Here we are providing Class 12 Biology Important Extra Questions and Answers Chapter 13 Organisms and Populations. Important Questions for Class 12 Biology are the best resource for students which helps in Class 12 board exams.

Class 12 Biology Chapter 13 Important Extra Questions Organisms and Populations

Organisms and Populations Important Extra Questions Very Short Answer Type

Question 1.
Define microclimate.
Answer:
Microclimate represents the climatic

Question 2.
Define habitat and niche.
Answer:
Habitat is the native environment of conditions that prevail at a local scale or in animal or place. area of limited size. Niche is the position or function of an organism in a community of plants and animals.

Question 3.
How is thermoregulation achieved in polar bears?
Answer:
Thermoregulation in the polar bear. By developing blubber in a subcutaneous zone prevents loss of body heat.

Question 4.
What are osmoconformers? Give one example.
Answer:
Osmoconformers. Animals that can change the osmolarity of their body fluids according to that of the surrounding medium are termed osmoconformers, e.g. Myxine.

Question 5.
What factors cause annual variations in the intensity and duration of temperature?
Answer:

  1. Rotation of the earth around the Sun.
  2. The tilt of the earth on its axis.

Question 6.
Species that can tolerate a narrow range of temperature are called ………………..
Answer:
Stenothermic.

Question 7.
What are eurythermic species?
Answer:
Species that can tolerate a wide range of temperature variations are called eurythermic.

Question 8.
Species that can tolerate a wide range of salinity are called ……………………
Answer:
Euryhaline.

Question 9.
Define stenohaline species.
Answer:
It is a species that lives within a narrow range of salinity.

Question 10.
Name two factors that cause the formation of major biomes.
Answer:

  1. Variation in the intensity and duration of temperature.
  2. Variation in precipitation.

Question 11.
What is the main cause of salinity?
Answer:
Salinity is due to the accumulation of soluble minerals on the surface or beneath the surface of the earth.

Question 12.
What does the stratification of community depict?
Answer:
Stratification of a community depicts vertical layering of vegetation.

Question 13.
From where are individual organisms derived?
Answer:
Individual organisms are always derived from pre-existing organisms through the mechanism of reproduction-may be vegetative, asexual or sexual.

Question 14.
List two negative interactions between two species.
Answer:
Parasitism, Predation.

Question 15.
What is the other term for facultative mutualism?
Answer:
Protocooperation.

Question 16.
Name the association in which one species produces a poisonous substance or a change in environmental conditions that is harmful to another species. (CBSE Delhi 2019)
Answer:
Amensalism.

Question 17.
What is mycorrhiza?
Answer:
It is a symbiotic or mutually beneficial association between a fungus and roots of higher plants.

Question 18.
Emergent land plants that can tolerate the salinity of the sea are called ………………..
Answer:
Mangrove plants.

Question 19.
Give an example of a commensal relationship.
Answer:

  1. Epiphytes and tree,
  2. Remora fish and shark.

Question 20.
Define community periodicity.
Answer:
The recurrence of daily or seasonal or lunar changes in a community is called community periodicity.

Question 21.
Mention any two significant roles predation plays in nature. (CBSE 2008)
Answer:
Role of predators:

  1. Transferring energy to a higher trophic level.
  2. They keep the prey population under control.

Question 22.
Why is the polar region not a suitable habitat for a tiny hummingbird? (CBSE 2008)
Answer:
Tiny animals like hummingbird have more surface area compared to their volume. So heat loss may occur which will not be good for the tiny bird in polar regions.

In polar regions, only animals with a thick layer of fat below the skin can survive.

Question 23.
Between birds and amphibians which will be able to cope with global warming? Give reason. (CBSE2008)
Answer:
Birds will be able to cope with global warming because they are homeotherms as body temperature remains constant irrespective to change in surroundings.

Question 24.
Which one of the two stenothermal or eurythermal shows a wide range of distribution on the earth? (CBSE 2008)
Answer:
Eurythermal shows a wide range of distribution on the earth because they can tolerate a wide range of temperature.

Question 25.
In a pond, there were 30 Hydrilla plants. Through reproduction 10 new Hydrilla plants were added in a year. Calculate the birth rate of the population. (CBSE 2010)
Answer:
A number of individuals added = 10 per 20 Hydrilla plants.

\frac{10}{30}= 0.33 offspring per hydrilla plant per year.

Question 26.
How is ‘stratification’ represented in a forest ecosystem? (CBSE Outside Delhi 2014)
Answer:
Stratification is a grouping of plants in a forest into two or more well-defined layers depending upon height like tall trees, medium-sized trees, small trees, bushes and herbs. In the rainforest, it is multi-storeyed.

Question 27.
Give an example of an organism that enters ‘diapause’ and why? (CBSE Delhi 2014)
Answer:
Under unfavourable conditions, many zooplanktonic species in lakes and ponds enter a stage of suspended development called diapause.

Organisms and Populations Important Extra Questions Short Answer Type

Question 1.
Certain species of wasps are seen to
frequently visit flowering fig trees. What type of interaction is seen between them and why? (CBSE 2008, 2014)
Answer:

  1. Mutualism.
  2. Flowers of a fig tree can only be pollinated by wasp species. In return, female wasp visits the fruit of fig as not only an egg-laying site but also uses the developing seed in fruit for nourishing its larvae.

Question 2.
Write a short note on microclimate.
Answer:
Microclimate: It represents the climatic conditions that prevail at a local scale, or in areas of limited size such as the immediate surroundings of plants and animals. Microclimate generally differs from the prevailing regional climatic conditions. For example, in a forest, dense foliage reduces the amount of light reaching the ground. This also results in a changed air temperature profile. The day-time air temperature inside the forests is lower than the outside temperature. Also, the interior of a forest may be more humid than a nearby non-forested area.

Question 3.
What are the effects of organisms on habitat?
Answer:
Effects of organisms on habitat:

  1. Excessive growth may lead to the death of the habitat as in the case of water hyacinth- an increase in population had led to the death of Hussain Sagar lake of Hyderabad.
  2. The predominance of a predator will reduce the population of prey.
  3. Man manipulates the habitat and has affected the forest causing serious problems due to deforestation. More so man has created pollution of air, water and land.

Question 4.
How do you differentiate habitat from the environment?
Answer:
Differences between habitat and environment:

Habitat Environment
(i) It is the living place of organisms. (i) It Is a specific region surrounding the population of organisms.
(ii) It is a part of the total environment. (ii) It is larger than a habitat.
(iii) It must offer food, shelter and climate condition suitable for the organism to live and flourish. (iii) It has a special condition suitable for specific organisms. Example. Desert rats of Rajasthan are not found in plains of U.P and M.P

Question 5.
Differentiate microhabitat. between habitat and
Answer:
Differences between habitat and microhabitat:

Habitat Microhabitat
(i) It is a living place of an organism. (i) It is a more localised part of the habitat.
(ii) It is a part of the total environment of the region. (ii) It is a part of the habitat.
(iii) It has a common climate for all organisms. Example. Sundarban forests are the habitat of Bengal tigers. (iii) It is mostly suited for specific animals, e.g. sediments of pond or togs.

Question 6.
List the means by which organisms survive at freezing temperatures.
Answer:

  1. The animals are usually white or light coloured. The light colouration of animals helps in camouflage with snow and thermal regulation.
  2. Animals are thickly coated with fur.
  3. Presence of a thick layer of fat below the skin.

Question 7.
Differentiate eurythermal, stenothermal and euryhaline animals.
Answer:
Differences between eurythermal, stenothermal and euryhaline animals:

Eurythermal Animals Stenothermal Animals Euryhaline Animals
These are the animals/ organisms which can tolerate a wide range of temperatures. These are the animals/ organisms which can tolerate only: a narrow range of temperatures. These are the animals that can tolerate a wide range of salinity of the medium.

Question 8.
Explain how is an orchid plant adapted to changes in temperature and humidity.
Answer:
An orchid plant, e.g.Venda, an epiphyte, is a native of tropical forests of India and South Asia.

Its adaptations are:

  • Stem stores water and leaves are adapted to keep water in and dryness out.
  • Roots of the plant secure their attachment to the bark of the tree and help in the absorption of moisture from humid air or rain.

Question 9.
Why do submerged plants receive weaker illuminations than exposed floating plants in a lake?
Answer:
Because light intensities of stronger illuminations are absorbed by the exposed floating plants of the lake/water-bodies, so only the weaker illuminations reach the submerged, Some light is reflected at the water surface and a part is absorbed by upper layers of water.

Question 10.
Categorise the following plants into hydrophytes, halophytes, mesophytes and xerophytes. Give reasons for your answers.
(i) Salvinia
Answer:
Hydrophyte. Because it is not able to tolerate deficiency of water. It is partially or completely submerged.

(ii) Opuntia
Answer:
Xerophyte. Because it prefers a dry and hot climate with low rainfall, thus has succulent leaves.

(iii) Rhizophora
Answer:
Halophyte. Because it grows in a saline habitat.

(iv) Marigifera
Answer:
Mesophyte. Because it prefers areas with high moisture content.

Question 11.
If a freshwater fish is placed in an aquarium containing seawater, will the fish be able to survive? Explain giving reasons. (CBSE Delhi 2015)
Answer:
No, a freshwater fish placed in the aquarium containing sea-water will not be able to survive. Because its body system is adapted to function normally in a narrow range of salinity and it cannot survive in the high salinity of sea-water.

Question 12.
Distinguish between population and community.
Answer:
Differences between population and community:

Population Community
(i) Group of individuals of same species inhabiting the same area. (i) Group of species lying in the same area.
(ii) Organism in a population undergoes the same life cycle. (ii) Different species have a different life cycle.

Question 13.
How does population size increase or decrease?
Answer:

  • Population (size/density) increases by birth, hatching, germination and immigration that add individuals.
  • Population (size/density) decreases by death and emigration. The population is regulated by food, space, disease, natural calamities and environmental factors.

Question 14.
Differentiate between natality rate and death/mortality rate.
Answer:
Differences between natality rate and mortality rate:

Character

 Natality rate  

Morality rate
(i) Definition A number of births per 1,000 individuals of a population per year. A number of death per 1,000 individuals of a population per year.
(ii) Population size and population density. Increases Decreases

Question 15.
Discuss the role of predators in an ecosystem.
Answer:
Predators are of great importance as they play the following important roles in an ecosystem:

  1. They act as ‘conduits’ for energy transfer to higher trophic levels.
  2. They keep the prey population under control, which otherwise can reach very high population density and cause an imbalance in the ecosystem,
  3. They help in maintaining species diversity in a community by reducing the intensity of competition among the competing prey species.

Question 16.
What is brood parasitism? Give an example.
Answer:
Brood parasitism. It refers to the phenomenon in which one (parasitic) bird species lays its eggs in the nest of another bird species.

Evolution has occurred in such a way that the eggs of the parasitic birds resemble those of the host bird in size, colour, etc. to avoid the host bird detecting the foreign eggs and ejecting them from the nest. Example. Cuckoo birds lay eggs in the nests of crows.

Question 17.
List a few adaptations that parasites have developed. (CBSE Delhi 2019)
Answer:
Parasites have evolved one or more of the following adaptations:

  1. Loss of unnecessary sense organs.
  2. Presence of hooks/adhesive organs and suckers.
  3. Loss of digestive system.
  4. High reproductive capacity.
  5. Produces antitoxins to counter toxins to the host.

Question 18.
How do parasites harm the host?
Answer:
The parasites harm the host in the following ways:

  1. Reduces the survival of the host.
  2. Growth and reproductive rates of hosts are reduced.
  3. Render the most vulnerable to its predators by making them physically weak.

Question 19.
Justify the statement “Predators and scavengers are markedly different.”
Answer:
Predators feed on another organism, i.e. prey, whereas scavenger feeds on dead animals or an animal killed by another animal. A predator can be a prey also, e.g. a frog eats insects and the frog may be eaten by a snake. But a scavenger such as jackals, hyenas and vultures cannot kill zebra or giraffe or deer but a lion kills them and leaves a part of it to be eaten by such scavengers.

Question 20.
Mention any two significant roles predators play in nature. (CBSE 2008, 2016)
Answer:
Role of predators:

  1. Transferring energy to a higher trophic level.
  2. Keeping the prey population under control.

Question 21.
How do organisms manage stressful conditions existing in their habitat for a short duration? Explain with the help of one example each. (CBSE (Delhi) 2008)
Answer:
Physiological adaptations allow some organisms to respond quickly to stressful conditions. At high altitude, mountain (altitude) sickness is experienced resulting in nausea, fatigue and heart palpitations. It is due to low atmospheric pressure at high altitude and the body does not receive the proper amount of oxygen. It is accommodated by increasing RBC production, decreasing the binding capacity of haemoglobin and increasing breathing rate.

Question 22.
Explain with the help of an example each of any three ways the ecologists use to measure the population density of different organisms rather than by calculating their absolute number. (CBSE Delhi 2019 C)
Answer:
Method to measure population density: Population density is the number of individuals of a species per unit area or volume.

PD= N/S

Where N = The no. of the individual in the region
S = No. of the unit area in a region

  1. Number of animals per square kilometre
  2. Number of trees per hectare
  3. A number of phytoplanktons per cubic litre of water. Sometimes the relative densities also serve the purpose, that is
  • The number of fish caught per trap is good enough to measure the population of fish.
  • Tiger census is based on pug marks and faecal pellets.

Question 23.
Name the interaction in each of the following:
(i) The cuckoo lays her eggs in the crow’s nest.
Answer:
Brood parasitism.

(ii) Orchid grows on a mango tree.
Answer:
Commensalism.

(ii) Ticks live on the skin of dogs.
Answer:
Parasitism.

(iv) Sea anemone is often found on the shell of a hermit crab. (CBSE Delhi 2008)
Answer:
Mutualism.

Question 24.
Name the interaction in each of the following:
(i) Cuscuta growing on a shoe flower plant.
Answer:
Parasitism.

(ii) Mycorrhizae living on the roots of higher plants.
Answer:
Mutualism.

(iii) Clownfish living among the tentacles of a sea anemone.
Answer:
CommensaLism.

(iv) Koel laying its eggs in the crow’s nest. (CBSE Delhi 2011)
Answer:
Brood parasitism.

Question 25.
Name the interaction In each of the following:
(i) Ascaris worms living in the intestine of human.
Answer:
ParasItism

(ii) Suckerfish attached to the shark.
Answer:
Commensalism

(iii) Smaller barnacles disappeared when Balanus dominated the coast of Scotland.
Answer:
Competition

(iv) Wasp pollinating fig inflorescence. (CBSE 2008)
Answer:
Mutualism.

Question 26.
Why do clownfish and sea anemone pair up? What is this relationship called? (CBSE 2012)
Answer:

  1. Commensalism,
  2. Interaction between sea-anemone that has stinging tentacles and the clownfish that lives among them in which fish gets protection from predators. The anemone does not appear to derive any benefit from the clownfish.

Question 27.
Some organisms suspend their metabolic activities to survive in unfavourable conditions. Explain with the help of any four examples. (CBSE 2012)
Answer:

  1. Thick-walled spores of bacteria and fungi help them to overcome unfavourable conditions.
  2. Frogs and lizards undergo hibernation during the winter season.
  3. Snails and fish undergo aestivation during summer to avoid summer-related problems of heat and desiccation.
  4. Species of zooplankton are known to enter a suspended state of development termed-diapause.

Question 28.
A moss plant is unable to complete its life cycle in a dry environment. State reason. (CBSE Delhi 2015)
Answer:

  1. Due to the absence of vascular tissue, water and minerals cannot be transported to various parts.
  2. Transfer of flagellated antherozooids to oospore of archegonium depends upon water.

Question 29.
Plants that inhabit a rain-forest are not found in wetlands. Explain. (CBSE Delhi 2016)
Answer:
A habitat is a specific place or area or locality having a combination of factors, physical features and barrier where a community resides. In rainforest plants adapt themselves to the specific conditions of their habitat and the characteristics adaptations related to a particular habitat are not found in those plants living in wetland and vice versa. Thus they can not inhabit a new habitat.

Question 30.
Answer:
In certain seasons we sweat profusely while in some other season we shiver. Explain. (CBSE Delhi 2016)
Answer:
Humans are a warm-blooded or homeothermic or endothermal organism. They maintain a fixed body temperature despite changes in the surrounding. The optimum temperature for maximum efficiency of enzymes is 37°C. Thus the human body maintains a body temperature near it.

Homoeostasis is the phenomenon of maintaining a constant internal environment. Thus during heavy exercise or summer, sweating occurs. It helps in cooling the body. During winter, shivering helps in retaining heat within the body.

Question 31.
How did David Tillman show that the “Stability of a community depends on its species richness”? Explain. (Outside Delhi 2019)
Answer:
The stability of a community depends on its species richness. David Tilman’s long-term ecosystem experiments using outdoor plots provided the answer for species richness. Tilman found that plots with more species showed less year-to-year variations in total biomass. He also showed that increased diversity contributed to higher productivity.

Thus he established that the stability of a community depends on its species richness.

Organisms and Populations Important Extra Questions Long Answer Type

Question 1.
(i) “Organisms may be conformers or regulators.” Explain this statement and give one example of each.
(ii) Why are there more conformers than regulators in the animal world? (CBSE 2017)
Or
Human is categorised as ‘regulators’. Explain how they maintain a constant body temperature. (CBSE Outside Delhi 2019)
Answer:
(i) Regulators are organisms that are able to maintain homoeostasis by physiological and behavioural means. All birds and mammals and few lower vertebrate and invertebrate species maintain homoeostasis by thermoregulation and osmoregulation, e.g. birds and mammals.

However, a majority (99%) of animals and nearly all plants cannot maintain a constant internal environment, i. e. their body temperature is not constant. They are conformers, e.g. fishes, frogs, etc.

(ii) There are more conformers than regulators in the animal world because conformers lack the capability to maintain a constant internal environment or homoeostasis. It is limited to birds, mammals and few lower vertebrate and invertebrate species only because it is a very energy-expensive process.

Question 2.
Explain the ecological hierarchy.
Answer:
Ecological hierarchy: It is a series of graded ecological categories.
Characteristics of ecological hierarchy:

  1. A biological unit at each level has a specific structure and function.
  2. In this hierarchy, smaller biological units coordinate to form the next higher level of organisation.
  3. Only the organisms show free existence.
  4. Organisms cannot live in isolation.
  5. At each level, different units show interdependence.
  6. At each level, the unit shows interaction with the physical environment (energy and matter).
  7. The biological units are successfully adapted to their environment.

Question 3.
In a seashore, the benthic animals live in sandy, muddy and rocky substrata and accordingly developed the following adaptations.
Find suitable substratum against each adaptation.
(i) Burrowing
Answer:
In a seashore, the benthic animals adapt to their mode of life according to the nature of the seafloor, e.g. Benthic animals become fossorial (burrowing) in the sandy substratum, e.g. tube worm.

(ii) Building cubes
Answer:
Benthic animals build cubes in the muddy substratum.

(iii) Holdfasts/peduncle
Answer:
Benthic animals develop holdfast/ peduncle if the substratum is rocky.

Question 4.
In a pond, we see plants that are free. floating, rooted-submerged, rooted-emergent, rooted with floating leaves. Write the type of plant against the following examples:
(i) Hydrilla
Answer:
Hydrilla is Rooted submerged.

(ii) Typha
Answer:
Typha is Rooted emergent.

(iii) Nymphaea
Answer:
Nymphaea is Rooted with floating leaves.

(iv) Lemna
Answer:
Lemna is Free-floating.

(v) Vallisneria.
Answer:
Vallisneria is Rooted submerged.

Question 5.
Why do all the freshwater organisms have contractile vacuoles whereas the majority of marine organisms lack them?
Answer:
Contractile vacuole helps in maintaining salt and water level called osmoregulation. Because of the cellular environment of a freshwater organism such as Amoeba, Paramecium etc. being hypertonic, the water diffuses inside the cell constantly and gets collected in the contractile vacuole, which squeezes the extra water out of the cell periodically.

While in the case of marine protozoans organisms, this does not occur due to high salt concentration. These organisms live in isotonic conditions in seawater. Thus there is no need for contractile vacuole.

Question 6.
Explain the following terms:
(i) Mimicry
Answer:
Mimicry: It is a 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.

The individual which shows mimicry is called a mimic, while the animate or inanimate object with which a mimic resembles is called a model. The concept of mimicry was first observed by an English naturalist, Henry Bates (1862 A.D.), so the phenomenon is also called Batesian mimicry.

Types: Mimicry is of three types:
(a) Protective mimicry,
(b) Aggressive mimicry,
(c) Feigning death or Conscious mimicry.

(ii) Acclimatisation
Answer:
Acclimatisation: The gradual physiological adjustment to slowly changing new environmental conditions is known as acclimatisation. If some factors shift beyond the tolerance range, the organism can come to a tolerance range or migrate to acclimatise.

Question 7.
DepIct the temperature-based thermal stratification in lakes.
Answer:
Thermal stratification in Lakes:
Class 12 Biology Important Questions Chapter 13 Organisms and Populations 1

Thermal stratification occurs in lakes, the seasonal mixing patterns of a lake are determined by its temperature profile.

Question 8.
1. How does age distribution help in the study of the population?
2. How does an age pyramid, for the human population at a given point of time helps the policymakers in planning the future? (CBSE Delhi 2016)
Or
Draw a stable human age pyramid. (CBSE Outside Delhi 2019)
Answer:
Age distribution (Age composition). The relative abundance of the organisms of various age groups in the population is called the age distribution of the population. With regard to age distribution, there are three kinds of populations:

1. Rapidly growing population. It has a high birth rate and low death rate, so there is more number of young individuals in the population. According to a recent survey, more than 42% of the Indian population consists of children below the age of 14 years, so the Indian population is called the young population.

Class 12 Biology Important Questions Chapter 13 Organisms and Populations 2
2. Stationary population: It has equal birth and death rates, so the population shows zero population growth.

3. Declining population. It has a higher death rate than the birth rate, so the population of young members is lower than that of old members.
(a) For the human population, the age pyramids generally show the age distribution of males and females in a combined diagram.
(b) The shape of the age pyramid reflects the growth status of the population. Thus age pyramid for the human population at a given time helps the policymakers in planning for the future.

Question 9.
Discuss life-history traits of an organism that have evolved in relation to the constraints imposed by biotic and abiotic factors in their habitat.
Answer:
According to ecologists, life-history traits of an organism have evolved in relation to the constraints imposed by the biotic and abiotic factors in their habitats.

  1. It can be illustrated with vast variations and life history.
  2. The evolution of populations aims at improving reproductive fitness or Darwinian fitness to the maximum in their habitats.
  3. They evolve towards the most efficient reproductive strategy.
  4. Organisms like Pacific Salmon fish and bamboo breed only once in their lifetime.
  5. Most birds and mammals breed many times during their lifetime.
  6. Oysters and pelagic fishes produce a large number of small-sized offspring.
  7. Birds and mammals produce a small number of large-sized offspring.

Question 10.
What is the predator-prey relationship? Give example. (CBSE 2016)
Answer:
Predation. It is an interspecific interaction, where one animal (called predator) kills Organisms and Populations and consumes the other weaker animal (called prey). Herbivores are predators of plants.

Predator-prey relationship. It is a kind of direct food relationship between two species of animals in which larger species, called predator, attacks, kills and feeds on the smaller species, called prey. It was proved by G.F. Gause (1934).

Although predation and competition appear to be very harmful processes, they are essential to keep a check on the size of the population of other species.

Predation is nature’s way of transferring the energy fixed by plants, to higher trophic levels.
Examples:

  1. A tiger killing and eating a deer.
  2. A snake eating a frog.
  3. A sparrow eating a fruit/seed.

Question 11.
Many prey organisms have developed different defence mechanism. Give a few examples.
Answer:
Prey species have developed various defence mechanisms to reduce the impact of predation; some of them are listed below:

  1. Certain insect species and frogs have camouflage (cryptic colouration) to avoid detection by their predators.
  2. Some animals (e.g. monarch butterfly) are highly distasteful to their predators. This butterfly species accumulates a chemical by feeding on a poisonous weed during its caterpillar stage.
  3. Some prey species are poisonous and hence are avoided by predators, e.g. Dart frogs like Phyllobates bicolour and Dendrobats.

Question 12.
“Herbivores are the predators of plants”. Discuss a few defence mechanisms of plants against herbivory.
Or
Write what do phytophagous insects feed on? (CBSE2012)
Answer:
Herbivores are the predators of plants. The problem of predation is more severe for plants than animals as the plants cannot move away from the predators. About 25% of the known insects are phytophagous and feed on the sap and other parts of plants.

Plants have developed certain morphological and chemical defence mechanisms against herbivores; a few of them are listed below:

  1. Morphological: Thorns (Bougainvillaea) and spines (Acacia, Cactus, etc.) are the most common morphological means of defence.
  2. Chemical: Plants produce and store certain chemicals which function in one or more of the following ways:
    (a) They make the animal feel sick.
    (b) They inhibit them from feeding.
    (c) They interfere with digestion.
    (d) They even directly kill them, e.g. Calotropis produces a highly poisonous glycoside, that is a cardiac poison.
    (e) Nicotine, strychnine, opium, quinine, etc. are the chemicals produced by plants for their defence against herbivores.

Question 13.
What is parasitism? Define parasite and host. What are the kinds of parasite?
Answer:
Parasitism: This is a relationship between two organisms in which one obtains its nourishment from the other and harms it at the same time.

Parasite: The organism which obtains its food from the other without directly killing it is known as a parasite. Parasites are host specific and parasite and host tend to co-evolve.

Host: The second organism which provides food to the parasite is named the host. Endoparasite. The parasite that lives inside the body of the host is known as endoparasite.

Ectoparasite. The parasite lives on the outside of the body of the host, e.g. Leech, Louse, Bedbug.

Question 14.
Define commensalism. Give examples.
Answer:
Commensalism: It is defined as the interspecific interaction where one species is benefitted while the other species is neither benefitted nor harmed.
Examples:

  1. Orchids grow as epiphytes on mango or other fruit trees. Orchids are benefitted by getting shelter, while the tree is neither benefitted nor harmed.
  2. The clownfish living among sea anemones get protection from their predators, which stay away from the stinging tentacles of the sea anemone.
  3. Barnacles growing on the whale are benefitted to move to where food is available.
  4. The cattle egrets always forage near where the cattle are grazing. As the cattle animals stir up, the insects are flushed out from the vegetation. The egrets are benefitted from this as otherwise, it might be difficult for the birds to detect and catch the insects.

Question 15.
Differentiate between the following:
(i) Mutualism commensalism.
Answer:
Differences between mutualism and commensalism:

Mutualism commensalism
(i) It is the inter-specific interaction, in which both the interacting species are benefitted. (i) It is the interspecific interaction, in which one species is benefitted white the other is neither benefitted nor harmed.
(ii) It may or may not involve physical association. (ii) The two individuals come in close physical contact.

(ii) Commensalism and amensalism.
Answer:
Differences between commensalism and amensalism:

Commensalism Amensalism
It is the interspecific interaction in which one species is benefitted while the other one is neither harmed nor benefitted. It is the interspecific interaction in which one species is harmed I affected, while the other is neither benefitted nor harmed.

(iii) Predators and parasites.
Answer:
Differences between predators and parasites:

Predators Parasites
(1) Predators are Larger and stronger animals that kill and consume the prey. (i) Parasites are small or microscopic organisms that depend on the host.
(ii) They do not take shelter on the prey (ii) They take shelter on the host.
(iii) Their biotic potential is Low. (iii) They have higher biotic potential.
(iv) They are mobile to capture the prey. (iv) They have poor means of dispersal.
(v) They are not specific for the Prey (v) They are host-specific.

Question 16.
Study the graph given below and answer the questions that follow:

Class 12 Biology Important Questions Chapter 13 Organisms and Populations 3

(i) Write the status of food and space in the curves (a) and (b).
Answer:
Food and space are unlimited in the curve (a), while they are limited in the curve (b).

(ii) In the absence of predators, which one of the two curves would appropriately depict the prey population?
Answer:
In the absence of a predator, any species will grow exponentially and show curve (b).

(iii) Time has been shown on X-axis and there is a parallel dotted line above it. Give the significance of this dotted line. (CBSE 2014)
Answer:
The dotted line represents (k). It is carrying capacity.

Question 17.
Explain parasitism and co-evolution with the help of one example of each. (CBSE Outside Delhi 2016)
Answer:

  1. Many parasites have evolved to be host-specific. They can live as parasite only in a single species of host.
  2. These organisms parasitize in such a way that both the host and the parasite tend to co-evolve; that is if the host evolves a special mechanism for rejecting or resisting the parasite, in such conditions parasite has to evolve a mechanism to counteract and neutralise them, in order to be successful with the same host species.
  3. In accordance with their mode of living, parasites evolve special adaptations such as loss of unnecessary sense organs, presence of adhesive organs or sucker so as to cling to the host. It also loses the digestive system. Parasites have a high reproductive capacity.
  4. The human liver fluke depends on the intermediate host (a snail and a fish ) to complete the life cycle.
  5. The malarial parasite needs a female anopheles mosquito as a vector to spread to other hosts.

Question 18.
(i) In a pond, there were 200 frogs. 40 more were born in the year. Calculate the birth rate of the population.
Answer:
Birth rate = No. of individuals born/ Total no. of individuals = 40/200 = 0.2 = 0.2 frog per year.

(ii) Population in terms of number is not always a necessary parameter to measure population density. Justify with two examples. (CBSE Outside Delhi 2019)
Answer:
To measure population density, the number is not always a necessary parameter.

For example:
(a) If there are 200 Parthenium plants but only a single huge banyan tree with a large canopy, the population density of banyan is low relative to that of Parthenium which amounts to underestimating the enormous role of the banyan in that community. In such cases, the per cent cover or biomass is a more meaningful measure of the population size.
(b) In a dense laboratory culture of a microbial population in a Petri dish, the total number of microbes is again not an easily adaptable measure because as the population is huge, counting is impossible and time¬consuming.

Question 19.
How do organisms which cannot migrate tend to overcome adverse environ¬mental conditions? Explain taking one example each from vertebrates and angiosperms respectively. (CBSE Delhi 2009)
Answer:
Organisms that cannot migrate tend to overcome adverse environmental conditions by developing several methods/ features. For example, some vertebrates escape the stress caused by unfavourable environmental conditions by escaping in time like bears go into hibernation during the winter months.

In angiosperms, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress. They reduce their metabolic activity and go into an inactive, i.e. ‘dormant’, state. They germinate to form new plant when the favourable conditions return.

Question 20.
Explain how tolerance to environmental factors determines the distribution of species.
Answer:
Range of tolerance: Biological species can show a range of tolerance to environmental factors. These factors show variation in their effects and anyone who is present in the least amount may become limiting. The response of an organism to a range of gradient of a single environmental factor such as temperature, sunlight or nutrient concentration forms a bell-shaped curve as shown in the figure.

Class 12 Biology Important Questions Chapter 13 Organisms and Populations 4
The response of an organism to a range or gradient of an environmental factor, (temperature, Light, nutrient)

Question 21.
Discuss in detail various adaptations found in plants and animals in snowy winter of polar regions.
Answer:
Adaptations in plant and animals of polar regions:

  1. Organisms living in polar regions have to face the severe snowy winter and show entirely different types of adaptations. Animals in polar regions are generally white or tight coloured. This light colouration helps in camouflage and in thermal regulation.
  2. During extreme winter they undergo hibernation.
  3. Animals eat a lot of food during summer and autumn and store a lot of energy in the form of fat.
  4. During hibernation, their metabolic activities are reduced considerably,
  5. The plants growing in these regions remain dormant during extreme winter months.
  6. They bear narrow leaves, spines, etc. which are shed easily and rapidly,
  7. Plants remain dwarf. Trees are usually not found in these regions as they cannot withstand the low temperatures.

Question 22.
How do desert plants prevent loss of water?
Answer:
Xeric adaptations of desert plants:

  1. Some xerophytes remain dormant either as seeds or as roots and during rains, they sprout up. It is termed ephemerals, e.g. Cassia, Argemone.
  2. Some desert plants develop succulent organs (stems, leaves and roots), e.g. Asparagus, Begonia, Bryophyilum.
  3. Presence of extensive root system.
  4. Either leaf is absent or small¬sized leaves present to minimise the rate of transpiration.
  5. The stomata get sunken to reduce the rate of transpiration.
  6. Presence of thick cuticle on stem and leaf surfaces. This reduces the rate of transpiration.
  7. Shedding of leaves in some desert plants. This helps in reducing the surface area and water loss.

Question 23.
(i) What is the competition?
Answer:
Competition is an interaction between organisms for life requirements (nutrition, shelter, sunlight, etc.). It is of two types: intraspecific and interspecific. The effort of a tiger and a leopard for prey is an interspecific competition. In general, it is believed that competition occurs among closely related species when they compete for the same resources that are limited.

(ii) Why is it not true always?
Answer:
But it is found to be not true always for the following reasons:
(a) Completely unrelated species can also compete for the same sources. For example in certain shallow lakes of South America, the visiting flamingoes and the native fishes compete for the same zooplankton as their food.

(b) Resources need not be limiting for the competition to occur, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, even if resources are plenty.

For example, the Abingdon tortoise in Galapagos island became extinct within a decade after goats were introduced into the island: this was due to the fact that the goats had greater browsing efficiency than the tortoise.

(iii) Explain competitive release and Gause’s competitive exclusion principle.
Answer:
Another evidence for the competition is competitive release, a phenomenon in which a species whose distribution is restricted to a small geographical area due to the presence of a competitively superior species, expands its distributional range when the competing species is experimentally removed.

Gause’s competitive exclusion principle states that two closely related species competing for the same resources cannot exist together as the competitively inferior one will be eliminated, but this is true only when the resources are limiting and not otherwise.

They have also pointed out that species facing competition might evolve mechanisms that promote co¬existence, rather than exclusion.

(iv) Write contribution of Mac Arthur.
Answer:
Mac Arthur had shown that five closely related species of warblers living on the same tree were able to avoid competition and co-exist to behavioural differences in their foraging activities.

Question 24.
Illustrate symbiosis with any four examples.
Answer:
Symbiosis: It is the relationship between two individuals where both partners are benefited.

That following are examples of symbiosis:

  1. The Rhizobium bacteria present in the root nodules of leguminous plant roots are 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 make it useful for plants.
  2. Trichonympha, a protozoan parasite, lives inside the intestine of termites. The presence of this protozoan helps the termites to digest the cellulose food and the parasite gets food and shelter.
  3. Lichen plants. Lichen is the result of a symbiotic relationship between algae and fungi. In this, algae depend on fungi for water, minerals, salts and safety, whereas fungi get food material prepared by algae.
  4. The human intestine contains a large number of symbiotic bacteria that help in the synthesis of vitamin B-complex. Bacteria get necessary nutritional substances from the human intestine and a safe place for living.

Question 25.
Define phenotypic adaptation. Give one example.
Answer:
Phenotypic adaptation involves non-genetic changes in individuals such as physiological modifications, acclimatisation 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 the body does not get enough oxygen at such a height and protect itself from atmospheric hypoxia. But after some time, the body acclimatises itself to the situation by increasing the number of RBC and decreasing the binding affinity of Hb and oxygen.

Question 26.
Name important defence mechanisms in plants against herbivores.
Answer:
Defence mechanisms 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 tobacco.

Question 27.
Distinguish between the following:
(i) Hibernation and aestivation
Answer:
Differences between hibernation and aestivation:

Hibernation Aestivation
(i) Spending the winter in an inactive? dormant state, Is called hibernation (winter sleep). (i) Spending dr hot period (summer) in an inactive period is called aestivation (summer sleep).
(ii) Example. Northern ground squirrels. (ii) Example. Ground squirrels in the South West deserts.

(ii) Ectotherms and endotherms.
Answer:
Differences between ectotherms and endotherms:

Ectotherms Endotherms
Ectotherms are those animals whose body temperature changes to match with that of the surroundings, in which they are living. They cannot maintain their internal environment constant. Endotherms are those animals whose body temperature is maintained relatively constant by physiological regulation.

Question 28.
Write short notes on:
(i) adaptations of desert plants and animals. (CBSE2011)
Answer:
Adaptations of desert plants and animals:
(a) Adaptations of desert animals.

  • Animals faced with water scarcity as found in arid or desert areas, show two types of adaptations, reducing water loss and the 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 daytime. 90% of its water requirement is met from metabolic water (water produced by respiratory breakdown) while 10% is obtained from food.
  • Loss of water is minimised by producing nearly solid urine and faeces.
  • Spiny skin and highly cornified in Phrynosoma (horned toad) and Moloch horridus.
  • Camels have long legs to stay away from the hot desert surface.

(b) Adaptations of desert plants.

  • Plants have thick cuticle, succulent organs where water and mucilage are stored.
  • Stomata are sunken to prevent water loss.
  • They have a well-developed branched root system.
  • They possess a waxy coating on the surface.
  • Crassulacean pathway of photosynthesis.

(ii) behavioural adaptations in animals. (CBSE 2011)
Answer:
Adaptations of plants to water scarcities. They are called xerophytes. The above-mentioned adaptation of plants are applicable (see part (a)).

(iii) Behavioural adaptations in animals,
(a) Hibernation
(b) Aestivation
(c) Periodic activity
(d) Camouflage
(e) Migration.

(iv) importance of light to plants.
Answer:
Importance of light to plants.
(a) Source of energy for photosynthesis
(b) Photoperiodism
(c) Pigmentation
(d) Daily rhythm
(e) Plant movements and (f) Growth.

(v) effect of temperature or water scarcity and the adaptation of animals.
Answer:
Effect of temperature or water scarcity and adaptation of animals. Optimum temperature is necessary for animals, survival as all the metabolic activities are driven by enzymes. And enzymes work actively only in a certain optimum range of temperature. Regulators can regulate their body temperature in case of temperature fluctuations in the external environment. Conformers also try to maintain temperature by certain methods but not internally.

Similarly, water is also necessary for the metabolic activities of animals. Various adaptations seen in animals to deal with temperature fluctuations are

  • thick fur,
  • sweating, short limbs and ears.

Various adaptation to counter the scarcity of water is the ability to use metabolic water, store water, reduced water loss, etc.

Question 29.
With the help of a suitable diagram describe the logistic population growth curve. (CBSE Delhi 2019 C)
Answer:
Logistic popuLation growth curve.

Class 12 Biology Important Questions Chapter 13 Organisms and Populations 5

S-shaped Population growth curve (Verhulst-Pearl logistic growth curve)

Unlimited resources result in exponential growth. Many countries have realised 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 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:

\(\frac{d \mathrm{~N}}{d t}=r \mathrm{~N}\left(\frac{\mathrm{K}-\mathrm{N}}{\mathrm{K}}\right)\)

where N = Population density at a time t; r = Intrinsic rate of natural increase and K = Carrying capacity,

\(\left[\frac{k-N}{k}\right]\) = Environmental resistance.

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

Question 30.
List any three important characteristics of a population and explain. (CBSE Delhi 2019)
Answer:
Three important characteristics of a population are:

  1. Population size and population density
  2. Birth or natality rate
  3. Death or mortality rate.
  • Population size: It is the actual number of individuals in the population. The size of the population keeps changing with time depending on the factors like:
    (a) Food availability
    (b) Weather
    (c) Predation pressure and
    (d) Competition.

Population density is a measure of population size per unit area. 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: Nt + 1 = Nt + [(B + I) – (D + E)],
where Nt = population density at time t.
B = No of birth I = No of Immigration D = No of death E = No of 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.

Birth or Natality rate. It is generally expressed as the number of 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 =

Class 12 Biology Important Questions Chapter 13 Organisms and Populations 6

Death or Mortality rate. It is the opposite of the natality rate. It is commonly expressed as the number of deaths per 1,000 individuals of a population per year.
Class 12 Biology Important Questions Chapter 13 Organisms and Populations 7

Question 31.
Define mutualism. Give examples.
Answer:
Mutualism: It is a symbiotic relationship between the members of two species in which the two partners are mutually benefited. There is a complete dependence of the partners on each other, and one cannot survive in the absence of the other.

Sometimes the term symbiosis is used as a synonym with mutualism.
Examples:
(a) Mutualism between animal and bacteria. Symbiotic bacteria like Ruminococcus are found in the rumen part of the compound stomach of cud-chewing mammals like cattle, sheep, goat, camel, etc. and secrete cellulase enzyme to digest the cellulose of plant food eaten by the ruminants which provide food and shelter to the bacteria.

(b) Mutualism between crab and sea anemone. In this case, a sea anemone gets attached to the back of the hermit crab.

(c) The Mediterranean orchid, Ophrys, employs sexual deceit to get its flowers pollinated. In this orchid, one petal of the flower resembles the female of a bee species in size, colour, markings, etc. The male bee perceives it as a female and pseudo copulate with it. During the process, the pollen grains get dusted on its body.

Question 32.
It is observed that plant-animal interactions often involve co-evolution. Explain with the help of a suitable example. (CBSE Delhi 2019 C)
Or
Mention four significant services that a healthy forest ecosystem provides. (CBSE Delhi 2019)
Answer:
Plants and animals interact for mutual ben¬efit. Plant-animal interactions often involve co-evolution of mutualism, that is, the evolution of flower and its pollinator species are tightly linked with one another. In many species of fig trees, there is a tight one to one relationship with the pollinator species of wasp. It means that a given fig species can be pollinated only by its partner wasp species and no other species.

The female wasp uses the fruit not only at the oviposition site but also uses the developing seeds within the fruit for nourishing its larvae. The wasp pol¬linates the fig inflorescence while searching for suitable egg-laying sites. In return for the favour of pollination, the fig offers the wasp some of its developing seeds as food for the developing wasp larvae.

Question 33.
(i) Compare, giving reasons, the J-shaped and S-shaped models of population growth of a species.
Answer:
There are two models of population growth-exponential growth and logistic growth.
(a) Exponential growth: This growth occurs where the resources, i.e. food and space, are unlimited. The equation can be represented as follows:
\(\frac{\mathrm{dN}}{\mathrm{dt}}\) = (b-d) x N

Let (b-d) = r
\(\frac{\mathrm{dN}}{\mathrm{dt}}\) = rN or Nt=Noert

N = population size
Nt = population density after time t.
No = population density at time zero
r = growth rate
e = base of natural log (2.71828)
b = birth rate
d = death rate
In this growth, when N in relation to time is plotted on a graph, the curve becomes J-shaped.

(b) Logistics growth model: This is a realistic approach as the resources become limited at a certain point in time.

Class 12 Biology Important Questions Chapter 13 Organisms and Populations 8

(a) J-shaped curve exponential growth
(b) S-shaped curve logistics growth Every ecosystem has limited resources to support a particular maximum carrying capacity (K). When N is plotted in relation to time t, a sigmoid S-shaped curve is obtained. It is also called Verhulst-Pearl logistic growth.

The equation is:
\(\frac{\mathrm{dN}}{\mathrm{dt}}\)=rN\(\left(\frac{K-N}{K}\right)\)

N = population density at time t
r = growth rate
K = carrying capacity.

(ii) Explain “fitness of a species” as mentioned by Darwin. (CBSE 2017)
Answer:
According to Darwin “Fitness of a species” means reproductive fitness. All organisms after reaching reproductive age have a varying degree of reproductive potential. Some organisms produce more offspring, whereas some organisms produce few offspring only. This phenomenon is also called differential reproduction. The species which produces more offspring are selected by nature.

Question 34.
(i) The following are the responses of different animals to various abiotic factors. Describe each one with the help of an example.
(a) Regulate
Answer:
Regulate: Some organisms are able to maintain homoeostasis by regulating their body temperatures. The mechanisms used by most mammals to regulate their body temperature are similar to what we humans use.

For example, our body temperature remains constant at 37°C. In summer, when the outside temperature is more than our body temperature, we sweat profusely to cool down and when the temperature is much lower than 37° C, we shiver to generate heat. Thus body temperature remains constant.

(b) Conform
Answer:
Confirm: Most of the animals cannot maintain a constant internal environment. Their body temperature changes with the ambient temperature. These are called conformers. Heat loss or heat gain is a function of surface area. Since small animals have a larger surface area relative to their volume, they tend to lose body heat very fast when it is cold outside, for example, shrews and hummingbirds.

(c) Migrate
Answer:
Migrate: Organism can move away temporarily from the stressful habitat to a more hospitable area and then return when the stressful period is over.

For example, every winter, the famous Keoladeo National Park in Bharatpur hosts thousands of migratory birds coming from Siberia and other northern regions,

(d) Suspend
Answer:
Suspend: Bacteria, fungi and some lower plants survive unfavourable conditions by forming thick-walled spores. In higher plants, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress. They do so by reducing their metabolic activity and going into a state of dormancy.

(ii) If 8 individuals in a population of 80 butterflies die in a week, calculate the death rate of the population of butterflies during that period. (CBSE 2018)
Answer:
Death Rate = \(\frac{8}{80}\) = 0.1 per week

Question 35.
(a) Identify the features of a stable biological community.
Answer:
(a) Features of the stable biological community.

  • It is stable.
  • A stable biological community is not replaced by any other community.
  • The environment becomes moister and shadier.
  • Communities should have greater biodiversity for greater stability.
  • It should be able to prevent invasion by alien species.
  • It should be able to restore itself in a short period of time.

(b) How did David Tilman’s findings link the stability of a biological community to its species richness? (CBSE Delhi 2019 C)
Answer:
David Tilman’s with his experiments established that the stability of a community depends on its species richness.