Class 12 Biology Notes
Complete exam notes for NCERT Class 12 Biology Chapter 11 — how organisms respond to their abiotic environment, key adaptations such as Allen's and Bergmann's rules, population attributes and growth curves, and the full set of species interactions from mutualism to amensalism.
Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali
It studies how individual organisms cope with abiotic factors and how populations change — through attributes, growth models (exponential and logistic) and interactions such as predation, competition, mutualism and parasitism.
An organism is the basic living unit; a population is a group of individuals of the same species living in a defined area that can potentially interbreed. Ecology studies how organisms and populations relate to their environment — the sum of all living (biotic) and non-living (abiotic) factors around them.
The most ecologically relevant environmental factor. Thermal tolerance varies widely: from microbes in hot springs (~100°C) and deep-sea hydrothermal vents, to plants and animals in polar ice. Most organisms function best in a narrow range, and temperature affects enzyme activity and metabolism.
An organism that maintains a constant internal body temperature and osmotic concentration despite external changes — e.g. mammals and birds, which use physiological, behavioural or morphological means to regulate their internal environment (homeothermy).
An organism that cannot regulate and whose internal conditions change with the environment — e.g. most fish and many invertebrates. Conformers cannot survive wide external fluctuations because their enzymes and metabolism are disrupted.
Partial regulators maintain homeostasis within a limited external range (e.g. many fishes and amphibians). Migrators avoid the unfavourable period entirely — e.g. birds and some animals move to a more favourable habitat during extreme seasons.
Why are there no true conformers at extremes?
There are few purely conforming species and almost no purely regulating ones — most combine strategies, and the ratio of regulators to conformers also varies with altitude and latitude.
Any physiological, morphological or behavioural attribute that helps an organism survive and reproduce in its environment. Adaptations are genetic, evolved over generations, and enable survival in particular habitats.
Common exam trap
Allen's rule is about shorter appendages (ears, limbs) in cold regions; Bergmann's rule is about large body size in cold regions. Many questions test which one applies to which body part.
Features that only exist at the population level, not for individuals. These include population size or density, birth rate (natality), death rate (mortality), immigration, emigration, age structure and sex ratio.
Age pyramids plot the proportion of pre-reproductive, reproductive and post-reproductive individuals. A broad base indicates an expanding population; a narrow base indicates a declining one; a balanced profile indicates a stable population.
When resources are unlimited, population size grows exponentially with time, giving a J-shaped curve. N is population size, t is time, r is the intrinsic rate of natural increase. The exponential rate varies between species — for example, an Escherichia coli division time is about 20 minutes.
In nature, resources are limited and growth slows as population approaches the carrying capacity K, giving an S-shaped logistic curve. The term (K−N)/K makes growth environmental-resource-dependent, and the population settles around the carrying capacity K of the habitat.
Carrying capacity K
K is the maximum population size a habitat can sustain indefinitely. Above K, resources run out and population declines; the logistic model captures this density-dependent braking that the exponential model ignores.
Life history traits — such as the age of first reproduction, number of offspring and parental care — vary by species and habitat. Species either invest in many small offspring with little care (like most fishes) or few large offspring with heavy care (like mammals and birds).
Sign conventions
Memorise the symbols: ++ mutualism, +− predation and parasitism, +0 commensalism, −0 amensalism, −− competition. Gause's principle is the classic competition takeaway for exams.
Example: A population growing exponentially has an intrinsic rate of increase r = 0.2 per year and a current size N = 1000. What does the exponential model dN/dt = rN predict for the instantaneous rate of change?
Solution: dN/dt = rN = 0.2 × 1000 = 200 individuals per year. This is the instantaneous rate of increase; because growth is exponential, this rate itself rises as N grows, producing the characteristic J-shaped curve.
Example: Distinguish between commensalism and amensalism with one example each.
Solution: Commensalism is a +0 interaction where one species benefits and the other is unaffected — e.g. barnacles attached to a whale (they get a moving base; the whale is unaffected). Amensalism is a −0 interaction where one species is harmed and the other unaffected — e.g. Penicillium secreting penicillin that kills neighbouring bacteria. The key difference is whether one partner benefits (+) or is only harmed (−).
Revision
Memorise these before attempting numericals — most exam questions hinge on one of them.
Exponential growth
Logistic growth
Carrying capacity
Natality
Mortality
Gause's principle
Interaction symbols
Exam tips
Where this topic appears in CBSE, JEE Main and NEET papers.
FAQ
Regulators maintain constant internal conditions regardless of outside changes (mammals and birds). Conformers allow internal conditions to change with the environment (most fish). Partial regulators maintain homeostasis within a limited range, and migrators avoid unfavourable periods by moving to a better habitat.
Allen's rule states mammals in colder climates have shorter ears and limbs to reduce heat loss. Bergmann's rule states animals in colder regions are generally larger-bodied, which lowers surface area relative to volume and conserves heat.
Exponential growth (dN/dt = rN) assumes unlimited resources and gives a J-shaped curve. Logistic growth (dN/dt = rN(K−N)/K) accounts for a finite carrying capacity K, slows as N nears K, and produces an S-shaped curve that levels off.
It states that two species competing for the same resources cannot coexist indefinitely in the same niche; one will competitively exclude the other. This is why closely related, co-occurring species typically occupy different niches.
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