NCERT Solution organisms and population class 12 Chapter 13

Organisms and population class 12 Biology Chapter 13 by NCERT Book

Organisms and Population class 12 taken from NCERT Book Biology in this chapter we covered all Important points related to Organisms and Population class 12 read and clear your all doubts

Organisms and Population Class 12 Introduction

Our living world is fascinatingly diverse and amazingly complex. We can try to understand its complexity by investigating processes at various levels of biological organization– macromolecules, cells, tissues, organs, individual organisms, populations, communities ecosystems, and biomes. At any level of biological organization, we can ask two types of

questions – for example, when we hear the bulbul singing early morning in the garden, we may ask – ‘How does the bird sing?’ Or, ‘Why does the bird sing ?’ The ‘how-type’ questions seek the mechanism behind the process while the ‘why-type’ questions seek the significance of the process. For the first question in our example, the answer might be in terms of the

operation of the voice box and the vibrating bone in the bird, whereas for the second question, the answer may lie in the bird’s need to communicate with its mate during the breeding season. When you observe nature around you with a scientific frame of mind you will

certainly come up with many interesting questions of both types – Why are night-blooming flowers generally white? How does the bee know which flower has nectar? Why does cactus have so many thorns? How does the chick recognize her own mother?, and so on

Organisms and Population Class 12 Author

Organisms and population class 12

Ramdeo Misra is revered as the Father of Ecology in India. Born on 26 August 1908, Ramdeo Misra obtained Ph.D. in Ecology (1937) under Prof. W. H. Pearsall, FRS, from Leeds University in the UK. He established teaching and research in ecology at the Department of Botany of the Banaras Hindu University, Varanasi.

His research laid the foundations for understanding tropical communities and their succession, environmental responses of plant populations and productivity, and nutrient cycling in tropical forest and grassland ecosystems.

Misra formulated the first postgraduate course in ecology in India. Over 50 scholars obtained a Ph.D. degree under his supervision and moved on to other universities and research institutes to initiate ecology teaching and research across the country.

He was honoured with the Fellowships of the Indian National Science Academy and World Academy of Arts and Science, and the prestigious Sanjay Gandhi Award in Environment and Ecology. Due to his efforts, the Government of India established the National Committee for Environmental Planning and Coordination (1972) which, in later years, paved the way for the establishment of the Ministry of Environment and Forests (1984)

Organisms and Population Class 12 Important Points

Population dynamics: Understanding how populations of organisms change over time. Factors such as birth rate, death rate, immigration and emigration affect the size and structure of the population.

Population growth models: Different models like exponential growth and logistic growth help in studying how the population changes under different conditions and limits.

Population Interactions: Organisms in a population interact in various ways with each other and with the environment. Predation, competition symbiosis and cooperation are examples of interactions that affect population dynamics.

Life history patterns: Organisms exhibit different life histories based on factors such as reproductive strategies, age at reproductive maturity, and lifespan. These patterns affect population growth and survival.

Population Regulation: Population is regulated by factors independent of density and density. Density-independent factors include competition for resources and disease, and density-independent factors include natural disasters and climate change.

Population Dynamics: Human development and its impact on the environment is an important aspect of population studies. Understanding human dynamics is critical to sustainable development and conservation efforts.

Conservation and management: Conservation and management strategies for populations, including threatened species and ecosystems, are essential to maintaining biodiversity and environmental sustainability

Main Characters of this chapter

  1. Population Dynamics: The study of how populations change in size, density, and structure over time.
  2. Population Growth Models: Understanding exponential and logistic growth models to predict population changes under different conditions.
  3. Population Interactions: Exploring the interactions between organisms within populations, such as predation, competition, symbiosis, and cooperation.
  4. Life History Patterns: Examining the life history strategies of organisms, including reproductive patterns, age at maturity, and lifespan.
  5. Population Regulation: Understanding the factors that regulate population growth, including density-dependent and density-independent factors.
  6. Human Population Dynamics: Analyzing the growth, distribution, and impact of the human population on the environment and other species

Organisms and Population Class 12 Explanation

You have already learned in previous classes that Ecology is a subject that studies the interactions among organisms and between the organism and its physical (abiotic) environment. Ecology is basically concerned with four levels of biological organization – organisms, populations, communities, and biomes. In this chapter, we explore ecology at organismic and population levels.


Ecology at the organismic level is essentially physiological ecology, which tries to understand how different organisms are adapted to their environments in terms of not only survival but also reproduction.

You may have learned in earlier classes how the rotation of our planet around the Sun and the tilt of its axis cause annual variations in the intensity and duration of temperature, resulting in distinct seasons. These variations, together with annual variation in precipitation (remember precipitation includes both rain and snow), account for the formation of major biomes such as desert, rainforests, and tundra (Figure 13.1).

Organisms and population class 12

Regional and local variations within each biome lead to the formation of a wide variety of habitats. Major biomes of India are shown in Figure 13.2. On planet Earth, life exists not just in a few favorable habitats but even in extreme and harsh habitats – scorching Rajasthan

desert, perpetually rain-soaked Meghalaya forests, deep ocean trenches, torrential streams, permafrost polar regions, high mountain tops, boiling thermal springs, and stinking compost pits, to name a few. Even our intestine is a unique habitat for hundreds of species of microbes

What are the key elements that lead to so much variation in the physical and chemical conditions of different habitats? The most important ones are temperature, water, light, and soil.

We must remember that the physico-chemical (abiotic) components alone do not characterize the habitat of an organism completely; the habitat includes biotic components also – pathogens, parasites, predators, and competitors – with which the organism interacts constantly.

Organisms and population class 12

We assume that over a period of time, the organism had through natural selection, evolved adaptations to optimize its survival and reproduction in its habitat.

13.1.1 Major Abiotic Factors

Temperature: Temperature is the most ecologically relevant environmental factor. You are aware that the average temperature on land varies seasonally, and decreases progressively from the equator towards the poles and from plains to the mountain tops.

It ranges from subzero levels in polar areas and high altitudes to >50°C in tropical deserts in summer. There are, however, unique habitats such as thermal springs and deep-sea

hydrothermal vents where average temperatures exceed 100°C. It is general knowledge that mango trees do not and cannot grow in temperate countries like Canada and Germany, snow leopards are not found in Kerala forests, and tuna fish are rarely caught beyond tropical

Latitudes in the ocean. You can readily appreciate the significance of temperature to living organisms when you realize that it affects the kinetics of enzymes and through it the basal metabolism, activity, and other physiological functions of the organism.

A few organisms can tolerate and thrive in a wide range of temperatures (they are called eurythermal), but, a vast majority of them are restricted to a narrow range of temperatures (such organisms are called stenothermal).

The levels of thermal tolerance of different species determine to a large extent their geographical distribution. Can you think of a few eurythermal and stenothermal animals and plants?

In recent years, there has been a growing concern about the gradually increasing average global temperatures (Chapter 16). If this trend continues, would you expect the distributional range of some species to be affected?

Water: Next to temperature, water is the most important factor influencing the life of organisms. In fact, life on earth originated in water and is unsustainable without water. Its availability is so limited in deserts that only special adaptations make it possible to live there. The productivity and distribution of plants are also heavily dependent on water.

You might think that organisms living in oceans, lakes, and rivers should not face any water-related problems, but it is not true. For aquatic organisms, the quality (chemical composition, pH) of water becomes important.

The salt concentration (measured as salinity in parts per thousand) is less than 5 in inland waters, 30-35 in the sea, and > 100 in some hypersaline lagoons. Some organisms are tolerant of a wide range of salinities (euryhaline) but others are restricted to a narrow range (stenohaline).

Many freshwater animals cannot live for long in seawater and vice versa because of the osmotic problems, they would face

Light: Since plants produce food through photosynthesis, a process which is only possible when sunlight is available as a source of energy, we can quickly understand the importance of light for living organisms, particularly autotrophs.

Many species of small plants (herbs and shrubs) growing in forests are adapted to photosynthesize optimally under very low light conditions because they are constantly overshadowed by tall, canopied trees.

Many plants are also dependent on sunlight to meet their photoperiodic requirement for flowering. For many animals too, light is important in that they use the diurnal and seasonal variations in light intensity and duration (photoperiod) as cues for timing their foraging, reproductive and migratory activities.

The availability of light on land is closely linked with that of temperature since the sun is the source for both. But, deep (>500m) in the oceans, the environment is perpetually dark and its inhabitants are not aware of the existence of a celestial source of energy called the Sun. What, then is their source of energy?.

The spectral quality of solar radiation is also important for life. The UV component of the spectrum is harmful to many organisms while not all the color components of the visible spectrum are available for marine plants

At different depths of the ocean. Among the red, green, and brown algae that inhabit the sea, which is likely to be found in the deepest waters? Why?

Soil: The nature and properties of soil in different places vary; it is dependent on the climate, the weathering process, whether soil is transported or sedimentary and how soil development occurred.

Various characteristics of the soil such as soil composition, grain size, and aggregation determine the percolation and water-holding capacity of the soils. These characteristics along with parameters such as pH, mineral composition, and topography determine to a large extent the vegetation in any area.

This in turn dictates the type of animals that can be supported. Similarly, in the aquatic environment, the sediment characteristics often determine the type of benthic animals that can thrive there.

13.1.2 Responses to Abiotic Factors

Having realized that the abiotic conditions of many habitats may vary drastically in time, we now ask – how do the organisms living in such habitats cope or manage with stressful conditions? But before attempting to answer this question, we should perhaps ask first why a highly variable external environment should bother organisms after all.

One would expect that during the course of millions of years of their existence, many species would have evolved a relatively constant internal (within the body) environment that permits all biochemical reactions and physiological functions to proceed with maximal efficiency and thus, enhance the overall ‘fitness’ of the species.

This constancy, for example, could be in terms of optimal temperature and osmotic concentration of body fluids. Ideally then, the organism should try to maintain the constancy of its internal environment (a process called homeostasis) despite varying external environmental conditions that tend to upset its homeostasis.

Let us take an analogy to clarify this important concept. Suppose a person is able to perform his/her best when the temperature is 25°C and wishes to maintain it so, even when it is scorchingly hot or freezingly cold outside.

It could be achieved at home, in the car while traveling, and at the workplace by using an air conditioner in summer and heater in winter. Then his/her performance would be always maximal regardless of the weather around him/her.

Here the person’s homeostasis is accomplished, not through physiological, but artificial means. How do other living organisms cope with the situation? Let us look at various possibilities (Figure 13.3).

(i) Regulate: Some organisms are able to maintain homeostasis by physiological (sometimes behavioral also) means which ensures constant body temperature, constant osmotic concentration, etc.

All birds and mammals, and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregulation and osmoregulation). Evolutionary biologists believe that the ‘success’ of mammals is largely due to their ability to maintain a constant body temperature and thrive whether they live in Antarctica or in the Sahara desert.

The mechanisms used by most mammals to regulate their body temperature are similar to the ones that we humans use. We maintain a constant body temperature of 37°C. In summer, when the outside temperature is more than our body temperature, we sweat profusely.

The resulting evaporative cooling, similar to what happens with a desert cooler in operation, brings down the body temperature.

In winter when the temperature is much lower than 37°C, we start to shiver, a kind of exercise which produces heat and raises the body temperature. Plants, on the other hand, do not have such mechanisms to maintain internal temperatures.

(iii) Migrate: The organism can move away temporarily from the stressful habitat to a more hospitable area and return when the stressful period is over.

In human analogy, this strategy is like a person moving from Delhi to Shimla for the duration of summer. Many animals, particularly birds, during winter undertake long-distance migrations to more hospitable areas.

Every winter, the famous Keolado National Park (Bharatpur) in Rajasthan hosts thousands of migratory birds coming from Siberia and other extremely cold northern regions

(iv) Suspend: In bacteria, fungi, and lower plants, various kinds of thick-walled spores are formed which help them to survive unfavorable conditions – these germinate on the availability of a suitable environment.

In higher plants, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress besides helping in dispersal – they germinate to form new plants under favorable moisture and temperature conditions.

They do so by reducing their metabolic activity and going into a state of ‘dormancy’. In animals, the organism, if unable to migrate, might avoid the stress by escaping in time.

The familiar case of bears going into hibernation during winter is an example of escape in time. Some snails and fish go into aestivation to avoid summer–related problems and desiccation. Under unfavorable conditions, many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development

13.1.3 Adaptations: While considering the various alternatives available to organisms for coping with extremes in their environment, we have seen that some are able to respond through certain physiological adjustments while others do so behaviorally (migrating temporarily to a less stressful habitat).

These responses are also actually their adaptations. So, we can say that adaptation is any attribute of the organism (morphological, physiological, behavioral) that enables the organism to survive and reproduce in its habitat.

Many adaptations have evolved over a long evolutionary time and are genetically fixed. In the absence of an external source of water, the kangaroo rat in North American deserts is capable of meeting all its water requirements through its internal fat oxidation (in which water is a by-product).

It also has the ability to concentrate its urine so that minimal volume of water is used to remove excretory products. Many desert plants have a thick cuticle on their leaf surfaces and have their stomata arranged in deep pits to minimize water loss through transpiration.

They also have a special photosynthetic pathway (CAM) that enables their stomata to remain closed during daytime. Some desert plants like Opuntia have no leaves – they are reduced to spines – and the photosynthetic function is taken over by the flattened stems.

Mammals from colder climates generally have shorter ears and limbs to minimize heat loss. (This is called Allen’s Rule.) In the polar seas, aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces loss of body heat.

Some organisms possess adaptations that are physiological which allow them to respond quickly to a stressful situation. If you had ever been to any high altitude place (>3,500m Rohtang Pass near Manali and Mansarovar, in China-occupied Tibet), you must have experienced what is called altitude sickness.

Its symptoms include nausea, fatigue, and heart palpitations. This is because in the low atmospheric pressure of high altitudes, the body does not get enough oxygen. But, gradually you get acclimatized and stop experiencing altitude sickness.

How did your body solve this problem? The body compensates for low oxygen availability by increasing red blood cell production, decreasing the binding affinity of hemoglobin, and by increasing breathing rate. Many tribes live in the high altitudes of the Himalayas.

Find out if they normally have a higher red blood cell count (or total hemoglobin) than people living in the plains.

In most animals, the metabolic reactions and hence all the physiological functions proceed optimally in a narrow temperature range (in humans, it is 37°C). But there are microbes (archaebacteria) that flourish in hot springs and deep-sea hydrothermal vents where temperatures far exceed 100°C. How is this possible?

Many fish thrive in Antarctic waters where the temperature is always below zero. How do they manage to keep their body fluids from freezing?

A large variety of marine invertebrates and fish live at great depths in the ocean where the pressure could be >100 times the normal atmospheric pressure that we experience.

How do they live under such crushing pressures and do they have any special enzymes? Organisms living in such extreme environments show a fascinating array of biochemical adaptations

Some organisms show behavioral responses to cope with variations in their environment. Desert lizards lack the physiological ability that mammals have to deal with the high temperatures of their habitat but manage to keep their body temperature fairly constant by behavioral means.

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 are capable of burrowing into the soil to hide and escape from the above-ground heat.


13.2.1 Population Attributes In nature, we rarely find isolated, single individuals of any species; the majority of them live in groups in a well-defined geographical area, share or compete for similar resources, potentially interbreed, and thus constitute a

A population. Although the term interbreeding implies sexual reproduction, a group of individuals resulting from even asexual reproduction is also generally considered a population for the purpose of ecological studies.

All the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate, and lotus plants in a pond are some examples of a population.

In earlier chapters, you have learned that although an individual organism is the one that has to cope with a changed environment, it is at the population level that natural selection operates to evolve the desired traits. Population ecology is, therefore, an important area of ecology because it links ecology to population genetics and evolution.

A population has certain attributes that an individual organism does not. An individual may have births and deaths, but a population has birth rates and death rates. In a population, these rates refer to per capita births and deaths, respectively.

The rates, hence, expressed is the change in numbers (increase or decrease) with respect to members of the population.

Here is an example. If in a pond there are 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year.

If 4 individuals in a laboratory population of 40 fruit flies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruit fly per week.

Another attribute characteristic of a population is sex ratio. An individual is either a male or a female but a population has a sex ratio (e.g., 60 percent of the population are females and 40 per cent are males).

A population at any given time is composed of individuals of different ages. If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid (Figure 13.4).

For the human population, the age pyramids generally show the age distribution of males and females in a combined diagram. The shape of the pyramids reflects the growth status of the population – (a) whether it is growing, (b) stable, or (c) declining.

The size of a population, whether influenced by the presence of a predator or the effect of a pesticide application, is always evaluated in terms of any change in the population size. The size, in nature, could be as low as <10 (Siberian cranes at Bharatpur wetlands in any year) or

go into millions (Chlamydomonas in a pond). Population size, more technically called population density (designated as N), need not necessarily be measured in numbers only. Although the total number is generally the most appropriate measure of population density, it is in some cases either meaningless or difficult to determine.

In an area, if there are 200 Parthenium plants but only a single huge banyan tree with a large canopy, stating that the population density of banyan is low relative to that of Parthenium amounts to underestimating the enormous role of the Banyan in that community.

In such cases, the percent cover or biomass is a more meaningful measure of the population size. Total number is again not an easily adoptable measure if the population is huge and counting is impossible or very time-consuming.

If you have a dense laboratory culture of bacteria in a petri dish what is the best measure to report its density? Sometimes, for certain ecological investigations, there is no need to know the absolute population densities; relative densities serve the purpose equally well.

For instance, the number of fish caught per trap is a good enough measure of its total population density in the lake.

We are mostly obliged to estimate population sizes indirectly, without actually counting them or seeing them. The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets.

The size of a population for any species is not a static parameter. It keeps changing in time, depending on various factors including food availability, predation pressure, and adverse weather.

In fact, it is these changes in population density that give us some idea of what is happening to the population – whether it is flourishing or declining. Whatever might be the ultimate reasons,

the density of a population in a given habitat during a given period fluctuates due to changes in four basic processes, two of which (natality and immigration) contribute to an increase in population density and two (mortality and emigration) to a decrease.

  • Natality refers to the number of births during a given period in the population that are added to the initial density.
  • Mortality is the number of deaths in the population during a given period.
  • Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.
  • Emigration is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.

So, if �N is the population density at time �t, then its density at time �+1t+1 is given by the equation:


You can see from the above equation that population density will increase if the number of births plus the number of immigrants (�+�B+I) is more than the number of deaths plus the number of emigrants (�+�D+E), otherwise, it will decrease.

Under normal conditions, births and deaths are the most important factors influencing population density, with the other two factors assuming importance only under special conditions.

For instance, if a new habitat is just being colonized, immigration may contribute more significantly to population growth than birth rates

Growth Models:

Does the growth of a population with time show any specific and predictable pattern? We have been concerned about unbridled human population growth and problems created by it in our country and it is therefore natural for us to be curious if

different animal populations in nature behave the same way or show some restraints on growth. Perhaps we can learn a lesson or two from nature on how to control population growth.

(i) Exponential growth: Resource (food and space) availability is obviously essential for the unimpeded growth of a population. Ideally, when resources in the habitat are unlimited, each

species has the ability to realise fully its innate potential to grow in number, as Darwin observed while developing his theory of natural selection.

Then the population grows in an exponential or geometric fashion. If in a population of size �N, the birth rates (not total number but

Organisms and Population Class 12 Summary

Organisms and population class 12 summary

As a branch of biology, Ecology is the study of the relationships of living organisms with the abiotic (physico-chemical factors) and biotic components (other species) of their environment. It is concerned with four levels of biological organization – organisms, populations, communities, and biomes.

Temperature, light, water, and soil are the most important physical factors of the environment to which organisms are adapted in various ways. Maintenance of a constant internal environment (homeostasis) by the organisms contributes to optimal performance, but only some organisms (regulators) are capable of homeostasis in the face of changing external environments.

Others either partially regulate their internal environment or simply conform. A few other species have evolved adaptations to avoid unfavorable conditions in space (migration) or in time (aestivation, hibernation, and diapause)

Evolutionary changes through natural selection take place at the population level, and hence, population ecology is an important area of ecology.

A population is a group of individuals of a given species sharing or competing for similar resources in a defined geographical area. Populations have attributes that individual organisms do not – birth rates and death rates, sex ratio, and age

Distribution. The proportion of different age groups of males and females in a population is often presented graphically as an age pyramid; its shape indicates whether a population is stationary, growing, or declining.

Ecological effects of any factors on a population are generally reflected in its size (population density), which may be expressed in different ways (numbers, biomass, percent cover, etc.,) depending on the species.

Populations grow through births and immigration and decline through deaths and emigration. When resources are unlimited, the growth is usually exponential, but when resources become progressively limiting, the growth pattern turns logistic.

In either case, growth is ultimately limited by the carrying capacity of the environment. The intrinsic rate of natural increase (r) is a measure of the inherent potential of a population to grow.

Organisms and Population Class 12 Question and Answer

Organisms and population class 12 In this chapter we are giving to to Important Question and Answer which is Exam level

Question:1  If a marine fish is placed in a freshwater aquarium, will the fish be able to survive? Why or why not?

Answer: The chances of survival of marine fish will reduce if placed in a freshwater aquarium, as their bodies are altered to higher salt concentrations as provided by marine environments. In a freshwater environment, fishes fail to regulate the water that enters the body through the process of osmosis. Due to the presence of a hypotonic environment outside the fish’s body, water enters its body which causes its body to swell, leading to the death of the marine fish.

Question:2 What is the ecological principle behind the biological control method of managing pest insects?


Predation is the ecological principle behind the biological control method of managing pest insects. Predation is referred to as the biological interaction between a predator and a prey wherein the predator feeds on the prey, thereby regulating the population of pest insects. Example: The Gawbusia fish checks the mosquito larvae in water bodies.

Question:3 An orchid plant is growing on the branch of a mango tree. How do you describe this interaction between the orchid and the mango tree?

Answer: An orchid that grows on a mango tree represents an interaction called commensalism. In this type of interaction, one species is benefited while another one remains unaffected. Orchid acts as an epiphyte on the mango tree as it does not derive nutrition from the mango tree but uses it for support while the mango tree remains unaffected

Question:4 Define population and community


Population– A group of individuals belonging to the same species and residing in a particular geographical area at a given period of time is called a population. All humans living in a region constitute the population.

Community– A community refers to groups of individuals of different species living in a particular area at a given period of time. Such individuals cannot breed with the members of other species.

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