Organisms and Populations: Class 12 Biology NCERT Chapter 13

Key Features of NCERT Material for Class 12 Biology Chapter 13 – Organisms and Populations

In the previous chapter: Biotechnology and its applications, you studied what is biotechnology and how it’s used for human welfare. In chapter 13 of NCERT class 12 biology: Organisms and Populations, you will study more about population and it’s growth models. As understudies of science, when you start to find out about nature, it gives a comprehensive point of view of the relationship every living organism with their biotic and abiotic parts. Nature digs into the more profound interdisciplinary parts of humanism, human sciences just as the ecological issues. 

At the point when organisms cooperate with the different ecological elements, be it the abiotic parts of the biotic segments, there comes when transformations start to shape, for the better endurance of the race.

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Any physiological, social or morphological property of an organism that empowers it to endure and duplicate in its individual living space is called as a transformation. Variations are significant for every single organism. Something else, the pace of endurance or proceeding with its descendants gets decreased.

Numerous creatures and plants have these transformations set up. This shows their prosperity rate in adjusting to the changing elements of the ecological conditions. Over the span of the transformative timetable of organisms, some significant variations have gotten hereditarily fixed. 

In nature, all organisms and their populations connect with each other somewhat or the other. That is the means by which an environment works impeccably! This population association is for the most part between two distinct species populations. These connections can be valuable or impartial or impeding. In like manner, there are six sorts of population collaboration.

The various ways populations of two unique species cooperate with one another can be summed up under the accompanying headings.

Quick revision notes

Investigation of biology is critical to find some kind of harmony among advancement and upkeep of characteristic natural and biotic networks, use and protection of assets, unravel neighbourhood, territorial and worldwide ecological issues. 

biotechnology and its applications

  • It is essentially worried about four degrees of natural association – organisms, populations, networks and biomes. 
  • Environmental chain of importance or biological degree of association associated with the natural gathering of organisms. 

Organism and its Environment

  • The aggregate of all biotic and abiotic elements, substances and conditions that encompass and possibly impact organisms without turning into their constituent part is called environment. At the organism level, physiological nature attempts to see how various organisms are adjusted to their condition as far as endurance and multiplication. The variety in the power and length of temperature alongside yearly varieties in precipitation brings about the arrangement of significant biomes like a desert, downpour backwoods and tundra. 

temperature and precipitation

  • Local and nearby varieties inside every biome lead to the arrangement of various types of living spaces like tropical downpour woodland, deciduous backwoods, desert, ocean coast and so forth.
  • The territory incorporates biotic segments like microorganisms, parasites, predators and contenders of the organism with which they communicate constantly.

Major Abiotic Factors

  •  Temperature: It is the most significant natural factor to decide the bio-mass of a spot. Normal temperature ashore changes occasionally and diminishes continuously from the equator towards the shafts and from fields to peaks. Temperature influences the energy of catalysts and basal digestion alongside physiological elements of the organisms.
  • The organisms that can endure a wide scope of temperature are called eurythermal, for instance, feline, canines, tigers and so forth.
  • The organism which can endure just a restricted scope of temperature is called stenothermal, for instance, Penguin, fishes, crocodile.etc.b) Water-life on earth is unreasonable without water. Profitability and conveyance of plants are vigorously reliant on water. For sea-going organisms the quality (synthetic structure, pH) of water gets significant. The salt focus (estimated as saltiness in parts per thousand), is under 5 in inland waters, 30-35 in the ocean and > 100 in some hypersaline tidal ponds
  • The organisms that can endure a wide scope of salinities are called euryhaline, for instance, salmon, herring and so forth.
  • The organism which can endure just a tight scope of salinities is called stenohaline, for instance, goldfish ( freshwater ) and haddock ( marine water )etc.c) Light-plants produce food through photosynthesis in the nearness of daylight. A few plants are adjusted to low light conditions since they are eclipsed by tall canopied trees. Blossoming in certain plants happens just in the nearness of basic sunshine called Photoperiodism. The accessibility of light and land is firmly connected to that of temperature as the sun is the wellspring of both. UV part of daylight is unsafe to plants and animals.d) Soil-Types of soil relies on atmosphere, enduring procedure, regardless of whether the soil is moved or sedimentary and how soil advancement happened. Soil creation, grain size and accumulation decide the permeation and water holding limit of the dirt alongside pH, mineral, piece and geography decide the vegetation in any area.

Responses to Abiotic Factor :

throughout advancement, numerous species have developed steady inner condition to grants every biochemical response and physiological capacities to work with most extreme proficiency to have overall wellness of species. Organisms attempt to keep up the consistency of their interior condition (homeostasis)in spite of changing outside condition. There are different approaches to build up hemostasis-

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  • Direct all flying creatures and creatures are fit for keeping up homeostasis by physiological methods which guarantee consistent body temperature, steady osmotic focus and so on. Thermoregulation and osmoregulation is the wellspring of achievement of warm-blooded animals in all the natural conditions. In summer, when the outside temperature is more than our body temperature, we sweat often, coming about evaporative cooling, which cuts down the body temperature. In winter we begin to shudder, a sort of activity which produces warmth and raises the body temperature.

biotechnology and its applications

  • Adjust the majority of creatures and plants, their body temperature change with surrounding temperature. In sea-going creatures osmotic convergence of the body liquid change with that of the encompassing water osmotic focus. These creatures are called conformer. Conformer can’t shoulder the vigorous costs to keep up the steady body temperature. Heat misfortune or warmth gain is an element of the surface zone. Since little creatures have a bigger surface territory comparative with their volume, they will, in general, lose body heat extremely quick when it is cold outside; at that point, they need to exhaust a lot of vitality to produce body heat through digestion. This is the principle motivation behind why little creatures are once in a while found in polar locales.
  • Move the organism move away for time being from the unpleasant horrible environment to more reasonable living space and return back when the upsetting period is finished. Numerous feathered creatures attempt significant distance to move to more accommodating territories. Siberia fowls move to Keoladeo National park, Bharatpur, India.
  • Suspend-in microorganisms like microscopic organisms, parasites and lower plants a thick-walled spore is framed which help them to endure negative conditions. These spores grow on the return of reasonable conditions. In higher plants, seeds and some other vegetative conceptive structures serves the way to hold over times of pressure and help them in dispersal moreover. The metabolic exercises are decreased to least during this torpid period. a)Hibernation – the condition or time of a creature or plant spending the winter in a lethargic state e.g bearb)Aestivation – the condition or time of a creature or plant spending the mid-year to dodge warmth and desiccation in a lethargic state e.g snails .c)Diapause – a phase of suspended improvement in zooplankton species in lakes and lakes.

Transformation is the trait of organism morphological, physiological and conduct changes that empower the organism to endure and repeat in its natural surroundings. 

  • Kangaroo rodent in North American deserts satisfies the water prerequisite by inner oxidation of fat without water. It likewise can think its pee so the negligible volume of water is utilized to expel excretory items.
  • Thick fingernail skin in numerous plants likewise forestalls loss of water. CAM plants open their stomata during the night to lessen the loss of water during photosynthesis.
  • Well evolved creatures from colder atmospheres have shorter ears and appendages to limit heat misfortune. This is called Allen’s Rule.
  • In polar oceans sea-going, well-evolved creatures like seals have a thick layer of fat called lard, beneath their skin that goes about as an encasing and diminishes loss of body heat.
  • Elevation ailment is seen at a higher height that incorporates manifestations like queasiness, exhaustion, heart palpitations because of less oxygen and environmental weight. The individual step by step get accustomed and quit encountering height disorder. This is sort of physiological variation.
  • Various marine invertebrate and fish live in temperature in every case under zero and a few lives in great depth in the sea where weight is extremely high by a cluster of biochemical variations.
  • A few organisms like desert reptile come up short on the physiological capacity that warm-blooded animals have however managed the high temperature of their living space by social methods. They lounge in the sun and assimilate heat and when their body temperature dips under the safe place, yet moves in conceal when the encompassing temperature begins expanding. 


Individuals of any species live in bunches in very much characterized geological territory, share or vie for comparable assets, conceivably interbreed and comprise a population.

Population Attributes:

A population has certain properties that an individual organism doesn’t, for example, an individual may have births and passings, yet a population has birth rates and demise rates. 

  • The birth and passing rates are alluded according to capita births or passings individually, which is increment and lessening as for individuals from the population.
  • Sex proportion is another characteristic of the population. An individual might be male or female however population engages in sexual relations proportion.
  • A population at a given time made out of various individual of various ages. In the event that the age dispersion is plotted for the population, the subsequent structure is called age pyramids. The state of pyramids mirrors the state of growth status of the population. Which might be

(I) Expending

(ii) Stable 

(iii) Declining 

population growth pyramid

  • Population size or population thickness (N) is estimated as far as number yet it might at some point not fitting.

Population Growth :

The size of the population isn’t static. It continues changing with time, contingent on food accessibility, predation pressure and lessens climate. The primary factors that decide the population growth are-

Natality (number of birth during a given period in the population)

 Mortality ( number of death during a given period in the population)

Immigration (individual of same species that have come into the environment)

Emigration ( individual of the population that have left the environment ) 

population density

In the event that ‘N’ is the population thickness at a time’t’, at that point its thickness at time t+1 is

Nt + 1 Nt + [(B + I) – (D + E)]

Population thickness will increment if the quantity of births in addition to the quantity of foreigners (B + I) is more than the quantity of passings in addition to the quantity of wanderers (D + E), else it will diminish.

Growth model

Growth of population happens as indicated by the accessibility of food, propensity condition and nearness of other biotic and abiotic factors. There are two principle kinds of models- 

  1. Exponential Growth-in these sorts of growth happen when food and space are accessible inadequate sum. At the point when assets in the natural surroundings are boundless, every species can understand completely its inborn potential to develop in number. The population develops in an exponential or mathematical manner. On the off chance that in a population of size N, the birth rates as spoke to as ‘b’ and demise rate as ‘d’. At that point increment and reduction in N during unit period time ‘t’ will be

dN/dt = (b – d) × N

Let (b – d) = r, at that point

dN/dt = rN

population growth curve

At that point, the r in this condition is called ‘characteristic pace of common increment’.

  1. Strategic Growth-there is an opposition between the people of a population for food and space. The fittest organism endures and repeats. In these sorts of growth at first, shows a leg stage followed by periods of increasing speed and de-quickening.

population growth formula

Where N = Population thickness at time t

R = Intrinsic pace of normal increment

K = Carrying limit

Since assets for growth for populations are limited and become constraining, the calculated growth model is viewed as a more reasonable one.

Population communication

All creatures, plants and organisms in a natural network communicate with one another. These connections might be advantageous, adverse or impartial to one of the species or both. Following sorts of communication is seen-

  1. Predation
  2. Rivalry
  3. Parasitism
  4. Commensalism
  5. Mutualism


It is an Interspecific Interaction where one creature executes and devours the other more vulnerable creature.

Jobs of Predators

  • Move vitality from plants to higher trophic levels (position of an organism in the evolved way of life) 
  • Control Prey population – Prickly pear desert flora moth
  • Organic control of Agricultural nuisance
  • Keep up species assorted variety by decreasing force of rivalry among contending prey species
  • Over misuse of prey by the predators brings about eradication of prey and predator.
  • Resistance to reduce the effect of predation

a)Insects and frog – disguise

b)Monarch butterfly – harmful 


  • Thistles desert flora and Acacia 
  • Produce and store synthetic – Calotropis
  • Nicotine, Caffeine, Quinin, Strychnine, opium – against slow eaters and programs


Connection either among people of same species or between people of various species.

Happens among firmly related species yet not in every case valid

  1. Irrelevant species additionally contend flamingo and fish seek zooplankton
  2. Taking care of the effectiveness of a species decrease because of different species regardless of whether assets are bounty – Abingdon turtle.

Proof for rivalry

Serious delivery – species dispersion confined to little regions because of seriously unrivalled species.


“Two firmly related species seeking same assets can’t exist together as the seriously sub-par one will be dispensed with.”

Asset segment Two contending species stay away from rivalry by diff. taking care of and scrounging designs Mc Arthur (larks rummaging exercises)


It is where one species (parasite) relies upon different species (have) for food and safe house, have is hurt.

Parasites and host self-develop.

Transformations of parasites

– Loss of pointless sense organs

– Hooks and sucker

– Loss of stomach related framework

– High Reproductive limit


(I) Reduce the endurance of host

(ii) Growth and conceptive rate are diminished

(iii) Render the host defenceless against its predators by making them powerless

Kinds of parasite

ECTOPARASITES–rely upon the outer surface of the host

Model – head lice on people, ticks on canines

ENDOPARASITES–take cover inside the body of the host organism

Model – Liverfluke, Plasmodium


It is cooperation in which both the connecting species are profited


  1. Lichen – parasites and green growth
  2. Mycorrhizae – parasites and underlying foundations of higher plants
  3. Fertilization of plants by creepy crawlies
  4. Mediterranean orchid-sexual trickery for fertilization shows up as a female honey bee


Interaction between two unique species, in which one species is hurt and different species is neither hurt nor profited. Model. Bacterial culture, following barely any days parasite growth, will be there on it like Penicillium, and its emissions of substance will eliminate microscopic organisms, yet no advantages to organisms.


Q: Give a few instances of the various types of population associations 

Ans: Examples of commensalism

Barnacles and Whales

Head Shrimp and Sea Cucumbers

Instances of predation

African Wild canine and Zebra

Cheetah and Gazelle

Instances of Parasitism

Aphids(insects) and Plants

Cuscuta and blueberry plant

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