynthesis, spiny leaves.
- Blubber in whales, seals — insulation.
- Allen's rule — mammals in cold have shorter limbs to reduce heat loss.
- Human high-altitude acclimatisation — more RBCs, heavier breathing.
Populations
Attributes
- Birth rate (natality) & death rate (mortality).
- Sex ratio.
- Age structure / pyramids:
- Expanding — pyramid-shaped (broad base).
- Stable — bell-shaped.
- Declining — urn-shaped (narrow base).
Population Density
Number per unit area/volume. Measured directly (count) or indirectly (pug marks, faecal pellets, nests).
Population Growth Models
Exponential (unlimited resources): dN/dt = rN → N_t = N_0 · e^(rt) — J-shaped curve.
Logistic (limited resources): dN/dt = rN · [(K − N)/K] → S-shaped (sigmoid) curve.
- K = carrying capacity — maximum sustainable population.
- r = intrinsic rate of natural increase.
Population Interactions
| Interaction | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | + | + | Lichen (algae + fungi); mycorrhiza |
| Competition | − | − | Barnacles on rocks |
| Predation | + | − | Tiger, lion |
| Parasitism | + | − | Tapeworm, lice, Cuscuta on plants |
| Commensalism | + | 0 | Orchid on mango tree; barnacles on whale |
| Amensalism | − | 0 | Penicillium releasing penicillin |
Predation Roles
- Predators keep prey populations in check.
- Help maintain species diversity (prevents monopoly by one prey).
- Plants have defenses: thorns (Acacia), poisons (Calotropis → cardiac glycosides).
Mutualism Examples
- Lichens: alga + fungus.
- Mycorrhiza: fungus + plant root.
- Fig & fig-wasp: mutually obligate reproduction.
- Sexual deceit: Ophrys orchid mimics female bee — pseudo-copulation transfers pollen.
Competition
- Same resource, two species → Gause's competitive exclusion principle — better competitor wins.
- Resource partitioning — species coexist by dividing niche (MacArthur's warblers).
- Interference competition — one species produces chemicals harmful to another (allelopathy).
Key Take-aways
- Organisms cope with abiotic stress via regulation, conformity, migration, or suspension.
- Populations grow exponentially without limits; realistically, they follow a logistic curve toward carrying capacity K.
- Species interactions (mutualism, competition, predation, parasitism, commensalism, amensalism) shape community structure.