By going through these Maharashtra State Board 12th Science Biology Notes Chapter 13 Organisms and Populations students can recall all the concepts quickly.

Maharashtra State Board 12th Biology Notes Chapter 13 Organisms and Populations


  • Ecology : Ecology is a study of the interactions : among organisms and between the organisms and their physical (abiotic) environment.
  • E. Haeckel introduced this term. Reiter used the term ecology for the first time.
  • Four sequential levels with increasing complexity of ecological (biological) organizations are organism, Populations, Communities and Biomes.
  • Organism : Individual which is the basic unit of ecological hierarchy is called organism.
  • Population : Organisms of same kind inhabiting a geographical area is called population.
  • Community : Several populations of different species in a particular area makes a community.
  • Land biome : A large regional terrestrial unit : delimited by a specific climatic zone with typical major vegetation and associated fauna.

Maharashtra Board Class 12 Biology Notes Chapter 13 Organisms and Populations

Organisms and the environment-

1. In ecology organismic level consists of its physiology, ways of adaptation to the surrounding environment, survival techniques and propagation, etc.

The rotation of earth along with its tilted axis, cause seasons. Due to seasons there are rain and snow which demarcate the major biomes of the earth. E.g. desert, tropical rain forest, temperate forest, coniferous forest, grassland, tundra, etc. are six major terrestrial biomes.

Biomes → Habitats → Biotic components. E.g. plants, pathogens, parasites and predators.

Maharashtra Board Class 12 Biology Notes Chapter 13 Organisms and Populations 1

3. Ethology : Study of behaviour of animals in relation to their environment is ethology. The term was coined by Hilaire. Term popularised by W. M. Wheeler.

4. Bionomics : The study of relation between. organisms to their environment is called bionomics. Lankester (1890) coined this term.

5. Environmental biology (Modern ecology) : Study of functional or physiological interrelationships between the organism and their surroundings. G. L. Clarke (1964) and Odum (1969) introduced this term.

6. Biosphere : All the ecosystems on earth constitute biosphere.

7. Habitat and Niche :

  • Habitat : Place or area where a particular  species lives is called habitat.
  • Factors deciding presence of organisms in a particular habitat : Sunlight, average : rainfall, annual temperatures, type of soil, topographic factors, etc.
  • Types of habitats : Arboreal, terrestrial, aerial, aquatic, etc.
  • Microhabitat : Small part of the habitat which forms immediate surrounding of an organism.
  • Niche : The functional role played by an organism in its environment is called niche. Term given by J. Grinnell. Niche includes various aspects of the life of an organism like diet, shelter, and its link with physical and biological environment.
  • Habitat is a postal address while niche is the profession of organism.

(7) Types of niche :
Maharashtra Board Class 12 Biology Notes Chapter 13 Organisms and Populations 2

Maharashtra Board Class 12 Biology Notes Chapter 13 Organisms and Populations

Major Abiotic Factors –

1. Key abiotic factors : Ambient temperature, availability of water, light and type of soil.

(1) Temperature :

(i) Ecologically relevant environmental factors showing seasonal variations.

  • Progressive decrease of the temperature from the equator towards the poles and from plains to the mountain tops.
  • In polar areas and at high altitude : below zero °C.
  • Tropical deserts : more than 50 °C in summer.
  • In thermal springs : 80 to 100 °C.
  • In deep sea hydrothermal vents : about 400 °C.

(ii) Distribution of animals and plant species is mainly dependent on the ambient temperature. Animals are geographically distributed according to their levels of thermal tolerance.

(iii) Temperature can affect the kinetics of enzymes in the body and thus alter the basal metabolism, organism’s activity and other physiological functions.

(iv) Eurythermal : Organisms that can tolerate and thrive in a wide range of temperature fluctuations are called eurythermal.

(v) Stenothermal : Organisms restricted to a narrow range of temperatures are called stenothermal.

(2) Water :

  • Water is the second most important factor influencing the life of organisms.
  • Life on earth originated in water and can sustain only due to water.
  • Availability of water changes according to geographical regions. It also decides the productivity and distribution of plants.
  • Even for aquatic organisms the chemical composition and pH of water are important qualities.

Salinity or the Salt concentration : Unit of salinity is parts per thousand. (%o)

  • Inland waters : Salinity is less than 5%o.
  • Sea : 30 – 35 %o
  • Hypersaline lagoons : 100%o
  • Euryhaline : Organisms that can tolerate wide range of salinities.
  • Stenohaline : Organisms that Eire restricted to a narrow range of salinity.
  • Many freshwater animals cannot live for long in sea water and vice versa because of the osmotic problems they would face.

(3) Light :

  • Sunlight is the ultimate source of energy. Photosynthesis depends upon the availability of sunlight. Hence for autotrophs it is a very essential abiotic factor.
  • Species of herbs and shrubs growing in forests are adapted to photosynthesis even under very low light conditions because they are constantly under a canopy of tall trees.
  • Flowering of plants is dependent on sunlight to meet their photoperiodic requirement of the plants.
  • In animals the diurnal and seasonal rhythms are dependent on the sunlight. Foraging, reproductive and migratory activities of animals depend upon photoperiod.
  • The availability of light on land is closely linked with that of temperature since the sun is the source for both.
  • In oceanic depths (> 500m) the environment is perpetually dark and its inhabitants are well adapted to this dark life. They are carnivorous.

Maharashtra Board Class 12 Biology Notes Chapter 13 Organisms and Populations

(4) Soil :

  • Climate of a place determines the nature and properties of the soil.
  • The weathering process, type of soil (sedimentary or transported), pattern of soil development, soil composition, grain size differs from place to place. Therefore, the soil characteristics are also varied.
  • The percolation and water holding capacity of the soils depend upon the soil composition, aggregation of particles and grain size.
  • The vegetation in many areas is dependent upon soil parameters such as pH, mineral composition and topography. Based on these characteristics the vegetation and the faunal pattern is seen.
  • The sediment-characteristics in the aquatic environment, determine the type of resident benthic animals.

2. Types of organisms according to abiotic factors : The abiotic factors change due to diurnal and seasonal variations. For survival, organisms adapt to these variations in the following four ways. By these mechanisms they maintain homeostasis or steady internal state.
Maharashtra Board Class 12 Biology Notes Chapter 13 Organisms and Populations 3


1. Adaptations enable the organism to survive and reproduce in its habitat.
2. Adaptations are of following types :

(1) Physiological adaptations : Thermoregulation and Osmoregulation.
(2) Behavioural adaptations :

  • Migration
  • Hibernation and aestivation
  • Behavioural responses to cope with variations in their environment.
  • Desert lizards → bask in the sun and absorb heat when temperature is cold. → move into shade → when the ambient temperature is more.
  • Burrowing into the sand for escaping heat.

(3) Morphological adaptations :

  • Desert plants : Thick cuticle on leaf surfaces, sunken stomata → minimizing loss of water through transpiration. CAM → Crassulacean acid metabolism pathway of photosynthesis. Modified leaves to spines, flattened green stems performing photosynthesis.
  • Mammals from cold climate region : Shorter snout, ears, tail and limbs to minimize the loss of body heat (Allen’s Rule).
  • Aquatic polar mammals → thick layer of blubber below their skin → insulator → minimize loss of body heat.


1. Population : Group of organisms in a well- defined geographical area which shares or competes for similar resources and which potentially interbreed with each other is called population. At the population level natural selection operates and desired traits are evolved.

2. Population ecology : An important area of ecology that links ecology to population dynamics, genetics and evolution.

3. Population attributes : Basic physical characteristics of population are called population attributes.

(1) Natality : Birth rate of a population.

  • Crude birth rate : Used for calculating population size (number of births per 1000 population/year).
  • Specific birth rate : Used when a specific criterion such as age has to be considered.
  • Absolute Natality : The number of births under ideal conditions when there is no competition and resources such as food and water, etc. are abundant.
  • Realized Natality: The number of births when environmental pressures are operating on the population.

(2) Mortality : Death rate of a population.

  • Absolute Mortality : Number of deaths under ideal conditions when there is no competition and resources, like food and water are abundant.
  • Realized Mortality : Number of deaths when environmental pressures are operating on the population.
    Absolute mortality will always be less than realized mortality.

Maharashtra Board Class 12 Biology Notes Chapter 13 Organisms and Populations

(3) Sex ratio : The ratio of the number of individuals of one sex to that of the other sex is sex ratio. Birth, death, immigration and emigration, etc. affect sex ratio.

Evolutionary stable strategy (ESS) : The males and females of a population should be in a ratio of 1 : 1.

(4) Age distribution and Age pyramid : Age pyramid is the figure plotted for a population to show age distribution. Age distribution is done in following way : Pre-reproductive (0-14 years), Reproductive (15-44 years) and Post-reproductive (45-85+ years).

(5) Population size or population density (N) : Population density is the number of individuals present per unit space in given time. Population’s status in the habitat indicate population size. The biomass is also more meaningful measure of the population size.

(6) Population Growth : The size of a population keeps changing with time, depending on various factors including food, predation pressure and adverse weather.

Density of population in a habitat during a given period, fluctuates due to changes in four basic processes : (i) New births (ii) Immigration (iii) Deaths (iv) Emigration.
Of these new births and immigration increase the population growth while deaths and emigration decrease population growth.

(7) Immigration (I) : Number of individuals of the same species that enter the habitat from elsewhere during specific time period under consideration.

(8) Emigration (E) : It is the number of individuals of the population who leave the habitat during specific time period.

So, if N is the population density at time ‘t’, then its density at time ‘t + 1′ can be calculated
as, Nt + 1 = Nt + [(B + I) – (D + E)]

4. Growth Models :

  • Exponential growth.
  • Logistic growth
  • Verhulst-Pearl Logistic Growth : A plot of N in relation to time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth.
  • Since resources for growth of most animal populations are finite and become limiting sooner or later, the logistic growth model is considered as a more realistic one.

Population Interactions-

1. Interactions are of two types in the living species :

  • Intraspecific : Interaction existing between organisms of same species’ population.
  • Interspecific : Interaction between members of different species.

2. Classification of population interactions :
Maharashtra Board Class 12 Biology Notes Chapter 13 Organisms and Populations 4

(3) Gause’s ‘Competitive Exclusion Principle’ :

  • This principle states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually.
  • The Gause’s principle may be true if resources are limiting, but not otherwise. More recent studies do not support such gross generalisations about competition.

Maharashtra Board Class 12 Biology Notes Chapter 13 Organisms and Populations

Important information :

  • Instrument used to measure the height of forest trees is called hypsometer.
  • World Environment day : 5th June
  • World Population day : 11th July
  • World Earth day : 22nd April
  • World Ozone day : 16th September