Organisms and Populations
'Ecology is the scientific study of the INTERACTIONS that determine the DISTRIBUTION and ABUNDANCE of organisms.'
1. Chapter Overview
This chapter covers the ECOLOGY of organisms (individuals) and POPULATIONS (groups of the same species). Topics include: ECOLOGICAL HIERARCHY (organism → population → community → ecosystem → biome → biosphere), ABIOTIC FACTORS (temperature, water, light, soil — their effects on organisms), ADAPTATIONS (how organisms cope with environmental stress), POPULATION ATTRIBUTES (density, natality, mortality, age distribution, growth rate), POPULATION GROWTH MODELS (exponential and logistic), and POPULATION INTERACTIONS (predation, competition, parasitism, commensalism, mutualism, amensalism).
2. The Ecological Hierarchy
- Organism: Individual living entity.
- Population: Group of SAME SPECIES individuals in a GIVEN AREA.
- Community: ALL populations of DIFFERENT SPECIES in an area.
- Ecosystem: Community + ABIOTIC ENVIRONMENT.
- Biome: LARGE-SCALE ecosystem (tropical rainforest, tundra, desert).
- Biosphere: ALL ecosystems on Earth.
3. Abiotic Factors
| Factor | Effects | Examples of Adaptation |
|---|---|---|
| Temperature | Enzymes have OPTIMUM temperature ranges. Too high → DENATURATION. Too low → SLOW metabolism | Eurythermal (tolerate wide range). Stenothermal (narrow range). Polar bears: thick fur. Cacti: reduced leaves. |
| Water | All life requires water. Availability determines habitat type | Aquatic: gills. Desert: KANGAROO RAT — produces CONCENTRATED URINE, does NOT drink water (metabolic water). |
| Light | Essential for PHOTOSYNTHESIS. Also affects behaviour, migration, reproduction | Photoperiodism: plants flower based on DAY LENGTH. Animal migration: birds, butterflies follow light cues. |
| Soil | Provides nutrients, water, and ANCHORAGE for plants. Composition varies | Plants: deep roots in dry areas, shallow roots in wet. Calcicoles (lime-loving). Calcifuges (lime-hating). |
4. Adaptations
- 'ADAPTATION is a GENETICALLY CONTROLLED feature that increases an organism's CHANCE of SURVIVAL and REPRODUCTION in its environment.'
Physiological Adaptations
- ALTITUDE: Humans at high altitude produce MORE RBCs (increased oxygen-carrying capacity).
- TEMPERATURE: Some fish produce ANTIFREEZE PROTEINS in cold waters. HIBERNATION (winter sleep — bears). AESTIVATION (summer sleep — lungfish).
- OSMOREGULATION: Salmon live in BOTH fresh and salt water — they REGULATE ion balance.
Morphological Adaptations
| Environment | Adaptation | Example |
|---|---|---|
| Desert | Thick cuticle, reduced leaves, CAM photosynthesis | Cactus |
| Cold | Thick fur, fat blubber, compact body | Polar bear, Penguin |
| Aquatic | Streamlined body, gills, fins | Fish |
| Arboreal | Grasping limbs, long tail | Monkey, Chameleon |
5. Population Attributes
| Attribute | Definition | Formula |
|---|---|---|
| Population density (N) | Number of individuals per unit area | — |
| Natality (birth rate, B) | NUMBER of births per individual per time | B = ∆N_b/∆t |
| Mortality (death rate, D) | NUMBER of deaths per individual per time | D = ∆N_d/∆t |
| Immigration (I) | Individuals ENTERING the population | — |
| Emigration (E) | Individuals LEAVING the population | — |
| Age distribution | Proportion of individuals in pre-reproductive, reproductive, post-reproductive stages | — |
Population Growth
- N_t₊₁ = N_t + (B − D) + (I − E) — the change in population size over time.
Age Pyramids
- Expanding (triangular) : Many young → GROWING population (e.g., developing countries).
- Stable (bell-shaped) : Equal distribution → STABLE population.
- Declining (urn-shaped) : Few young → SHRINKING population (e.g., Japan).
6. Population Growth Models
6.1 Exponential (J-shaped) Growth
- dN/dt = rN — where r = intrinsic rate of natural increase.
- N_t = N₀ e^(rt) — 'Under ideal conditions, population grows EXPONENTIALLY.'
- 'Exponential growth is UNSUSTAINABLE — no population grows exponentially forever.'
6.2 Logistic (S-shaped) Growth
- dN/dt = rN [(K − N)/K] — where K = CARRYING CAPACITY (maximum population the environment can support).
- 'As N APPROACHES K, growth rate DECREASES. At N = K, growth = ZERO.'
- Phases: Lag phase → Log phase (exponential) → Deceleration phase → STATIONARY phase (at K).
| Feature | Exponential | Logistic |
|---|---|---|
| Shape | J-shaped | S-shaped |
| Carrying capacity | NO (ignored) | YES (built-in) |
| Realistic? | NO (temporary, ideal conditions) | YES (most populations) |
| Equation | dN/dt = rN | dN/dt = rN(K−N)/K |
7. Population Interactions
| Interaction | Species A | Species B | Description | Example |
|---|---|---|---|---|
| Predation (+/−) | BENEFITS | HARMED | One organism (predator) EATS another (prey) | Tiger eats deer. Lion eats zebra. |
| Competition (−/−) | HARMED | HARMED | BOTH species HARMED by the interaction. Compete for SAME resources | Lions and hyenas compete for prey. Plant species compete for light |
| Parasitism (+/−) | BENEFITS | HARMED | Parasite lives ON or IN host, OBTAINING nutrients at host's EXPENSE | Tapeworm in intestine. Plasmodium (malaria). |
| Commensalism (+/0) | BENEFITS | UNAFFECTED | One BENEFITS, the other is NOT affected | Barnacles on whale. Orchids on tree. |
| Mutualism (+/+) | BENEFITS | BENEFITS | BOTH BENEFIT | Lichens (fungus + algae). Mycorrhiza. Pollination by insects. |
| Amensalism (−/0) | HARMED | UNAFFECTED | One is INHIBITED, the other UNAFFECTED | Penicillium mould kills bacteria. Walnut trees release juglone. |
8. Common Mistakes
- Population growth is NOT always exponential: Real populations are LIMITED by resources. Logistic growth with carrying capacity is MORE realistic.
- Competition is NOT always obvious: Gause's COMPETITIVE EXCLUSION PRINCIPLE states that TWO SPECIES with IDENTICAL ecological niches CANNOT coexist. One will OUTCOMPETE the other.
- Predation is NOT just about killing: It is a KEY SELECTIVE FORCE — prey evolve DEFENCES, predators evolve better HUNTING strategies. 'Evolutionary arms race.'
- Parasitism is NOT mutualism: In parasitism, the host is HARMED. In mutualism, BOTH benefit. They are fundamentally DIFFERENT.
9. CBSE Exam Focus
- Abiotic factors — temperature, water, light, soil — their effects and adaptations
- Population attributes — density, natality, mortality, age distribution, age pyramids
- Population growth — exponential (J-shaped) and logistic (S-shaped) — formula and graphs
- Carrying capacity (K) — meaning, significance
- Population interactions — predation, competition, parasitism, commensalism, mutualism, amensalism — with examples
10. Self-Test
Q1: What is the difference between a HABITAT and a NICHE? A1: HABITAT: WHERE an organism lives (the address). NICHE: HOW an organism interacts with its environment — what it eats, what eats it, when it is active, its role in the ecosystem. 'Habitat is the address — niche is the profession.'
Q2: Write the logistic growth equation and explain each term. A2: dN/dt = rN(K−N)/K. r = intrinsic rate of natural increase. N = current population size. K = CARRYING CAPACITY. (K−N)/K = the fraction of carrying capacity remaining — slows growth as N approaches K.
Q3: Give TWO examples of mutualism. A3: (1) LICHENS — fungus (absorbs water/minerals) + algae (photosynthesises). (2) MYCORRHIZA — fungus (increases water/nutrient absorption) + plant roots (provides carbohydrates).
Q4: How does a kangaroo rat survive in the desert without drinking water? A4: The kangaroo rat produces METABOLIC WATER from the oxidation of food. It has HIGHLY CONCENTRATED URINE (minimal water loss) and is NOCTURNAL (avoids daytime heat). It does NOT need to drink water.
Q5: What is Gause's Competitive Exclusion Principle? A5: 'Two species CANNOT coexist if they occupy the SAME NICHE indefinitely — one will OUTCOMPETE and EXCLUDE the other.' This is why species evolve DIFFERENT niches to reduce competition (resource partitioning).
11. Conclusion
Ecology is the SCIENCE OF INTERACTIONS:
- INDIVIDUALS: 'Adaptations allow organisms to SURVIVE in diverse environments — from deserts to polar regions.'
- POPULATIONS: 'Populations GROW, SHIFT, and INTERACT — these dynamics shape the distribution of life on Earth.'
- 'Ecology teaches us that NO organism exists in ISOLATION — everything is CONNECTED.'
