Notes-Class-12-Biology-Chapter-13-Organisms and Population-Maharashtra Board

Organisms and Population

Maharashtra Board-Class-12th-Biology-Chapter-13


Topics to be Learn :

  • Introduction
  • Organisms and the environment around
  • Major Abiotic Factors
  • Adaptation
  • Population
  • Population Interactions


  • There are amazing diversity of forms and complexity of relations in natural world.
  • Levels of organization in the living world are, macromolecules, cells, tissues, organs, individual organism, population, communities, ecosystems and biomes.
  • Ecology : Ecology is a study of the interactions among organisms and between the organisms and their physical (abiotic) environment.
  • Term ecology was first used by Reiter but E. Haeckel introduced ecology.
  • 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.

 Organisms and the environment around

  • Ecology at the level of organism is the study of animal or plant physiology, ways of adaptation to the surrounding environment, their survival techniques, and propagation (multiplication).
  • 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.
  • Regional and local variations within each biome lead to the formation of a variety of habitats.
  • Habitat includes biotic components like plants, pathogens, parasites and predators.

  • Ethology – The term denotes the study of behaviour of animals in relation to their environment. The term was coined by Hilaire (1854) but was popularised by W. M. Wheeler (1902).
  • Bionomics : The study of relation between organisms to their environment is called bionomics. Lankester (1890) coined this term.
  • Environmental biology (Modem ecology): Study of functional or physiological interrelationships between the organism and their surroundings. G. L. Clarke (1964) and Odum (1969) introduced this term.
  • Biosphere :All the ecosystems on earth constitute biosphere.

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 surroundings 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.

Types of niches :

Types of niches :

  • Spatial or habitat niche : Spatial or habitat niche means the physical space occupied by the organisms.
  • Trophic niche : This kind of niche is based on the trophic level of an organism in a food chain.
  • Multidimensional or hypervolume niche : In multidimensional niche, number of abiotic and biotic environmental factors are considered. The resulting space by the niche is called hypervolume. Therefore it is also called hypervolume niche. It shows the position of an organism in the environmental gradient.
  • Fundamental niche : It is the niche in the absence of all competitors, this is highly improbable in nature.
  • Realized niche : Niche with the competitors. For the available resources in the habitat.


Characteristics of ecological niche :

Characteristics of ecological niche :

  • A niche describes how that organism is linked with its physical and biological environment.
  • Niche is described as a position of a species in the environment. It gives the idea about how the organisms are surviving and fulfilling their needs of shelter and food.
  • By studying niche one can get idea of the flow of energy from one organism to another. This helps to understand the feeding habits and interactions involving food chain and food web.
  • If any niche is left vacant, other organisms fill that position.
  • The niche is specific to each species. Two species can never share the same niche. By having specific niche, every organism tries to reduce competition for resources.
  • E.g. In birds, each one is specific in their eating habits, some are insectivorous, while some are frugivorous. Some are omnivorous, in this way birds living in the same habitat differ in their niches because of different eating habits.


The differences between Habitat and Niche :

The differences between Habitat and Niche :

Habitat Niche
A habitat is an area, where a species lives and interact with the other factors and prosper. A niche is a concept, of how an organism lives or survives in the environmental conditions.
Habitat consists of numerous niches. Niches do not contain such components.
Effect of temperature, rainfall and other abiotic factors. Flow of energy from one organism to other through ecosystem.
Habitat supports numerous species at a time. Niche supports a single species at a time.
Habitat is a physical place. Niche is an activity performed by organisms.
Habitat is not species specific. Niche is species specific.


Major Abiotic Factors :

Key abiotic factors : Key abiotic factors that influence any habitat are ambient temperature, availability of water, light and type of soil.

(1) Temperature :

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 0°C.
  • Tropical deserts : more than 50°C in summer.
  • In thermal springs : 80 to 100 °C.
  • In deep sea hydrothermal vents : about 400 °C.

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

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

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

  • Inland waters : Salinity is less than 5 ppt
  • Sea : 30 — 35 ppt
  • Hypersaline lagoons : 100 ppt
  • Euryhaline : Organisms that can tolerate wide range of salinities.
  • Stenohaline : Organisms that are 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.

(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.

Types of organisms according to abiotic factors :

The abiotic factors change due to diurnal and seasonal variations. For survival, organisms adapt to these variations on of the four possible ways viz. regulate, conform migrate and suspend. By these mechanisms they maintain homeostasis or steady internal state.

  • Regulate : In this method, organisms maintain homeostasis by physiological and behavioural changes. Due to homeostatic regulation, they can perform thermoregulation or osmoregulation. E.g. All birds and mammals show constant body temperature and osmotic concentration irrespective of external temperature.
  • Conform : Most of the animals and plants are unable to maintain a constant internal environment. Their body parameters change according to outside environment. E.g. Poikilothermic animals cannot maintain body temperature but they are simple conformers. In few aquatic animals, the osmotic concentration of the body fluids changes according to surrounding osmotic concentration. Few conformers can regulate the parameters in limited range.
  • Migrate : When organism is unable to cope up with surrounding temperatures, they migrate temporarily from such stressful habitat to a more favourable habitat. After the stressful period is over, they return back. Birds show long-distance migrations during severe winter.
  • Suspend : Suspending the life activities for particular period is one of the methods to cope up with stressful conditions. Seeds of plants remain dormant over unfavourable period and once favourable conditions are resumed they start growing. This state is called dormancy during which metabolic activities are suspended.
  • Hibernation and aestivation seen in someanimals is also for escaping severe winter or summer respectively. E.g. Polar bear shows hibernation while snails and fish show aestivation. These are also suspension measures.

 Adaptation :

Adaptation is an attribute of the organism (morphological, physiological, and behavioural) that enables the organism to survive and reproduce in its habitat.

Types of adaptations :

Adaptations are of following types :

(1) Physiological adaptations : Thermoregulation and Osmoregulation.

(2) Behavioural adaptations :

  • Behavioural responses to cope with variations in their environment are shown by few animals.
  • Migration : Migrations shown by the birds and mammals are also behavioural responses for adapting to severe winter temperatures.
  • Hibernation and aestivation
  • Desert lizards manage to keep their body temperature fairly constant by behavioural adaptations. They bask in the sun and absorb heat, when their body temperature drops below the comfort zone, but move into shade, when the ambient temperature starts increasing.
  • Some species burrow into the sand to hide and escape from the heat.

(3) Morphological adaptations :

Morphological adaptation is a structural change which gives an organism a greater chance of survival in its habitat.

  • Many desert plants have a thick cuticle on their leaf surfaces and have their stomata in deep pits to minimize loss of water through transpiration. They also have a special photosynthetic pathway (CAM – Crassulacean acid metabolism) that enables their stomata to remain closed during daytime.
  • Some desert plants like Opuntia, have their leaves reduced (modified) to spines and the photosynthetic function is taken over by the flattened stems.
  • Mammals from colder climates generally have shorter snout, ears, tail and limbs to minimize the loss of body heat (Allen’s Rule.)
  • In the polar seas, aquatic mammals like seals have a thick layer of fat (blubber) below their skin acting as an insulator to reduce loss of body heat.


Population :

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 naturalselection operates and desired traits are evolved.

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

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

  • Basic physical characteristics of population are - its size and density.
  • Besides size and density, the other characteristics include natality, mortality, immigration, emigration, age pyramids, expanding population, population growth forms and biotic potential.
  • An individual has birth and death, but a population has birth rate and death rate.

(1) Natality :

  • Natality is the birth rate of a population. Due to increased natality the population density rises.
  • Natality is a crude birth rate or specific birth rate.
  • Crude birth rate: Number of births per 1000 population/year gives crude birth rate. Crude birth rate is helpful in calculating population size.
  • Specific birth rate : Crude birth rate is relative to a specific criterion such as age. E.g. If in a pond, there were 200 carp fish and their population rises to 800. Then, taking the current population to 1000, the birth rate becomes 800/200 = 4 offspring per carp per year. This is specific birth rate.
  • Absolute Natality : The number of births under ideal conditions when there is no competition and the resources such as food and water are abundant, then it give absolute natality.
  • Realized Natality : The number of births under different environmental pressures give realized natality. Absolute natality will be always more than realized natality.

(2) Mortality :

  • Mortality is the death rate of a population.
  • It gives a measure of the number of deaths in a particular population, in proportion to the size of that population, per unit of time.
  • Mortality rate is typically expressed in deaths per 1,000 individuals per year.
  • A mortality rate of 9.5 (out of 1,000) in a population of 1,000 would mean 9.5 deaths per year in that entire population or 0.95% out of the total.
  • Absolute Mortality : The number of deaths under ideal conditions when there is no competition, and all the resources such as food and water are abundant, then it gives absolute mortality.
  • Realized Mortality : The number of deaths under environmental pressures come into play gives realized mortality.
  • It must be remembered that absolute mortality will always be less than realized mortality.

(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.

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).

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.

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

Growth Models :

(i) Exponential growth :

(i) Exponential growth :

When the resources are abundant, organisms show continuous growth of a population without any hindrance. With unlimited resources, each species has the ability to realize fully its innate potential to grow in number. Such growth of a population is called an exponential or geometric growth.

Any species growing exponentially under unlimited resource conditions can reach enormous population densities in a short time. E.g. Human population shows such exponential growth. Exponential growth shows J-shaped curve.


(ii) Logistic growth :

(ii) Logistic growth :

  • Naturally all populations of any species always have limited resources to permit exponential growth. Due to this there is always competition between individuals for limited resources. The most fit organisms succeed by survival and reproduction.
  • A given habitat has enough resources to support a maximum possible number, but beyond a particular limit the further growth is impossible.
  • This limit is called nature's carrying capacity (K) for that species in that habitat.

  • A population growing in a habitat with limited resources show following phases in a sequential manner. (a) A lag phase (b) Phase of acceleration (c) Phase of deceleration (d) An asymptote, when the population density reaches the carrying capacity.
  • 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.
  • Logistic growth thus always shows sigmoid curve.


 Population Interactions :

  • In nature, animals, plants and microbes do not and cannot live in isolation but interact in myriad ways to form a biological community.
  • In nature, every species requires interactions with at least one other species for its food.
  • Even autotrophic plant species needs soil microbes to break down the organic matter in soil and return the inorganic nutrients for absorption.
  • The plants need animal agents for pollination.

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.
  • The interspecific interactions occur between minimum two organisms- plants/ animals/ plant and animal.
  • Such interaction may be classified as four types viz, neutralism, negative (harmful), positive (benificial), and both positive and negative interactions.
  • Interspecific interactions arise from the interaction of populations of two different species. These interactions could be beneficial, detrimental or neutral (neither harm nor benefit) to one of the species or both.

The various types of interactions are classified as per the nature of these interactions to one or both the species.

Classification of population interaction :

Classification of population interaction :

Nature of Interaction Species




Mutualism + + Both the species benefit in their interactions
Competition Both the species lose in their interactions with each other.




Only one species benefits (predator and parasite, respectively) and the other species (prey and host, respectively) is harmed as the interactions are detrimental to them.
Commensalism + O One species is benefited and the other is neither benefited nor harmed.
Amensalism O One species is harmed whereas the other is unaffected

+ = benefited, − = inhibited, O = not affected



  • Mutualism is an obligatory and interdependent interaction. It is an association of two species in which both of them are benefited.
  • The classic example of mutualism is lichens.
  • Lichen is an intimate, mutualistic relationship between a fungus and photosynthetic algae or cyanobacteria.
  • Most of the plant and animal interactions are of mutualistic type.
  • For pollination and seed dispersal, plants depend on the animals.
  • Animals in turn feed on pollen and nectar during pollination. During seed dispersal juicy and nutritious fruits are used by the animals.
  • In animal-animal interactions also mutualism is seen in many instances.


  • Competition is the type of interaction where both the species are at a loss.
  • Totally unrelated species may compete for the same resource e.g. in shallow creeks on the westcoast of Mumbai, visiting flamingos and resident fish compete for their common food, the zooplankton.
  • In competition, the feeding efficiency of one species is reduced due to the interference or inhibitory presence of the other species, even if resources (food and space) are abundant, e.g. Leopards do not hunt in close proximity of lion pride.
  • Therefore, competition is best defined as a process in which the fitness of one species is significantly lower in the presence of another species.

Gauses Competitive Exclusion Principle :

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.



Parasitism has evolved in so many taxonomic groups from plants to higher vertebrates.

Effects of parasite on the host :

  • Most of the parasites cause harm to the host.
  • Host is affected by reducing its survival, growth and reproduction.
  • Some parasites can also be fatal to the host causing death of the host.
  • The population density of host species is reduced by parasites.
  • The host species become more vulnerable to predation by making it physically weak.

Special adaptations of endoparasites :

  • Endoparasites show loss of unnecessary sense organs as these are not needed for the parasite.
  • There are adhesive organs or suckers always present in the endoparasites which are needed to cling on to the host.
  • Endoparasites show loss of digestive system.
  • They have very high reproductive capacity.
  • The complex life cycles are seen in such parasites which involve intermediate hosts or vectors to facilitate transfer to the host.

Ectoparasites :

Ectoparasites. : Parasites that feed on the external surface of the host organism are called ectoparasites.

Examples :

  • The lice on humans and ticks on dogs.
  • Cuscuta, a parasitic plant that is commonly found growing on hedge plants, has lost its chlorophyll and leaves in the course of evolution. It derives its nutrition from the host plant which it parasitizes.


Brood parasitism :

Brood parasitism :

  • Brood parasitism is a type of parasitic behaviour shown by Asian Koel. Koel lays its eggs in the nest of crow.
  • Crow acts as a host bird and incubate the eggs of koel.
  • The eggs of koel show resemblance to the host’s egg in size and colour. This reduces the chances of the crow detecting koel’s eggs and ejecting them from the nest.
  • Eggs of koel hatch before the host’s eggs and hence parasitic bird is in advantage.



  • Predators play many important roles.
  • Predators keep prey population under control. If predators are lacking from the ecosystem, the prey population will rise without any control. Their high density may cause instability in ecosystem.
  • Predators also help in maintaining the species diversity in a community. This is done by reducing the intensity of competition among competing prey species.
  • Predators control the pest species and thus can be used for natural biological control measures in an ecosystem. E.g. frog controlling the locust population.
  • Predators also control the invading exotic species and stop their rapid spread of such species.

Defence mechanisms of prey species :

Defence mechanisms of prey species :

  • Prey species show following defence mechanisms :
  • Showing camouflage for concealment.
  • Moving at faster speed for escape.
  • Cryptic colouration to avoid the detection.
  • This is seen in some insects. Also predators display such cryptic colouration to avoid detection. E.g. Colouration in frog.
  • Bad taste due to accumulated chemicals.
  • E.g. The Monarch butterfly is highly distasteful to its predator bird as it stores a special chemical in the body during its caterpillar stage by feeding on a poisonous weed.


Defence mechanisms in plants against herbivores :

Defence mechanisms in plants against herbivores :

Herbivores feed on plants and thus they are predators for the vegetations.

Plants therefore have evolved variety of morphological and chemical defences against herbivores.

Examples :

  • Thorns seen in cactus or acacia makes the plant inedible and thus this is the most common morphological means of defence.
  • Many plants produce and store chemicals which are toxic and unpalatable. They can affect the feeding herbivore.
  • When these chemicals / produce are consumed, there is inhibition of feeding or digestion of predator. It also disrupts the reproduction or even kill it.
  • Plants like Calotropis produces highly poisonous cardiac glycosides which can act toxic to the grazers.
  • A Wide Variety of chemical substances such as nicotine, caffeine, quinine, strychnine, opium etc. are secondary metabolites produced by plants which act as defences against grazers and browsers.


Commensalism :

Commensalism is the interaction between two species in which one species derives benefit and the other one is neither harmed nor benefited.

Examples of commensalism :

  • Orchid grows as epiphyte on other big trees.The tree do not get any benefit but is neither harmed. But orchid gets support.
  • Cattle egret is the insectivorous bird which forage close to cattle. When cattle move, the hidden insects in the grass are flushed out. These insects are then captured by egrets. Cattle do not get benefit but birds do.
  • Sea anemone has stinging cells on the tentacles which offer protection to clown fish. Clown fish gets the protection from other predators, whereas, sea anemone does not derive any benefit from this association.
Know This :

  • The instrument used to measure the hight 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

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