Important Notes For NEET Biology - Organisms and Populations

Find below the important notes for the chapter, Organisms and Populations, as per NEET Biology syllabus. This is helpful for aspirants of NEET and other exams during last-minute revision. Important notes for NEET Biology- Organisms and Populations covers all the important topics and concepts useful for the exam. Check BYJU’S for the full set of important notes and study material for NEET Biology and solve the NEET Biology MCQs to check your understanding of the subject.

Name of the NEET sub-section Topic Notes helpful for
Biology Organisms and Populations NEET exams

Organisms and Populations – Important Points, Summary, Revision, Highlights

Organisms and Populations

Ecology is the study of interaction among biotic and abiotic components. Ernst Haeckel coined the term ‘Ecology’.

At the organismic level, ecology tells us how organisms are adapted for their survival and reproduction.

There are different kinds of habitat present on earth. It is due to variation in heat and temperature they are exposed to, as a result of rotation around its own axis and revolution around the sun. Life exists at favourable as well as extreme conditions.

Habitat: Habitat is the natural environment of an organism, where it thrives. Organisms develop adaptations, which is best suited for survival.

Niche: It includes all the specific requirements such as resources and conditions required by an organism. It tells about the functional role that each organism plays in an ecosystem.

Major Abiotic Factors

The major abiotic factors of an ecosystem are temperature, water, light and soil.

Eurythermal: Tolerance to a wide range of temperatures.

Stenothermal: Intolerance to a wide range of temperatures.

Euryhaline: More tolerance to different salinity.

Stenohaline: Less tolerance to changing salinity.

Photoperiodism: Flowering is dependent on the duration of light exposure. Some are long-day plants, some are short-day plants and some day-neutral (not impacted by duration of light).

Response to Abiotic Factors

Regulate: Homeotherms can regulate their internal temperature, i.e. maintain homeostasis.

Conform: Most plants and animals cannot maintain their internal temperature and they are known as poikilotherms.

Migrate: Some organisms temporarily move to avoid environmental stress. E.g. Siberian cranes migrate to Keoladeo National Park of Bharatpur to avoid extreme cold winters of Siberia.

Suspend: Thick-walled spores are formed in microorganisms, fungi, algae to avoid unfavourable conditions.

Seed dormancy protects the seed from desiccation.

Amphibians Hibernate to save themselves from winters. Some fish and snails go into aestivation in summers.

Diapause is a stage of suspended development of zooplankton under unfavourable conditions.

Adaptations: Over a period of time, all organisms develop certain adaptations to survive and reproduce in their habitats. They can be morphological, behavioural or physiological.

Allen’s rule: Mammals living in colder climates have smaller limbs and ears to minimize heat loss.

Examples of adaptations: Blubber found in seals reduces the loss of body heat so they are adapted to live in colder regions.

Adaptations like thick cuticle, sunken stomata and CAM cycle of photosynthesis, etc. help in reducing water loss in desert plants.

Himalayan tribes have a high R.B.C. content to compensate for low oxygen levels in the hills.

Basking in the sun by desert lizards to absorb heat and hiding in a burrow to escape the heat of the desert are some of the behavioural adaptations.


Population refers to the members of an inbreeding species living in a geographical area. They compete for similar resources.

Population Attributes: Birth rates, death rates, sex ratio, age pyramid, etc. are population attributes that help understand population make up of a geographical area.

Population density or population size refers to the biomass or percent cover of a population. Measuring it by the number of organisms will not be a good idea for all the species.

Most of the time we estimate population size indirectly rather than counting each and every organism. E.g. Population density of fish in a lake can be estimated by fish caught per trap. Tiger census is estimated by faecal pellets and pugmarks.

Population Growth

The main factors determining growth of a population is natality (birth rate), mortality (death rate), Immigration (no. of individuals moving in) and emigration (no. of individuals moving out)

Nt+1 = Nt + [{(B + I) – (D + E)}]

Where Nt = population density at time t

Nt+1 = density at t + 1

(B+I) = total addition, i.e. birth and immigrants

(D+E) = total death and emigrants

Growth Models

Growth of a population follows a specific pattern. There are two main types of models:

  1. Exponential Growth: When there is an unlimited supply of food and other resources, growth is exponential. In the case of exponential growth, a species will increase enormously, e.g. microorganisms growing in culture, spoilage of food, pandemics, etc.The change in population size in a time t (dN/dt) can be calculated by:

    dN/dt = (b-d) x N

    Where N = population size

    (b-d) = difference in birth and death rates (per capita)

    Let, (b-d) = r, then

    dN/dt = rN, r is known as ‘intrinsic rate of natural increase’

    We get J-shaped curve on plotting N Vs time, graph

    It can also be represented as Nt = N0 ert

  2. Logistic Growth: In nature, no species can grow exponentially without check. Exponential growth ultimately leads to competition between individuals for limited space and resources at its disposal. It leads to survival of the fittest.Logistic growth shows a sigmoid curve. There is an initial lag phase followed by acceleration and deceleration. When the density reaches its carrying capacity (K), the graph reaches its limiting value. It is a more realistic model of growth.

    dN/dt = rN(K – N/K)

    Logistic growth is also known as Verhulst-Pearl logistic growth.

Population Interactions

Name of Interaction Species A Species B Examples
Mutualism benefitted benefitted Flowers and butterflies, water fern Azolla with nitrogen-fixing cyanobacteria Anabaena, lichens (association between fungi and algae or cyanobacteria), mycorrhizae (fungi and higher plant roots)
Competition harmed harmed Any two species competing for the same resources
Parasitism benefitted harmed Disease pathogens inside animals and plants, Cuscuta is a parasite on hedge plant, parasitism for breeding in birds, e.g. cuckoo and crow
Predation benefitted harmed Prey and predator relationship
Commensalism benefitted not affected Epiphytes, e.g. orchids, barnacles on whales, egret and cattle, sea anemone and clownfish
Ammensalism not affected harmed Penicillium and a black walnut tree, fig and wasp, strangler fig is an epiphyte but it kills the host plant by crushing its secondary phloem

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