Board exam focus — Organisms and Populations (CBSE & HBSE)

CBSE focuses on organism-environment interactions (abiotic factors, thermoregulation), population attributes, growth models (exponential vs logistic), interspecific interactions (mutualism, predation, parasitism, competition, commensalism, amensalism). HBSE emphasises definitions, population growth equations, carrying capacity, and examples of biotic interactions.

Organism and its Environment

Ecology: Study of Organism-Environment Interactions

Ecology (from Greek: oikos = house; logos = study) is the scientific study of the interactions between organisms and their environment. Hierarchy of study: organism → population → community → ecosystem → biome → biosphere.

Environment: everything that surrounds and affects an organism — biotic (living) and abiotic (non-living) components.

Major Abiotic Factors:

1. Temperature:

• Most ecologically relevant physical factor
• Affects rates of enzymatic reactions (Q10 effect: reaction rate approximately doubles for every 10°C rise)
• Most organisms function in a temperature range of 0–40°C; exceptions: hot springs bacteria (up to 90°C), Antarctic fish (below 0°C)
• Eurythermal: organisms tolerant of a wide temperature range (most temperate and polar organisms)
• Stenothermal: organisms tolerant of only a narrow temperature range (most tropical organisms — adapted to stable temperatures)

2. Water:

• Essential for all life processes
• Productivity of terrestrial habitats largely determined by water availability
• Euryhaline: tolerant of a wide range of salinity (salmon, bull shark — freshwater to seawater)
• Stenohaline: intolerant of significant changes in salinity (most freshwater fish, most marine fish)
• Salinity effects: osmotic stress; organisms in saline environments have adaptations to maintain osmotic balance (osmoregulation)

3. Light:

• Solar radiation drives photosynthesis — ultimate energy source for almost all ecosystems
• Photoperiod: day length controls flowering (photoperiodism), reproductive cycles, migration, and dormancy in many organisms
• UV radiation: harmful (damages DNA, causes mutations); protective ozone layer essential
• Deep-sea organisms: no light below 200m; chemosynthetic producers; bioluminescence

4. Soil:

• Composition, pH, texture, mineral content determine what plants grow → determines what animals live there
• Soil erosion, compaction, salinity → major environmental problems

Responses of Organisms to Abiotic Factors:

Regulation (Homeostasis): Organisms maintain internal conditions constant despite external changes.

• Thermoregulation (temperature regulation):
• Endotherms (homeotherms): birds and mammals; maintain constant body temperature (37°C in humans); generate heat internally; can be active in cold environments; expensive (high food requirement)
• Ectotherms (poikilotherms): fish, amphibians, reptiles; body temperature fluctuates with environment; behavioural thermoregulation (basking, burrowing); less costly
• Osmoregulation: maintaining water/ion balance; kidneys (vertebrates), contractile vacuoles (protozoa), salt glands (marine birds, reptiles)

Conformers: Allow internal environment to change with external environment; no homeostasis; less costly but limits activity range. Most invertebrates are conformers for temperature and osmolarity.

Migration: Seasonal movement to more favourable habitats. Arctic tern: 71,000 km annual migration between Arctic and Antarctic. Humpback whale, wildebeest (Serengeti), monarch butterfly, bar-headed goose (crosses Himalayas at 8,000+ metres).

Suspension: When environmental conditions are intolerable, organisms enter a dormant state.

• Hibernation: winter dormancy in mammals (bears, groundhogs) — reduced metabolism, temperature, heart rate
• Aestivation: summer dormancy in response to heat and drought (lungfish, snails, crocodiles)
• Diapause: suspended development in insects (delay of embryonic development until conditions improve; e.g., copepods, silkworm eggs)

Adaptations: Long-term evolutionary changes in response to environments.

• Desert plants (xerophytes): sunken stomata, waxy cuticle, CAM photosynthesis, succulent water storage, deep roots
• Desert animals: nocturnal activity, concentrated urine (kangaroo rat), metabolic water production
• Aquatic adaptations: streamlined body, gills, lateral line system, swim bladder
• Polar adaptations: thick fur, blubber insulation, countercurrent heat exchange (flipper arteries), antifreeze proteins (Antarctic fish — AFGP in blood plasma)
• Allen's rule: warm-blooded animals in colder climates have shorter extremities (ears, limbs, tails) to reduce heat loss
• Bergmann's rule: warm-blooded animals in colder climates tend to be larger (smaller surface area:volume ratio → less heat loss)

Allen Bergmann Rule Examples: Arctic fox has small ears and rounded body (Bergmann + Allen); African elephant has large ears (for heat dissipation).

Population Growth and Attributes

Population Ecology

A population is a group of individuals of the same species occupying a defined area at a given time and capable of interbreeding.

Population Attributes:

1. Population Size (N) and Density:

• Total number of individuals in a given area at a given time
• Measurement methods: direct count (birds, large mammals), quadrat sampling (plants, small animals), mark-recapture (Lincoln-Petersen method): N = (M x n) / m (M = marked individuals, n = recapture sample size, m = marked recaptures)

2. Birth Rate (Natality, b):

• Number of births per individual per unit time
• Increases population size

3. Death Rate (Mortality, d):

• Number of deaths per individual per unit time
• Decreases population size

4. Immigration (I):

• Number of individuals entering the population from outside
• Increases population size

5. Emigration (E):

• Number of individuals leaving the population
• Decreases population size

Change in population size: dN/dt = (b - d) + (I - E) Where N = population size, b = birth rate, d = death rate, I = immigration, E = emigration. For closed populations (no migration): dN/dt = (b - d) x N = r x N

6. Age Structure (Age Pyramid):

• Ratio of different age groups (pre-reproductive: young; reproductive: adults; post-reproductive: old)
• Growing population: broad base, triangular (India, Africa — many young individuals)
• Stable population: roughly equal in all age groups (bell-shaped)
• Declining population: narrow base (Japan, many European countries — few young)
• Indicates future population trend

7. Sex Ratio:

• Ratio of males to females in a population
• Affects reproductive rate; skewed sex ratios (due to selective harvesting, poaching, or sex-selective practices) can threaten population viability

8. Population Density:

• Number of individuals per unit area (or volume)
• Can use biomass instead of number (useful for algae, bacteria, parasites)

Population Growth Models

Exponential Growth (J-shaped / Malthusian growth):

When resources are unlimited (food, space), populations grow at their maximum intrinsic rate — exponential growth.

Equation:

• Differential form: dN/dt = rN
• N = population size
• r = intrinsic rate of natural increase (r = b - d for closed population)
• t = time

• Integral form: Nt = N0 e^(rt)
• N0 = initial population size
• e = base of natural logarithm (2.718)
• Nt = population size at time t

• When plotted against time → J-shaped curve (exponential, geometric growth)
• Characteristic of organisms with unlimited resources: bacteria doubling in culture, human population (historically), invasive species newly introduced
• Doubling time = 0.693/r = ln(2)/r

Logistic Growth (S-shaped / Sigmoidal / Verhulst-Pearl logistic growth):

Resources are ALWAYS limited in nature; as population size increases, competition for limited resources intensifies → growth rate slows down.

Equation: dN/dt = rN x (K-N)/K

• K = Carrying Capacity — the maximum population size that the environment can sustainably support (equilibrium population size when birth rate = death rate for that environment)
• When N << K: (K-N)/K ≈ 1 → growth is approximately exponential
• When N = K/2: growth rate is maximum (inflection point of S-curve)
• When N approaches K: (K-N)/K → 0 → growth rate → 0 → population stabilises at K
• When N > K: (K-N)/K is negative → population declines back toward K

When plotted: S-shaped (sigmoid) curve (logistic growth curve / Verhulst-Pearl equation)

Phases of logistic growth:

1. Lag phase: slow initial growth (small population establishing)
2. Exponential (log) phase: rapid growth when N << K
3. Deceleration phase: growth rate decreasing as K approached
4. Stationary (plateau) phase: N ≈ K; population stable

r vs K selection:

Survivorship Curves: Three types describe how a cohort of individuals survive over time:

• Type I (Convex/Late loss): low mortality throughout life until old age; most individuals survive to old age; examples: humans, elephants, many large K-strategists
• Type II (Linear/Constant loss): constant mortality rate throughout life; examples: birds, some lizards, hydra
• Type III (Concave/Early loss): very high early mortality; few survive to adulthood; but those that do, live to old age; examples: fish, oysters, insects, most plants — produce many offspring (r-selected)

Interspecific Interactions

Interspecific Interactions (Biotic Interactions)

Interspecific interactions occur between individuals of different species. They can be categorised based on whether each species benefits (+), is harmed (-), or is unaffected (0).

1. Mutualism (+/+): Both species benefit from the association.

Examples:

• Lichens: symbiosis between fungi and algae (or cyanobacteria); fungi provide shelter and moisture; algae/cyanobacteria provide photosynthate (carbohydrates). Classic obligate mutualism.
• Mycorrhiza: fungi and plant roots; fungi get carbon from plant; plant gets phosphorus and water from fungal hyphae
• Rhizobium and legumes: nitrogen-fixing bacteria in root nodules; bacteria get sugars; plant gets fixed nitrogen
• Fig and fig wasp: obligate mutualism; female wasp enters fig to lay eggs; pollinates fig flowers in the process; larvae develop inside; essential relationship — neither can reproduce without the other (R.I. Ricklefs calls it "most intricate mutualism")
• Ophrys orchid and bee: orchid flower mimics female bee (colour, shape, scent — pheromone mimicry); male bee attempts to mate with flower → pollinates orchid; bee gets nothing (pseudocopulation — NOT mutualism actually but fascinating coevolution)
• Pollination by insects, birds, bats: animals get nectar/pollen; plants get pollination services — diffuse mutualism

2. Commensalism (+/0): One species benefits; the other is neither helped nor harmed.

Examples:

• Barnacles on whale: barnacles get transport and access to food-rich waters; whale unaffected
• Orchids on trees (epiphytes): orchids (and bromeliads, mosses) growing on trees for support and access to light; trees unaffected
• Remora fish and shark: remora attaches to shark; feeds on scraps from shark's meals; shark unaffected
• Cattle egret and livestock: cattle disturb grassland insects as they walk; cattle egrets follow and catch flushed insects; cattle unaffected

3. Predation (+/-): One organism (predator) kills and eats another (prey).

Role of predation in ecosystem:

• Maintains prey population size and prevents overgrazing (e.g., wolves in Yellowstone)
• Maintains prey genetic health (removes sick and slow individuals)
• Predator: important in food chain energy transfer

Prey defences:

• Cryptic colouration (camouflage): stick insects, walking leaf insects, Arctic hare in winter
• Warning colouration (Aposematism): monarch butterfly (orange/black); poison dart frogs (vivid colours) → signal toxicity to predators
• Mimicry: Batesian mimicry — palatable species (mimic) resembles an unpalatable/toxic species (model); viceroy butterfly mimics monarch; milk snake mimics coral snake. Mullerian mimicry — multiple unpalatable species share similar warning patterns; any predator learning to avoid one avoids all (e.g., bumblebees and yellow jackets share yellow-black warning pattern)
• Mechanical defences: spines (hedgehog, porcupine), shells (turtle, molluscs), chemical defences (skunk spray, bombardier beetle, nettles)
• Alarm calls and group vigilance (meerkats)

Coevolution of predator-prey: arms race → prey evolves better defences; predator evolves better predation abilities

4. Parasitism (+/-): Parasite lives in/on host, derives nutrition, and harms host (but usually does not kill immediately — would eliminate own habitat).

• Ectoparasites: on surface of host; lice, ticks, mites, leeches, mistletoe (Viscum album — partial parasite on tree branches)
• Endoparasites: inside host's body; tapeworm, Plasmodium, Ascaris, liver fluke, Fasciola hepatica
• Brood parasitism: Cuckoo (Cuculus canorus) lays eggs in nests of other bird species; host incubates and raises the cuckoo chick (which pushes out host's eggs/chicks); cuckoo chicks mimic host chick calls; host species evolves egg recognition — cuckoo evolves egg mimicry (coevolution)

5. Competition (-/-): Both species are harmed; occurs when they compete for the same limited resources (food, space, mates, territory).

• Intraspecific competition: between individuals of the SAME species; most intense (most similar resource needs)
• Interspecific competition: between individuals of DIFFERENT species
• Gause's Competitive Exclusion Principle: two species with identical ecological requirements (same ecological niche) cannot coexist indefinitely in the same habitat — one will eventually outcompete the other. The superior competitor excludes the inferior one.
• Resource partitioning: coexisting competing species avoid competitive exclusion by dividing resources (different microhabitats, feeding times, food sizes) → allows coexistence
• Warblers in spruce trees (Robert MacArthur): 5 warbler species coexist in same spruce trees by feeding in different zones (different height, inner vs outer branches)

6. Amensalism (0/-): One species is harmed, the other is unaffected.

• Penicillium producing penicillin: bacteria in environment killed by penicillin; Penicillium unaffected
• Allelopathy in plants: some plants release chemicals (allelochemicals) from roots or leaves that inhibit germination and growth of nearby plants; Black walnut (Juglans nigra) produces juglone → inhibits growth of many neighbouring plants; Eucalyptus releases terpenes; sunflower seeds contain allelopathic compounds

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