Organisms and Populations Class 12 Notes Biology Chapter 13 - CBSE


What are Organisms and Populations?


The branch of biology that deals with the relations of organisms with each other and with their physical surroundings are known as ecology.

Different factors that affect physical and chemical conditions of habitats

Abiotic Factors

Temperature Water Light Soil
  • Temperature decreases from equator towards pole and from plane to mountain tops.
  • Distribution of plants depends on availability of water.
Sun light is the major source of energy for photosynthesis. pH, mineral composition, topography, water holding capacity affects the type of vegetation that grows in that area.
  • Organisms are classified on the basis of tolerance of temperature:
  • Organisms are classified on the basis of tolerance to salinity:
➥ Eurythermal: Tolerate wide range of temperature ➥ Euryhaline:Tolerate wide range of salinity.
➥ Stenothermal: Tolerate only narrow range of temperature. ➥ Stenohaline: Can only tolerate narrow range of salinity.

Responses To Abiotic Factors

Homeostasis: The ability of an organism to maintain its internal environment constant even when external environmental conditions vary.

Regulators Conformers Migratory Suspend
Maintain constant body temperature (thermoregulation), and constant osmotic concentration (osmoregulation). Examples – mammals Small animals have large surface area compared to volume and this helps in loosing less amount of heat which turns out to be less expensive process. Hence small size animals are found more in polar regions. Animals migrate temporarily to avoid stressful conditions. Mechanisms adopted by animals to deal with stressful situations.

Biotic Factors

Animals Plants Bacteria Moss
It plays an essential roles in the formation and maintenance of the ecosystems they live in. For example, one ecological role of animals in their environment is to act as consumers, which is a vital part of the community dynamics and energy flows of the ecosystem. These are also biotic factors in an environment. Their dependency, however, on other biotic factors, in terms of food is not as extensive as the animals that hunt and prey. They produce their own food by photosynthesis. Bacteria are the most abundant and genetically diverse organisms on the planet, and their activity is vital for many aspects of biogeochemical cycling and ecosystem function. It also play a vital role in the development of new ecosystems. They are among the first plant colonisers of disturbed sites, such as when an area is deforested or affected by forest fires. They stabilise the soil surface and retain water, helping new plants to grow.

Responses To Biotic Factors

Biotic factors are living things within an ecosystem; such as plants, animals, and bacteria, while abiotic are non-living components; such as water, soil and atmosphere. The way these components interact is critical in an ecosystem. Biotic factors such as the presence of autotrophs or self-nourishing organisms such as plants, and the diversity of consumers also affect an entire ecosystem. Abiotic factors affect the ability of organisms to survive and reproduce. Abiotic limiting factors restrict the growth of populations.


Pollen grains/male gametophyte - spherical in shape and made up of 2 layers+ 1 Germ Pore+ 2 Cells

  • Desert plants: Thick cuticle, sunken stomata to minimize transpiration.

  • Cold climate mammals: short ears and short limbs to minimize heat loss. This is Allen’s Rule.

  • Polar region aquatic mammals: Thick layer of fat below the skin to provide insulation. e.g. seals.
    Physiological activity that helps in better adaptation

  • People living at high altitude: More RBC production and high breathing rate.
Behavioural Response

Desert lizards: Bask in sun to avoid coldness and move to shade when hot.
Some burrow and hide into the soil to escape from the ground heat.


Group of individuals which share or compete for similar resources and interbreed among themselves.

Population Attributes

  • Sex ratio: Male is to Female ratio in a population.
  • Birth rate: Average no. of young ones born in a period of time.
  • Death rate: Average no. of deaths in a period of time.
  • Population density: Population density is the number of individuals per unit geographical area, for example, number per square meter, per hectare, or per square kilometer.
  • Age pyramid: Plot of age distribution (%) of individuals of a given age or age group. It reflects whether growth is:

Density Changes By Change In Four Basic Processes

  • Natality (N): Increase population (due to birth)
  • Immigration (I): Increase population (individuals of same species which have come from some other place)
  • Mortality (M): Decrease population (due to death)
  • Emigration (E): Decrease population (individuals of a population who have gone elsewhere)

Population density will increase if (B+I) > (D+E)

Change in population size = (Births + Immigration) – (Deaths + Emigration)

Growth Models

Pattern of a population growth can be shown with the help of growth model.

Types of Growth Model

  • ➥ Exponential
  • ➥ Logistic
Exponential Growth Logistic Growth
When resources are unlimited, population grows exponentially. dN/dt – increase/decrease in N during time t Then, dN/dt = (b – d)*N Let (b – d) = r, then dN/dt = r*N When a population has limited resources it showslag phase, phase of acceleration, asympotecarrying capacity = K
Exponential Growth Logistic Growth

Population Interactions

There are more than one kind of species which are available in a habitat and these species interact mutually.

S.No. Types of Interaction Species 1 Species 2 General Nature
of Interaction
1. Amensalism 0 Biological interaction where one species causes harm
to another organism without any cost or benefits to itself.
Penicillum killing bacteria
2. Mutualism + + Interaction favourable to both species and obligatory. Between fungus & algae (Lichen)
3. Commensalism + 0 HRelationship between two living organisms in which
one organism benefits from the other without harming it.
Sucker Fish on shark
4. Competition Direct inhibition of each species by the other. Birds compete with squirrels for units and seeds
5. Parasitism + Symbiotic relationship where the parasite (smaller in size),
lives on or inside the host (larger in size), causing it some harm.
Ascaris and tapeworm in human digestive tract
6. Predation + Biological interation where predator (larger in size), kills and
eats another organism, its prey (smaller in size).
Lion Predatory on deer

Types of Growth Model

Ecto-parasitism Endo-parasitism Brood Parasites
Ectoparasitism is a form of parasitism where the parasite lives outside the body of the host. E.g. Lice and ticks Endoparasitism are parasites that live within the body of their host. E.g. hook worms The brood parasite manipulates a host, either of the same or another species, to raise its young as if it were its own, using brood mimicry. E.g. Cuckoo, Mochokidae cat fish