Organisms and Populations — Long Notes

Ecology studies the interactions among organisms and between organisms and their physical environment. It operates at four levels of biological organisation:

Organism → Population → Community → Biome

Two related terms:

• Habitat — the physical location where an organism lives (e.g. a pond, a forest floor).
• Niche — the ecological "role" of the organism in its habitat: what it eats, when it is active, whom it competes with, whom it is preyed upon by. Two species can share a habitat but rarely a niche.

1. Major Abiotic Factors

1.1 Temperature

• The most ecologically relevant abiotic factor.
• Affects enzyme kinetics, metabolism, distribution ranges.
• Eurythermal species tolerate wide temperature ranges; stenothermal ones can only survive in narrow bands.
• Sets latitudinal and altitudinal limits of species.

1.2 Water

• Critical for all life; drives distribution on land (deserts vs forests) and salt concentration in aquatic systems.
• Euryhaline / stenohaline describes tolerance to salinity.

1.3 Light

• Powers photosynthesis in producers.
• Photoperiod cues flowering, migration, breeding, moulting.
• Understory plants adapt to low light (shade tolerance).

1.4 Soil

• Substrate for vegetation; nature depends on climate, rocks, and biotic components.
• Texture (sand/silt/clay), pH, water-holding capacity, mineral content — all determine which plants grow, hence which animals live there.

2. Responses to Abiotic Factors

Different organisms cope in different ways:

2.1 Regulate

• Homeostasis — maintain constant internal environment despite external changes.
• All mammals and birds are regulators — they thermoregulate (via metabolism, sweating, shivering) and osmoregulate.
• Very few lower vertebrates and plants regulate.

2.2 Conform

• Conformers — internal condition changes with the environment.
• ~99% of animals and nearly all plants are conformers.
• Cost of regulation is very high in energy — conforming is easier when environment is not too harsh.

2.3 Migrate

• Move temporarily to a more favourable habitat.
• Siberian cranes migrate to Keoladeo National Park, Bharatpur in India during winter.
• Fish migrate for spawning; whales for feeding/breeding.

2.4 Suspend

• Enter dormant states when conditions are bad:
• Hibernation — winter dormancy (bears in cold regions).
• Aestivation — summer dormancy (snails, some fish).
• Diapause — halted development stage in zooplankton (under stress).
• Seeds and spores in plants remain dormant for years.

3. Adaptations — Special Cases

• Kangaroo rat — never drinks water; gets it from metabolic water, concentrates urine, remains in cool burrows during the day.
• Desert plants — thick cuticle, sunken stomata, CAM photosynthesis (stomata open at night), some like Opuntia have modified stems that photosynthesise while leaves are reduced to spines.
• Aquatic mammals (whales, seals) — insulating blubber to conserve heat.
• Allen's rule — mammals in cold climates tend to have shorter ears/limbs to reduce heat loss.
• High-altitude acclimatisation in humans — more RBCs, higher haemoglobin, faster breathing to overcome low pO₂.

4. Populations

A population is a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources, and potentially interbreeding.

4.1 Population Attributes

• Birth rate (natality) — new individuals per unit time.
• Death rate (mortality) — individuals lost per unit time.
• Sex ratio — proportion of males to females.
• Age structure / age pyramid — proportion in each age class.

Age pyramid shapes:

• Expanding — pyramid with a wide base (many young).
• Stable — bell shape.
• Declining — narrow base, indicating fewer young.

4.2 Population Density

• Number of individuals per unit area (or volume).
• Directly measured for large, easy-to-see species (deer, trees).
• Indirect measures for cryptic species: pug marks, fecal pellets, calls, nests.

4.3 Population Growth Models

Exponential Growth (unlimited resources)

$$ \frac{dN}{dt} = rN $$ $$ N_t = N_0 e^{rt} $$ Shape: J-curve — accelerates without limit. Unrealistic long-term.

r = intrinsic rate of natural increase (birth − death per capita).

Logistic Growth (with resource limits)

$$ \frac{dN}{dt} = rN \cdot \frac{K - N}{K} $$ Shape: S-curve (sigmoid). Starts slowly, accelerates, then slows and plateaus at K.

K = carrying capacity — maximum population sustainable in that environment.

Real populations follow logistic-like curves, with fluctuations around K driven by biotic and abiotic factors.

5. Population Interactions

Two species can interact in six main ways. The sign for each species indicates effect:

Interaction	A	B	Example
Mutualism	+	+	Lichen (alga + fungus); mycorrhiza; fig-wasp
Competition	−	−	Barnacles on rocky shores; MacArthur's warblers
Predation	+	−	Lion & deer; sea star & mussel
Parasitism	+	−	Cuscuta on plants; ticks; brood parasitism
Commensalism	+	0	Orchid on mango tree; barnacles on whale; cattle egret & grazing cattle
Amensalism	−	0	Penicillium releasing penicillin

5.1 Predation

Predators keep prey populations in check.

• Prevents any one prey species from monopolising resources — maintains diversity.
• Keystone predators (e.g. the sea star Pisaster) — if removed, the community shifts drastically.

Prey defenses:

• Camouflage, mimicry, spines (Acacia).
• Chemical defenses — Calotropis has cardiac glycosides; monarch butterfly stores them from milkweed.
• Behavioural — escape, warning colouration.

5.2 Competition

• Interspecific competition — occurs when two species share a limiting resource.
• Gause's Competitive Exclusion Principle: two species with identical resource requirements cannot coexist indefinitely — the better competitor eliminates the other.
• Resource partitioning — in nature, coexisting species often divide the resource. MacArthur showed five warbler species on the same tree feed at different heights and behaved differently — reducing competition.

5.3 Parasitism

• Parasite benefits, host suffers.
• Various strategies: ectoparasites (lice, ticks) live outside; endoparasites (tapeworms, roundworms) live inside.
• Brood parasitism — cuckoo lays eggs in the crow's nest; the crow raises the chick.

5.4 Commensalism

• One benefits, other neither helped nor harmed.
• Orchid on mango tree, barnacles on whale, cattle egret + grazing cattle (eats insects stirred up).

5.5 Mutualism

• Both benefit; often obligate.
• Lichens — algae (photosynthesis) + fungi (structure, water absorption).
• Mycorrhiza — fungi in roots help absorb P; plant supplies sugars.
• Fig & fig-wasp — wasp pollinates fig; fig provides food & nursery for wasp larvae.
• Sexual deceit: Ophrys orchid mimics the female of a bee species; male bees "pseudo-copulate" and transfer pollen.

5.6 Amensalism

• One is inhibited; the other is unaffected.
• Allelopathy: Penicillium releases penicillin, harming nearby bacteria but Penicillium itself is unaffected.

Key take-aways

1. Ecology examines the fit between organism and environment — driven by abiotic factors (temperature, water, light, soil) and biotic interactions.
2. Organisms respond to abiotic stress by regulating, conforming, migrating, or suspending.
3. Population dynamics: exponential (unlimited) → logistic (with carrying capacity K) growth models.
4. Species interactions can be positive (+), negative (−), or neutral (0); real communities are woven from all six interaction types.
5. Predation and competition shape community structure; mutualism, commensalism, and parasitism illustrate the range of intimacy of interactions.