NCERT Class 12 Biology Organisms and Populations

ORGANISMS AND POPULATIONS

Our living world is fascinatingly diverse and amazingly complex. We can try to understand its complexity by investigating processes at various levels of biological organisation–macromolecules, cells, tissues, organs, individual organisms, population, communities and ecosystems and biomes. At any level of biological organisation 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 ‘whytype’ 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 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 recognise her own mother?, and so on. You have already learnt in previous classes that Ecology is a subject which studies the interactions among organisms and between the organism and its physical (abiotic) environment. Ecology is basically concerned with four levels of biological organisation – organisms, populations, communities and biomes. In this chapter we explore ecology at organismic and population levels.

13.1 ORGANISM AND ITS ENVIRONMENT

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 learnt 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, rain forest and tundra (Figure 13.1). 

Figure 13.1 Biome distribution  with respect to annual temperature and precipitationRegional 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 favourable 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.

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, 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 >500C in tropical deserts in summer. There are, however, unique habitats such as thermal springs and deep-sea hydrothermal vents where average temperatures exceed 1000 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 Figure 13.2 Major biomes of India :

(a) Tropical rain forest;

(b) Deciduous forest;

(c) Desert;

(d) Sea coast

latitudes in the ocean. You can readily appreciate the significance of temperature to living organisms when you realise 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 (suchorganisms are called stenothermal). The levels of thermal tolerance ofdifferent species determine to a large extent their geographical distribution.

EXERCISES

1. How is diapause different from hibernation?

2. If a marine fish is placed in a fresh water aquarium, will the fish be able to survive? Why or why not?

3. Define phenotypic adaptation. Give one example.

4. Most living organisms cannot survive at temperature above 450C. How are some microbes able to live in habitats with temperatures exceeding 1000C?

5. List the attributes that populations but not individuals possess. 

6. If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?

7. Name important defence mechanisms in plants against herbivory.

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

9. What is the ecological principle behind the biological control method of managing with pest insects?

10. Distinguish between the following:

 (a) Hibernation and Aestivation

 (b) Ectotherms and Endotherms

11. Write a short note on

 (a) Adaptations of desert plants and animals

 (b) Adaptations of plants to water scarcity

 (c) Behavioural adaptations in animals

 (d) Importance of light to plants

 (e) Effect of temperature or water scarcity and the adaptations of animals.

12. List the various abiotic environmental factors.

13. Give an example for:

 (a) An endothermic animal

 (b) An ectothermic animal

 (c) An organism of benthic zone

14. Define population and community.

15. Define the following terms and give one example for each:

 (a) Commensalism

 (b) Parasitism

 (c) Camouflage

 (d) Mutualism

 (e) Interspecific competition

16. With the help of suitable diagram describe the logistic population growth curve.

17. Select the statement which explains best parasitism.

 (a) One organism is benefited.

 (b) Both the organisms are benefited.

 (c) One organism is benefited, other is not affected.

 (d) One organism is benefited, other is affected.

18. List any three important characteristics of a population and explain

Please refer to attached file for NCERT Class 12 Biology Organisms and Populations