Organisms and Populations covers the levels of ecological organisation and major abiotic factors, the ways organisms respond to and cope with their environment, population attributes such as density and age pyramids, the exponential and logistic models of population growth, life-history traits, and the different kinds of interactions between species. It is a high-yield NEET chapter where questions test the growth equations, carrying capacity, the categories of population interactions, and adaptations like regulators and conformers.
Class 12 Biology · Chapter 13 – swipe through all 10 cards to understand the whole chapter.
Ecology & Its Levels
Ecology studies how organisms interact with one another and with their physical environment, at four nested levels.
Direction is smallest → largest; a scrambled order is a common trap.
- Order goes organism → population → community → biome
- Studied from a single individual up to whole climatic regions
- Interactions are both with living and non-living surroundings
Abiotic Factors
Four non-living factors shape where life can survive, with temperature being the most ecologically relevant.
Eurythermal/stenothermal (temperature); euryhaline/stenohaline (salinity).
- Major factors: temperature, water, light, soil
- Temperature affects enzyme kinetics → controls metabolism & distribution
- For aquatic life, salinity is the key water variable
Responses to Stress
To keep a constant internal milieu (homeostasis), organisms use one of four strategies.
~99% of animals and nearly all plants are conformers (cheapest strategy).
- Regulate = hold internal state constant (all birds & mammals); costly
- Suspend = dormancy: spores, hibernation (winter), aestivation (summer), diapause
- Migrate = leave temporarily (Siberian cranes → Keoladeo)
Adaptations
Any heritable trait that improves survival and reproduction in a given habitat.
Kangaroo rat makes water by fat oxidation + concentrated urine, no drinking.
- Desert plants: thick cuticle, sunken stomata, CAM photosynthesis
- Opuntia has spines (not leaves) + flattened photosynthetic stem
- High-altitude humans make more RBCs to offset low oxygen
Population Attributes
A population is one species in an area; it has group-level properties no individual has.
Density may be % cover or biomass (e.g. Lantana), not always a head count.
- Natality = birth rate added; mortality = death rate removed
- Tigers counted indirectly via pug marks & faecal pellets
- A single birth/death is an event, not a population attribute
Age Pyramids
Stacking a population by age groups gives a shape that predicts its future trend.
Read the BASE first: young at the base mean future growth.
- Triangular/expanding = large pre-reproductive cohort
- Bell-shaped/stable = births balance older groups
- Urn-shaped/declining = few young replacing the old
Exponential Growth
With unlimited resources, population grows in proportion to its current size, giving a J-shaped curve.
r = b − d (intrinsic rate); e^0.693 ≈ 2, so 0.693/r = doubling time.
- J-shaped curve, shoots up without limit (unrealistic long-term)
- Nₜ = N0 eʳᵗ gives size after time t
- If births = deaths then r = 0 and size stays steady
Logistic Growth
Real habitats have finite resources, so growth slows toward a carrying capacity K, giving an S-shaped curve.
Verhulst–Pearl model; sigmoid: lag → acceleration → deceleration → plateau at K.
- When N = K → growth = 0 (population is largest)
- Growth RATE is maximum at N = K/2
- More realistic than exponential; carrying capacity exists only here
Life History & r
Natural selection tunes how, when and how often to reproduce to maximise reproductive fitness (high r).
Bamboo & Pacific salmon = semelparous; most birds/mammals = iteroparous.
- Unpredictable habitats favour many small offspring
- Stable habitats favour fewer, larger offspring
- r values: Norway rat ≈ 0.015, flour beetle ≈ 0.12, human(US 1981) ≈ 0.0205
Population Interactions
Name any two-species interaction by its effect (+/−/0) on each partner.
Predation usually kills prey; parasitism keeps the host alive & is host-specific.
- Mutualism: lichen, mycorrhiza, fig–wasp, Ophrys pseudocopulation
- Gause’s exclusion: same limited resource → cannot coexist; resource partitioning dodges it
- Cuscuta = parasite; barnacle-on-whale & orchid-on-tree = commensalism
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Frequently Asked Questions
A population is a group of individuals of the same species living in a defined geographical area, sharing or competing for resources and capable of interbreeding. Populations show attributes that individuals do not, such as density, natality, mortality, sex ratio and age distribution.
Exponential growth occurs when resources are unlimited and follows dN/dt = rN, giving a J-shaped curve. Logistic growth occurs when resources are finite, so the habitat has a carrying capacity K, and follows dN/dt = rN((K minus N)/K), giving an S-shaped or sigmoid curve that levels off at K.
Carrying capacity, denoted K, is the maximum population size of a species that a particular habitat can support with its available resources. In logistic growth the population grows quickly at first and then slows as it nears K, finally stabilising around this value.
A regulator keeps its internal conditions, such as body temperature and osmotic concentration, constant regardless of the external environment, which is energetically costly. A conformer allows its internal conditions to change with the environment. About 99 percent of animals and plants are conformers, while birds and mammals are regulators.
Gause’s competitive exclusion principle states that two species competing for the same limited resource cannot coexist indefinitely, and the competitively inferior species is eventually eliminated. Species may avoid this through resource partitioning, as shown by MacArthur’s warblers feeding in different parts of the same tree.