NCERT Solutions for Class 12 Biology Chapter 13 - Organisms and Population

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Differences between diapause and hibernation.

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The marine fish cannot live in fresh water because of the osmotic problems, it would face death.
Rapid endosmosis occurs which increases blood volume and blood pressure that causes death of fish.

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Phenotypic adaptation involves non-genetic changes in individuals such as physiological modifications, acclimatization or behavioural changes. Some organisms possess adaptations that are phenotypic, which allow them to respond quickly to a stressful situation.
Example. If a person had ever been to any high altitude place (Rohtang pass near Manali or Mansrover in Tibet), he or she must have altitude sickness because body does not get enough oxygen at such a height and protect themselves from atmospheric hypoxia.

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1. Some microorganisms of hot springs and vents can survive at high temperature.
2. They have minimum amount of free water in their bodies. Removal of water provides resistance to high temperature.
3. They also have heat tolerant molecules such as branched chain lipids in their cell walls that reduces fluidity and
4. They have thermal resistant enzymes.

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(i) Natality
(ii) Mortality
(iii) Growth forms
(iv) Population density
(v) Population dispersion
(vi) Population age distribution

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The intrinsic rate of increase (r) of such a population will be towards maximum. Intrinsic rate of increase (r) of population can be calculated by applying following equation :

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Defence mechanism of plants against herbivorous animals 1. Formation of thick cuticle on their leaf surface.
2. Formation of leaf spines e.g. leaf spines in Opuntia
3. Modification of leaves into thorns. e.g. Bougainvillea and Duranta
4. Development of spiny margins on leaves.
5. Development of sharp silicated edges in leaves.
6. Many plants produce and store toxic chemicals which cause discomforts to herbivores e.g. cardiac glucoside by Calotropis, Nicotine by tobacoo.

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The orchid grows as an epiphyte on branch of mango tree. The interaction between orchid and the mango tree is termed commensalism where in orchid derives benefit of interaction while mango tree is not affected.

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Predation is the means of biological control to manage pest insects where predators prey upon pests and regulate their numbers in the habitat. Gambusia fish preys upon larvae of mosquito thus regulate the number of insects.

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Predation is the means of biological control to manage pest insects where predators prey upon pests and regulate their numbers in the habitat. Gambusia fish preys upon larvae of mosquito thus regulate the number of insects.

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(a) Differences between hibernation and aestivation

(b) Differences between ectotherms and endotherms.

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Adaptations of desert plants and animals. 1. Adaptations of desert animals.
1. Animals faced with water scarcity as found in arid or desert areas, show two types of adaptations, reducing water loss and ability to tolerate arid conditions. Kangaroo/Desert Rat seldom drinks water. It has a thick coat to minimise evaporative desiccation. The animal seldom comes out of its comparatively humid and cool burrow during the day time. 90% of its water requirement is met from metabolic water (water produced by respiratory breakdown) while 10% is obtained from food.
2. Loss of water is minimised by producing nearly solid urine and faeces.
3. Spiny skin and highly cornified in Phrynosoma (horned toad) and Maloch horridus.
2. Adaptations of desert plants. 1. Plants have thick cuticle, succulent organs where water and mucilage are stored.
2. Stomata are sunken.
3. They have well developed branched root system.
4. They possess waxy coating on surface.
5. Crassilucean pathway of photosynthesis.
6. They are C₄ plants.

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Adaptations of plants to water scarcities. They are called xerophytes. The above mentioned adaptation of plants are applicable (see part (a).

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Behavioural adaptations in animals. (i) Hibernation (ii) Aestivation (iii) Periodic Activity (iv) Camouflage migration

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Importance of light to plants. (1) Source of energy (2) Photoperiodism (3) Pigmentation (4) Daily rhythm (5) Plant movements and (6) Growth.

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Effect of temperature or water scarcity and adaptation of animals. As discussed in (c) and reducing loss of water from body, and ability to tolerate arid condition.

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(a) Birds and mammals;
(b) Amphibians and reptiles ;
(c) Bacteria, sponges, corals, star fishes, etc.

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Population. A population is a unit of biotic community made up of near permanent group of interbreeding individuals of a species found in a geographical area or space at a particular time.
Community. It is a grouping of different but independent and interacting populations of different species which live in a specific locality, e.g., pond community.

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(a) Commensalism is the relationship of two species of which the smaller one is benefited and the larger one is unaffected. Example: sucker fish and shark.
(b) Parasitism is an association of two species in which the smaller species is benefited and the larger one is harmed. Example: Tapeworm in human intestine.
(c) Camouflage. It is the ability to blend with the surroundings or background. It is difficult to distinguish leaf insect from the surroundings.
(d) Mutualism. It is an association of two species both of which are benefited but cannot live separately. Example: Termites and flagellates.
(e) Interspecific competition. Interactions among the species of a community are based on modes of obtaining food, shelter and habits. Interactions between two species may be positive, such as scavenging, commensalism, protocooperation, mutualism and plant-animal interdependent, or negative, such as amensalism, competition, parasitism and predation.
(f) Symbiosis. It is the relationship between two individuals where both partners are benefited. Following are the examples of symbiosis. The Rhizobium bacteria present in the leguminous plant roots is an example of symbiosis. Bacteria live in the roots in the form of nodules and avail carbohydrate and other food substances. In exchange bacteria fix the nitrogen present in the atmosphere and makes it useful for plants.
(g) Mimicry. It is phenomenon in which a living organism modifies its form, appearance, structure or behaviour and looks like another living organism or some inanimate (non-living) object so as to defend from its predators, or to increase the chances of capturing the prey.

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Logistic population growth curve.Unlimited resources result in exponential growth. Many countries have realized this fact and introduced various restraints to limit human population growth. In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. This limit is called as carrying capacity (K) for that species in that habitat.
A population growing in a habitat with limited resources shows initially a lag phase, followed by phases of increase and decrease and finally the population density reaches the carrying capacity. 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 as explained by the following equation:

Since resources for growth for most animal populations become limiting sooner or later, the logistic growth model is concerned a more realistic one.

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One organism is benefited, other is affected.

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Three important characteristics of a population are:
(i) Population size and population density
(ii) Birth or natality rate
(iii) Death or mortality rate.
(i) Population size. The size of population keeps changing with time depending on the factors like:
(i) Food availability
(ii) Weather
(iii) Predation pressure and
(iv) Competition.
The population density in a given habitat during a given period, changes due to four basic processes namely (1) Natality, (2) Mortality, (3) Immigration and (4) Emigration.
While natality and immigration contribute to an increase in the size and density of a population, mortality and emigration contribute to a decrease in them.
So the equation for population growth is:
N_{t + 1} = N_t + [(B + I) – (D + E)],
where N_t = population density at time t.
B = birth rate.
I = Immigration.
D = death rate.
E = emigration.
If B + I is more than D – E, the population density increases.
If B + I is less than D – E, the population density decreases.
(ii) Birth or Natality rate. It is generally expressed as number or births per 1,000 individuals of a population per year. It increases the population size (total number of individuals of a population) and population density.
Natal or Birth Rate

(iii) Death or Mortality rate. It is opposite of natality rate. It is commonly expressed as the number of deaths per 1,000 individuals of a population per year.
Mortality (Death) Rate