CBSE Class 12 Biology Organisms and Populations Assignment Set A

POINTS TO REMEMBER

Adaptation : Any attributes of the organism (morphological, physiological, behavioural) that enables the organism to survive and reproduce in its habitat.

Aestivation : Strategy to escape in time during summers (summer sleep). E.g., Snails and some fishes.

Allen’s Rule : Mammals from colder climates generally have shorter ears and limbs to minimise heat loss.

Carrying Capacity : Maximum number of individuals of a population which can be provided with all the necessary resources for their healthy living.

Commensalism : One organism is benefitted while the other is neither harmed nor benefitted except to a negligible extent.

Competition : Rivalry between two organisms for obtaining the same resources.

Ectoparasite : Parasites which live on the surface of their host.

Emigration : Number of individuals of the population who have left the habitat and gone elsewhere during a given time period. Exponential Growth Curve : Shows that if food and space for a population are unlimited and each species has the ability to grow, then the population grows in exponential or geometric ratio.

Hibernation : Strategy to escape in time during winters (winter sleep). E.g., Polar bears.

Homeostasis : Maintaining constancy of internal environment despite varying external environmental conditions.

Immigration : Number of individuals of the same species that have come into the habitat from elsewhere during a given time period.

QUESTIONS

1. Which are the factor responsible for the wide variety of habitat formed within each biome?

2. Fresh water animals are unable to survive for long in sea water. Give reason.

3. With which population growth model is the Verhulst Pearl equation associated?

4. Define diapause. Which organisms exhibit it?

5. Calculate the death rate if 6 individuals in a laboratory population of 60 fruit flies died during a particular week.

6. In biological control method, one living organism is used against another to check its uncontrolled growth. Which kind of population interaction is involved in this?

7. An organism has to overcome stressful condition for a limited period of time. Which strategies can it adopt to do so?

8. Write what do phytophagous insects feed on?

9. What are the four levels of biological organisation with which ecology basically deals?

10. Differentiate between stenohaline and euryhaline organisms.

11. List four features which enable the Xeric plants to survive in the desert conditions.

12. Mention the attributes which a population has but not an individual organism.

13. Differentiate between stenothermal and eurythermal organisms. 

14. What are the four ways through which the living organisms respond to abiotic factors?

15. Why do clown fish and sea anemone pair up? What is this relationship called?

 

Important Questions for NCERT Class 12 Biology Organisms and Populations

Question. The famous 'Australian havoc' is associated with which of the following invasive species :-
(a) Nile pearch
(b) Princkly pear cactus
(c) Red fox
(d) Rabbit

Answer : B

Question. In rocky intertidal communities removal of which of the following predator became the cause of destruction of 10 species of invertebrates?
(a) Monarch butterfly
(b) Starfish pisater
(c) Paramecium aurelia
(d) Abingdon tortoise

Answer : B

Question. Behavioural response to cope with variations in the environment can be seen in
(a) CAM plants
(b) Opuntia plant
(c) Desert lizards
(d) C4 - plants

Answer : C

Question. Find out the correct match with reference to their habitat -
(a) Mango tree - Canada
(b) Snow leopards - Kerela forest
(c) Tuna fish - Temperate latitudes in oceans
(d) Lion - Gujarat

Answer : D

Question. Temperature is one of the important abiotic factor.Significance of temperature on living beings can be realised through -
(a) Kinetics of enzymes
(b) Basal metabolism
(c) Physiological functions
(d) All the above

Answer : D

Question. Population ecology is an important area of ecology because it links ecology with
(a) Population genetics
(b) Evolution
(c) Physiognomy
(d) Both 1 and 2

Answer : D

Question. The tiger counting in our national parks and tiger reserves is often based on
(a) Pug marks
(b) Manual counting
(c) Fecal pellets
(d) Both 1 and 3

Answer : D

Question. Match the following
(a) Breeding once in life (i) Mammals
(b) Breeding several times (ii) Oysters in life
(c) Large number of small (iii) Most of birds sized offsprings
(c) Small number of large (iv) Pacific salmon sized offsprings fish
(a) a(iv), b(ii), c(iii), d(i)
(b) a(iv), b(iii), c(ii), d(i)
(c) a(iii), b(iv), c(ii), d(i)
(d) a(ii), b(iii), c(iv), d(i)

Answer : B

Question. Annual migration does not occur in the case of
(a) arctic tern
(b) salmon
(c) siberian crane
(d) salamander. 

Answer: D

Question. Praying mantis is a good example of
(a) camouflage
(b) mullerian mimicry
(c) warning colouration
(d) social insects. 

Answer: A

Question. More than 70% of world’s freshwater is contained in
(a) polar ice
(b) glaciers and mountains
(c) Antarctica
(d) Greenland. 

Answer: A

Question. At which latitude, heat gain through insolation approximately equals heat loss through terrestrial radiation?
(a) 22 1/2 ° North and South
(b) 40° North and South
(c) 42 1/2 ° North and South
(d) 66° North and South 

Answer: B

Question. Animals have the innate ability to escape from predation. Examples for the same are given below.
Select the incorrect example.
(a) Colour change in Chamaeleon
(b) Enlargement of body size by swallowing air in puffer fish
(c) Poison fangs in snakes
(d) Melanism in moths 

Answer: C

Question. Which one of the following pairs is mismatched?
(a) Tundra – Permafrost
(b) Savanna – Acacia trees
(c) Prairie – Epiphytes
(d) Coniferous forest– Evergreen trees 

Answer: C

Question. In which one of the following pairs is the specific characteristic of a soil not correctly matched?
(a) Laterite – Contains aluminium compound
(b) Terra rosa – Most suitable for roses
(c) Chernozems – Richest soil in the world
(d) Black soil – Rich in calcium carbonate

Answer: D

Question. In which one of the following habitats does the diurnal temperature of soil surface vary most?
(a) Shrub land
(b) Forest
(c) Desert
(d) Grassland

Answer: C

Question. What is true for individuals of same species?
(a) Live in same niche
(b) Live in same habitat
(c) Interbreeding
(d) Live in different habitat 

Answer: C

Question. Cause of mimicry is
(a) concealment
(b) offence
(c) defence
(d) both (b) and (c).

Answer: D

Question. Which part of the world has a high density of organisms?
(a) Deciduous forests
(b) Tropical rainforests
(c) Grasslands
(d) Savannah 

Answer: B

Question. In desert grasslands, which type of animals are relatively more abundant?
(a) Aquatic
(b) Fossorial
(c) Diurnal
(d) Arboreal 

Answer: B

Question. Plants such as Prosopis, Acacia and Capparis represent examples of tropical
(a) deciduous forests
(b) evergreen forests
(c) grasslands
(d) thorn forests. 

Answer: D

Question. Which of the following communities is more vulnerable to invasion by outside animals and plants?
(a) Temperate forests
(b) Oceanic island communities
(c) Mangroves
(d) Tropical evergreen forests 

Answer: D

Question. During adverse season, therophytes survive by
(a) rhizomes
(b) seeds
(c) bulbs
(d) corms 

Answer: B

Question. Benthoic animals are those, which
(a) are submerged in area
(b) float on the sea surface
(c) are deep dweller in sea
(d) are floating (free) organisms.

Answer: C

Question. The ‘niche’ of a species is meant for
(a) habitat and specific functions of a species
(b) specific place where an organism lives
(c) specific species function and its competitive power
(d) none of these. 

Answer: A

Question. The animals of cold countries have relatively shorter and poorly developed ears, eyes, hairs and other phenotypic characters. This is known by which law?
(a) Cope’s Law
(b) Dollo’s Law
(c) Allen’s Law
(d) Bergamann’s Law

Answer: C

Question. Study of inter-relationships between organisms and their environment is
(a) ecology
(b) ecosystem
(c) phytogeography
(d) ethology. 

Answer: A

 

Very Short Answer Questions

Question. Why are cattle and goats not seen browsing on Calotropis growing in the fields?
Answer. Calotropis produces highly poisonous cardiac glycosides. Therefore, cattle and goats do not browse on them.

Question. When and why do some animals like snails go into aestivation?
Answer. Snails undergo aestivation if they are unable to migrate in order to avoid stressful condition of high temperature.

Question. What is mycorrhiza?
Answer. Mycorrhiza is a symbiotic association between a fungus and the roots of higher plants.

Question. Why are mammals the most successful animals on earth?
Answer. The mammals are most successful animals on earth because they can maintain a constant body temperature with high range of tolerance whether in Antarctica or in Sahara desert.

Question. Give an example of an organism that enters ‘diapause’ and why. 
Answer. Many species of Zooplankton under unfavourable conditions enters diapause which delay overall development and hence they can pass unfavourable conditions.

Question. What is a tree line? 
Answer. When we go up the altitude, beyond a particular height no tree are found and the vegetation comprise only of shrubs and herbs. The altitude beyond which no tree is seen is known as tree line.

Question. Why are green algae not likely to be found in the deepest strata of the ocean?
Answer. The wavelength of light at the deepest strata is unsuitable for growth of green algae.

Question. Why has life history of variation evolved? 
Answer. (a) Life history of variation has evolved.
(b) In order to maximise reproductive fitness of individual.

Question. What is the ecological principle behind the biological control method of managing with pest insects?
Answer. The ecological principle operating in the biological control method of managing with pest insect is through their natural enemies, i.e., predators and parasites.

Question. Name the interaction between sea anemone and the hermit crab that grows on it.
Answer. Commensalism

Question. State Gause’s Competitive Exclusion principle. 
Answer. Gause’s Competitive Exclusion Principle states that two closely related species competing for same resources, cannot coexist indefinitely, (the inferior will be eliminated) by the superior one.

Question. Why has life history of variation evolved?
Answer. (a) Life history of variation has evolved.
(b) In order to maximise reproductive fitness of individual.

Question. Name the interaction between a whale and the barnacles growing on its back. 
Answer. Commensalism

Question. State Gause’s Competitive Exclusion principle. 
Answer. Gause’s Competitive Exclusion Principle states that two closely related species competing for same resources, cannot coexist indefinitely, (the inferior will be eliminated) by the superior one.

Question. What does nature’s carrying capacity for a species indicate?
Answer. In nature, a given habitat has enough or limited resources to support a maximum possible number of population and nature’s carrying capacity indicates that how much growth is possible in a population. 

Question. Name two organisms (one plant and one animal) which breed only once in their life time.
Answer. Pacific salmon fish and bamboo.

Question. Write the basis on which an organism occupies a space in its community/natural surroundings.
Answer. Feeding relationships with other organisms.

Question. Why do predators avoid eating Monarch butterfly? How does the butterfly develop this protective feature? 
Answer. The Monarch butterfly is highly distasteful to its predator (birds) because of a special chemical present in its body. It acquires this chemical during its caterpillar stage by feeding on a poisonous weed.

Short Answer Questions

Question. In certain seasons we sweat profusely while in some other season we shiver. Explain.
Answer. Human beings maintain a constant body temperature of 37°C. OU In summers: The outside temperature is much higher than our body temperature. Therefore, we sweat profusely. This results in evaporative cooling and our body temperature is brought down to normal (37°C). OU In winters: The outside temperature is much lower than our body temperature. Therefore, we start to shiver; this action (of shivering) is a kind of exercise (work) that produces heat and raises the body temperature.

Question. Explain Verhulst-Pearl Logistic Growth of a population. 
Answer. According to Verhulst–Pearl Logistic growth, a population growing in a habitat with limited resources initially shows a lag phase, followed by phases of acceleration and deceleration and finally an asymptote when the population density reaches the carrying capacity. It is given by the following equation:
dN /dt = rN [K-N/K]
where, N = population density at time t, r = intrinsic rate of natural increase, K = carrying capacity.

Question. Why are small animals rarely found in the polar regions? Explain.
                                                      OR 
Why are small birds like humming birds not found in polar regions? Explain. 
Answer. Small animals like humming birds have a large surface area relative to their volume. So they tend to lose body heat very fast when it is cold outside. Then, these animals have to use their energy (generated by metabolic reactions) to generate body heat. That is the reason why small sized animals are rarely found in the polar regions.

Question. Co-evolution is a spectacular example of mutualism between an animal and a plant. Describe co-evolution with the help of an example. 
                                                                                                                        OR
Describe the mutual relationship between fig tree and wasp and comment on the phenomenon that operates in their relationship. 
Answer. Co-evolution can be observed in Fig (plant) and wasp (animal). The female wasp uses the fruit for oviposition or egg laying. It also uses developing seeds within the fruit for nourishing its larvae.
The wasp in turn pollinates the fig inflorescence. The given Fig species can be pollinated by its ‘partner’ wasp species and no other species.

Question. Many fresh water animals cannot survive in marine environment. Explain.
Answer. Marine environment has high salt concentration. These fresh water animals in such hypertonic surroundings suffer from osmotic problems. Their bodies start losing water by exosmosis.

Question. How do seals adapt to their natural habitat? Explain. 
Answer. Seals adapt to the cold climate by developing a thick layer of fat (blubber) below their skin that acts as an insulator and reduce excess loss of body heat.

Question. Some organisms suspend their metabolic activities to survive in unfavourable conditions. Explain with the help of any four examples. 
Answer. (i) Polar bear: They hibernate during winter to escape the cold weather.
(ii) Snails/fishes: They go into aestivation during summer to avoid heat related problems and desiccation.
(iii) Seeds of higher plants/spores of bacteria/fungi: They become dormant in unfavourable conditions and in case of Amoeba cyst formation takes place.
(iv) Some species of zooplankton: They undergo diapause.

Question. How does a desert plant adapt to the dry, warmer environmental conditions?
Answer. Adaptations of a desert plant:
(i) Leaf surface has a thick cuticle.
(ii) Stomata are situated in deep pits.
(iii) Stem is flattened and performs photosynthesis.
(iv) Leaves are modified into spines aOs puinn tia. (Any two)

Question. Why do clown fish and sea anemone pair up? What is this relationship called?
Answer. The clown fish gets protection from predators which stay away from stinging tentacles of anemone but anemone does not derive any benefit from the fish. This relationship is called commensalism.

Question. (a) Explain “birth rate” in a population by taking a suitable example.
(b) Write the other two characteristics which only a population shows but an individual
Answer. (a) Birth rate is expressed as the number of births per 1,000 individuals of a population per year.
For example, in a pond there were 200 frogs and 40 more were born in a year. Then, the birth rate of the population will be 40/200 = 0.5 frogs per year.
(b) Sex ratio, age distribution, population density, population growth. (Any two)

Question. How is the normal human body temperature of 37°C maintained during (i) Summer, and
(ii) Winter? Explain.
Answer. Human beings maintain a constant temperature of 37°C.
In summers, the outside temperature is much higher than our body temperature. Therefore, we sweat profusely. This results in evaporative cooling, and our body temperature is reduced to normal.
In winters, the outside temperature is much lower than our body temperature. Therefore, we shiver, which is a kind of exercise, which produces heat and raises the body temperature. 

Question. Plants that inhabit a rain-forest are not found in a wetland. Explain.  
Answer. Plants that inhabit a rain forest are not adapted to survive in aquatic conditions / wetlands.
Plants are conformers / stenothermal / cannot maintain constant internal environment/ temperature / osmotic concentration of the body fluids affects kinetics of enzymes through basal metabolism / activity and other physiological functions of the organisms 

Question. If in a population of size ‘N’ the birth rate is represented as ‘b’ and the death rate as ‘d’, the increase or decrease in ‘N’ during a unit time t’ will be :
dN/dt = (b – d) × N
The equation given above can also be represented as :
dN/dt = r × N where r = (b – d)
What does ‘r’ represent ? Write any one significance of calculating ‘r’ for any population. 
Answer. r = intrinsic rate of natural increase; it is an important parameter for assessing the impacts of any biotic or abiotic factor on population growth .

Question. Why are the plants that inhabit a desert are not found in a mangrove? Give reasons.
Answer. Desert plants are not adapted to survive in saline / aquatic conditions.  Plants are conformers / stenothermal / cannot maintain constant internal environment / temperature / osmotic concentration of the body fluids affects kinetics of enzymes through basal metabolism / activity and other physiological functions of the organisms. 

Question. What is mutualism ? Mention any two examples where the organisms involved are commercially exploited in agriculture.
Answer. Interaction between two species in which both are benefitted
(i) Rhizobium in the roots (nodules) of legumes
(ii) Mycorrhiza / Glomus with the roots of higher plants.

Question. Why do people suffer from altitude sickness after reaching the high altitude regions ? How does their body acclimatize after a couple of days ? 
Answer. Altitude sickness” is because of low atmospheric pressure at high altitude, the body does not get sufficient oxygen.
The body compensates low oxygen availability by increasing RBCs production, decreasing the binding capacity of haemoglobin, by increasing breathing rate. 

Question. Many freshwater animals cannot survive in marine environment. Explain. 
Answer. Freshwater animals are not able to maintain their osmotic concentration in marine conditions. If they are transferred in marine conditions, their body will shrink due to exosmosis. Hence, they cannot survive in marine environment. 

Question. Shark is eurythermal while polar bear is stenothermal. What advantage does the former have and what is the constraint the latter has ?
Answer. Shark : Tolerates wide range of temperature so widespread / survives in all waters.
Polar bear : Restricted occurrence in a narrow range of temperature so constraint to live in a very cold icy environment. 

Question. Apart from being part of the food chain, predators play other important roles. Mention any two such roles supported by examples.
Answer. (i) Predators act as conduits for energy transfer across trophic levels.
(ii) They keep prey population under control.
(iii) They help in maintaining species diversity in a community by reducing intensity of competition among competing prey species.
(iv) An efficient predator may cause extinction of prey species. 

Question. How do mammals living in colder regions and seals living in polar regions able to reduce the loss of their body heat ?
Answer. Mammals from colder climates generally have shorter ears and limbs which minimise heat loss (Allen’s rule).
In the polar region, seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduce loss of body heat. 

Question. How do kangaroo rats and desert plants adapt themselves to survive in their extreme habitat? Explain.
Answer. Kangaroo rats : Internal fat oxidation where water is a byproduct, excretes concentrated urine. 
Desert Plants : Thick cuticle / sunken stomata / leaves reduced to spines / deep roots / Special photosynthetic pathway / CAM (Any four) 

Question. A moss plant is unable to complete its life cycle in a dry environment. State two reasons.
Answer. Mosses cannot complete their life cycle in a dry environment because of the following reasons :
(i) They need water for sexual reproduction as water acts as a medium for flagellated sperm to reach the egg and undergo fertilization.
(ii) Since their roots are rudimentary, they cannot absorb water. Therefore, they need to grow in a moist environment for their survival.

Question. Heat loss or heat gain depends upon the surface area of the organism’s body. Explain with the help of a suitable example.
Answer. Small animal such as humming bird / shrew, have a larger surface area relative to their volume, they tend to lose heat when it is cold outside, hence spend much energy to generate body heat through metabolism. 

Question. Are hummingbirds and fish, regulators or conformers ? Give reasons in support of your answer. 
Answer. Conformers.
Heat loss or 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, they have
to expend much energy, to generate body heat through metabolism. (cannot maintain a constant body temperature).

Question. Explain by taking three different examples how do certain organisms pull through the adverse conditions when unable to migrate under stressful period.
Answer. Hibernation—winter sleep to escape cold weather e.g., bears.
Aestivation—summer sleep to avoid heat and desiccation e.g., snails / fish.
Diapause—suspended development / activity e.g., zooplanktons.
Spore formation—to tide over unfavourable conditions e.g., fungi / bacteria / lower plants.
Dormancy—By reducing metabolic activity e.g., seeds.

Question. Highlight the differences between the population interactions given below. Give an example of each. 
(i) Parasitism
(ii) Amensalism
(iii) Mutualism
Answer. Parasitism : Only one species benefits e.g.,
Cuscuta / Tapeworm 
Amensalism : One species is harmed whereas the other is unaffected e.g., Penicillium growing on bacterial culture / Trichoderma - biological control agent and plant pathogen 
Mutualism : Both species are benefitted e.g., lichens exhibit mutualistic relationship with fungus that absorbs water and nutrients from soil and photosynthesizing algae / cyanobacteria

Question. When do you describe the relationship between two organisms as mutualistic, competitive and parasitic ? Give one example of each type.
Answer. Mutualistic : Both the interacting organisms are benefitted from each other e.g., Lichens - Algae and fungi mutually help each other. (any other appropriate example). 
Competition : When two organisms belonging to closely related species / unrelated groups compete for the same resources that are limited, both are
losers e.g., superior barnacle dominates and excludes the small barnacles / in some South American lakes visiting flamingoes and resident fishes compete for their common food (zooplankton) in the lake / any other appropriate example. 
Parasitic : One of the two organisms is dependent on the other (host) for nutrition and support / the host is harmed and the parasite is benefited e.g.,
Malarial parasite and human / Cuscuta on host plant / or any other appropriate example. 

Question. In certain seasons we sweat profusely while in some other season we shiver. Explain.
Answer. To regulate body temperature. 1
In summer, outside temperature is higher than body temperature, sweating causes cooling by evaporation of sweat and thus lowering the body temperature. 
In winter, outside temperature is much lower than body temperature, shivering is an (involuntary) exercise which produces heat and raises the body temperature in winter.

Question. Name and explain the type of interaction that exists in mycorrhizae and between cattle egret and cattle. 
Answer. Mycorrhiza is an association between fungi and the roots of higher plants. It is an example of mutualism in which both fungi and plants are dependent on each other for nutritional needs.
The fungi help the plant in the absorption of essential nutrients from the soil while the plant provides the fungi with carbohydrates.
The interaction that exists between cattle egret and cattle is known as commensalism. In this type of interaction, one species is benefitted whereas the other is neither benefitted nor harmed. The cattle egret (bird) usually forages near the grazing cattle. As cattle moves in the grass they stir up the grass and flush out the insects which then become an easy target for the egret. In this way, the cattle are neither benefitted nor harmed but the egret is benefitted.

Long Answer Questions

Question. Differentiate between mutualism, parasitism and commensalism. Provide one example for each of them. 
Answer. Predation
• It is an interspecific interaction, where an animal, called predator, kills and consumes the other weaker animal called prey.
• Predation is nature’s way of transferring energy to higher trophic levels, e.g., a tiger (predator) eating a deer (prey), a sparrow (predator) eating fruit or seed (prey), etc.
Commensalism
• Commensalism is referred to as the interaction between two species where one species is benefited and the other is neither harmed nor benefited.
Mutualism
• Mutualism is referred to as the interspecific interaction in which both the interacting species are benefited.
• Some examples of mutualism

Question. (a) Following are the responses of different animals to various ecological factors. Describe each one with the help of an example
(i) Regulate
(ii) Conform
(iii) Migrate
(iv) Suspend
(b) If 18 individuals in a population of 80 butterflies die in a week, calculate the death rate of population of butterflies during the period. 
Answer. (a) (i) Regulate : Maintain constant internal temperature / osmotic concentration / homeostasis.
e.g., birds / mammals.
(ii) Conform : Do not maintain constant internal temperature / osmotic concentration / No homeostasis.
e.g., any one example of animal other than birds and mammals.
(iii) Migrate : Temporary movement of organisms from the stressful habitats to hospitable areas and return when stressful period is over.
example : Bar headed geese
(iv) Suspend : Reducing / minimising the metabolic activities during unfavourable conditions.
e.g. Polar bear / amphibian / snails / fish / any other example of animals .
(b) Death rate = 18/80 = 0.225 therefore, death rate percentage will be = 0.225 × 100 = 22.5% butterfly death per week

Question. “The population of a metro city experiences fluctuations in it’s population density over a period of time.”
(a) When does the population in a metro city tend to increase?
(b) When does the population in metro city tend to decline?
(c) If ‘N’ is the population density at the time ‘t’, write the population density at the time
Answer. (a) Population in a metro city will tend to increase when natality and immigration will be higher.
(b) Population in metro city will tend to decline when mortality and emigration will be higher.
(c) The equation Nt+1 = Nt + [(B + I) – (D + E)] represents the population density at time t + 1.
Here,
Nt+1 = Population density at time t+1.
Nt = Population density at time t.
B = Natality
I = Immigration
D = Mortality
E = Emigration

Question. Mention how have plants developed mechanical and chemical defence against herbivores to protect themselves with the help of one example of each.
Answer. Several plants have evolved various mechanisms both morphological and chemical to protect themselves against herbivores.
Morphological defence mechanisms :
• Cactus plants (Opuntia) are modified into sharp spines (thorns) to deter herbivores from feeding on them.
• Sharp thorns along with leaves are present in Acacia to deter herbivores.
• In some plants, the margins of their leaves are spiny or have sharp edges that prevent
herbivores from feeding on them. (Any one)
Chemical defence mechanisms :
• All parts of Caloptropis weeds contain toxic glycosides, which can prove to be fatal if ingested by herbivores.
• Chemical substances such as nicotine, caffeine, quinine and opium are produced in plants as a part of self-defence.

Question. (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 ?
                                               OR
Why are certain organisms called regulators or conformers ? Explain with the help of one example of each.
Answer. (i) Regulators maintain / regulate constant body temperature irrespective of external condition. 
Conformers changing / varying body
temperature as per external condition. Regulators : Humans maintain a constant body temperature of 37°C. In summer when temperature is more outside we sweat, when evaporates causes cooling / in winter when outside temperature is much lower than 37°C, we start shivering / a kind of exercise which produce heat and raises body temperature (any other suitable appropriate example). Conformers : In aquatic animals, the osmotic concentration of body fluid changes with that of the ambient water osmotic concentration / Internal body temperature of reptiles, amphibians, fishes change with that of external temperature. (Any other suitable appropriate example.) 

Question. Mention the special adaptations evolved in parasites and why ? 
Answer. Loss of unnecessary sense organs, since they do not interact with external environment (e.g., lacks eyes as they are found in an environment that lacks light).
- Presence of adhesive organs / suckers / hooks, to cling to the host. 
- Loss of digestive system , to absorb (digested) food from the host body. 
- High reproductive capacity , to increase the chances of the survival.
- If the host evolves special mechanism for resisting or rejecting the parasite
- the parasite also evolves mechanism to counteract and neutralise them , in order to be successful with the same host species.
- Presence of more than one host , to facilitate parasitisation of its primary host. 
- Loss of chlorophyll & leaves (Cuscuta) , to derive its nutrition from the host plant which it parasitise. 
- Eggs resembles the host egg (crow) in size and colour , to reduce the chances of host bird detecting / ejecting the foreign eggs (koel)

Question. (i) What is an age-pyramid ?
(ii) Name three representative kinds of agepyramids for human population and list the characteristics for each one of them.
                    OR
(i) What is an age-pyramid ?
(ii) Explain with the help of figures the three different types of age pyramids represented by a human population.
Answer. (i) If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called the age pyramid.
(ii) Expanding : Pre-reproductive population is greater than reproductive or post reproductive population / growing with maximum no. of individuals in pre-reproductive phase and least no. in post reproductive phase.
Stable : Pre-reproductive and reproductive population are almost similar / ideal for population / maintains balanced continuity / no. of individuals in reproductive phase is almost same, less no. of individuals in post reproductive phase.
Declining : Pre-reproductive population is less than reproductive population / less no. of individuals in pre reproductive phase than reproductive phase. 
                                            OR
In lieu of the above explanation the following diagram can be considered.
Diagram : Refer Topic 2/ Revision Notes/
Important diagrams/ Fig 5.3. 

Question. (a) What is “population” according to you as a biology student?
(b) “The size of a population for any species is not a static parameter.” Justify the statement with specific reference to fluctuations in the population density of a region in a given period of time. 
Answer. (a) The total number of organisms of a species in a particular area at a particular time. 1
(b) The size of a population for any species is not a static parameter because of the factors like :
• Birth rate/ Natality, number of births during a given period 
• Death rate/ Mortality, number of deaths during a given period 
• Immigration, number of individuals of the same species that have come into the habitat from elsewhere during the period under consideration
• Emigration, number of individuals of the population who left the habitat and gone elsewhere during the period under consideration

Question. (i) What is population density ? Why are ecologists interested in measuring it ?
(ii) Write the different ways of measuring population density. Explain any two with the help of specific examples.
Answer. (i) The population density is the number of individuals of a population found per unit area at a given time. 
Whatever ecological process we wish to investigate in a population (competition / pesticide applicable) we always evaluate in terms of any change inpopulation size (numbers / biomass). 
(ii) Number of organism  Biomass of organism
Example : Three ways of measuring population density of a habitat
(a) Per cent cover for trees with larger canopy.
(b) Number of fishes caught per trap.
(c) Pug marks or faecal pellets for tiger census.

Question. (a) Why are herbivores considered similar to predators in the ecological context? Explain.
(b) Differentiate between the following interspecific interactions in a population:
(i) Mutualism and Competition (ii) Commensalism and Amensalism 
Answer. (a) Herbivores are animals feeding on plants. Although they are classed differently they are considered predators. Like predators, for transfer of energy across trophic levels, herbivores also do the same. Besides this, they also keep the population of their prey under control. For example, when the prickly pear cactus was introduced in Australia in early 1920s, they spread rapidly, causing havoc. Their population was controlled by introducing cactusfeeding predator (a moth).
(b) (i) Differences between mutualism and competition

(ii) Commensalism and ammensalism Refer to Basic concept 1i3v ) (and 13v )

Question. (a) Explain any two defence mechanisms plants have evolved against their predators.
(b) How does predation differ from parasitism?
Answer. (a) Plants have developed the following defence mechanisms:
(i) Thorns as means of defence.
(ii) Plants may produce chemicals such as nicotine, caffeine, quinine, strychnine, opium for defence.
(b) Table 13.8 Differences between predation and parasitism

Question. How do organisms like fungi, zooplanktons and bears overcome the temporary short-lived climatic stressful conditions? Explain.
Answer. Fungi form thick-walled spores which help them survive in unfavourable conditions. On availability of suitable environment, these germinate. Zooplanktons in lakes and ponds under unfavourable conditions, enter diapause, a stage of suspended development.
Bears in extreme low temperatures, escape winter time by hibernating.

Question. Highlight the differences and a similarity between the following population interactions: Competition, predation and commensalism.
Answer. Table 13.9 Differences between competition, predation and commensalism

Similarity: All these interactions leads to evolution as the fittest organism survives.

Question. Study the graph given alongside and answer the questions which follow:
(i) The curve ‘b’ is described by the following equation:
dN/dt = rN [K-N/K]

What does ‘K’ stand for in this equation? Mention its significance.
(ii) Which one of the two curves is considered a more realistic
one for most of the animal populations?
(iii) Which curve would depict the population of a species of deer if there are no predators in the habitat? Why is it so?
Answer. (i) ‘K’ stands for carrying capacity. Carrying capacity is defined as the maximum number of individuals of a population that can be sustained by the given habitat/environment.
(ii) Curve b‘’ is more realistic.
(iii) Curve ‘a’. When the predators are absent, the prey population grows exponentially.

Question. Interspecific interactions of two species of any population may be beneficial, detrimental or neutral. Explain each of them with the help of suitable examples.
Answer. Population interactions:

i) + = Beneficial; – Detrimental; 0 Neutral. Both species ben, ee.fig.t licihne nsmutualism
(ii) Both species lose in competition, e.g. (detrimental)
(iii) In predation and parasitism the predator and the parasite gets benefit but it is detrimental to the other species (host and prey respectively)
(iv) In commensalism one species is benefitted but the other is neither harmed nor benefitted.

Question. (a) List the different attributes that a population has and not an individual organism.
(b) What is population density? Explain any three different ways the population density can be measured, with the help of an example each.
Answer. (a) Attributes of population Birth rate, death rate, sex ratio, age pyramids/age distribution. (Any two)
(b) Population density: Number of individuals per unit area at a given time/period.
(i) Biomass/%Cover, e.g., Hundred Parthenium plants and 1 huge banayan tree.
(ii) Relative Density, e.g., Number of fish caught per trap from a lake.
(iii) Numbers, e.g., Human population
(iv) Indirect estimation, e.g., without actually counting/seeing them, e.g., tiger census based on pug marks and faecal pellets. (Any three)

Question.

(a) Which of the above represents the increase or decrease of population?
(b) If N is the population density at time t, then what would be its density at time (t+1)? Give the formula.
(c) In a barn there were 30 rats. 5 more rats enter the barn and 6 out of the total rats were eaten by the cats. If 8 rats were born during the time period under consideration and
7 rats left the barn, find out the resultant population at time (t+1).
(d) If a new habitat is just being colonized, out of the four factors affecting the population growth, which factor contributes the most?
Answer. (a) a and d represents increase of population and b and c represent decrease of population.
(b) Nt+1 = Nt + [(B + I) – (D + E)]
(c) Here, Nt = 30; I = 5; E = 7 ; D = 6; B = 8
Putting the value in Nt+1 = Nt + [(B + I) – (D + E)]
Nt+1 = 30 + [(8 + 5) – (6 + 7)]
= 30 + [13 – 13]
= 30 + 0
= 30 rats
(d) Immigration contributes the most.

Question. The following diagrams are the age pyramids of different populations. Comment on the status of these populations.

Answer. Fig. A: It is a ‘pyramid’ shaped age pyramid. In this figure, the base, i.e., pre-reproductive stage is very large as compared with the reproductive and post-reproductive stages of the population.
This type of age structure indicate that the population would increase rapidly.
Fig. B: It is an ‘inverted bell’ shaped pyramid. In this figure, the pre-reproductive and reproductive stages are same. This type of age structure indicates that the population is stable.
Fig. C: It is ‘urn’ shaped pyramid. In this figure, the pre-reproductive and reproductive stages are less than the post-reproductive stage of this population. In this population more older people are present. This type of age structure indicates that the population definitely is declining.

Question. List the various abiotic environmental factors.
Answer. (i) Atmospheric factors: Light, temperature, wind and water.
(ii) Lithosphere: Rock, soil.
(iii) Hydrosphere: Pond, river, lake and ocean.
(iv) Edaphic factors: Soil texture, soil water, soil air, soil micro-organisms, soil pH, minerals.
(v) Topographic factors: Slope, altitude, valley.

Question. (a) Write the importance of measuring the size of a population in a habitat or an ecosystem.
(b) Explain with the help of an example how the percentage cover is a more meaningful measure of population size than mere numbers. 
Answer. (a) By measuring the size of a population, following can be predicted:
(i) Status of the population in a habitat.
(ii) Outcome of competition with other species.
(iii) Impact of predator or pesticides.
(iv) Increase or decrease of population size.
(iv) Effect of pesticide application (Any two)
(b) Example: Banyan tree and Parthenium plants.
When 1 banyan tree is compared with 100 Parthenium plants, the population of banyan in terms of number is very much low as compared to Parthenium. But in terms of percentage cover or biomass, the banyan tree provides a much larger cover in comparison to 100 Parthenium plants. Thus, the percentage cover or biomass is a more meaningful measure of population size.

Question. How do organisms cope with stressful external environmental conditions which are localised or of short duration?
Answer. The following methods are employed by organisms to cope with stressful conditions:
(i) Migrate temporarily from the stressful habitat to a hospitable area,
(ii) suspend activities,
(iii) form thick walled spores,
(iv) form dormant seeds,
(v) hibernate during winter,
(vi) aestivate during summer,
(vii) planktons undergo diapause. 

Question. Describe the specific adaptation of xerophytes with respect to root system, stem and leaves.
Answer. (i) The root system is deep-rooted, that help to reach up to the level of water table.
(ii) Stem become modified into fleshy, spongy flat and green structure called phylloclade. Stem can perform photosynthesis and store water.
(iii) Leaves have waxy layer called cuticle, contain sunken stomata and leaves are modified into hard pointed spines to reduce transpiration. Leaves become fleshy in succulents.

Question. (a) “Organisms may be conformers or regulators.” Explain this statement and give one example of each.
(b) Why are there more conformers than regulators in the animal world? 
Answer. (a) Conformers are organisms which cannot maintain a constant internal environment under varying external environmental conditions. They change body temperature and osmotic concentration with change in external environment. For example, all plants, fishes, amphibians etc.
Regulators are organisms which can maintain homoeostasis (by physiological means or behavioural means) i.e., they maintain constant body temperature and osmotic concentration.
For example, birds and mammals.
(b) Thermoregulation is energetically expensive for animals. Therefore, more conformers are found.

Question. (a) State how the constant internal/environment is beneficial to organisms.
(b) Explain any two alternatives by which organisms can overcome stressful external conditions.
Answer. (a) Constant internal environment permits all biochemical reaction and physiological functions to proceed with maximal efficiency and thus enhance the overall fitness of the species.
(b) Organisms can overcome stressful external conditions with the following ways:
(i) Regulation: Maintaining internal environment by maintaining constant body temperature or osmotic concentration.
(ii) Suspend (conform): By suspending metabolic activities through hibernation or aestivation or diapause.
(iii) Migration: Organisms migrate temporarily to more hospitable areas

Question. Interspecific interactions of two species of any population may be beneficial, detrimental or neutral. Explain each of them with the help of suitable examples. 
Answer. Population interactions: 

Species A	Species B	Name of interaction
+	+	Mutualism
–	–	Competition
+	–	Predation
+	–	Parasitism
+	0	Commensalism

(i) + = Beneficial; – Detrimental; 0 Neutral. Both species benefit in mutualism, e.g. lichens
(ii) Both species lose in competition, e.g. (detrimental)
(iii) In predation and parasitism the predator and the parasite gets benefit but it is detrimental to the other species (host and prey respectively)
(iv) In commensalism one species is benefitted but the other is neither harmed nor benefitted. 

Question. Different animals respond to changes in their surroundings in different ways. Taking one example each, explain “some animals undergo aestivation while some others hibernate”. How do fungi respond to adverse climatic conditions? 
Answer. Some animals go into aestivation to avoid summer problems like, heat and dessication. For example, snails and fish.Some animals go into hibernation to avoid winter related problem like, extreme cold. For example, bear.Fungi form thick walled spores and suspend their activities to respond to adverse climatic conditions.

Question. (a) List any three ways of measuring population density of a habitat.
(b) Mention the essential information that can be obtained by studying the population density of an organism. 
Answer. (a) By physical counting, percent cover or total biomass, from relative density, counting pugmarks, counting faecal pellets. (Any three)
(b) Status of habitat, whether competition for survival exists or not, whether population is increasing or declining, natality, mortality, emigration, immigration.

Question. (a) Explain any two defence mechanisms plants have evolved against their predators.
(b) How does predation differ from parasitism?
Answer. (a) Plants have developed the following defence mechanisms:
(i) Thorns as means of defence.
(ii) Plants may produce chemicals such as nicotine, caffeine, quinine, strychnine, opium for defence.
(b) Differences between predation and parasitism

Predation	Parasitism
The predator only feeds on prey.	The parasite lives and feeds on the host.
The predator is not prey specific.	The parasite is host specific.
The predator keeps a check on prey population.	The parasite coevolves with the host.

Question. Highlight the differences and a similarity between the following population interactions:
Competition, predation and commensalism.
Answer. Differences between competition, predation and commensalism

Competition	Predation	Commensalism
In this type of interaction both the species suffer.	In this type of interaction the predator kills and consumes the prey.	In this type of interaction one species is benefited and the other is neither harmed nor benefited.
It occurs due to limited resources between closely related species.	It is the nature’s way of transferring energy to higher trophic level.	It is not particularly for any gain of energy or resources.
For example, In American lakes visiting flamingos and resident fish.	For example, tiger (predator) and deer (prey).	For example, sucker fish and shark.

Question. (a) List the biotic components an organism interacts with in its natural habitat.
(b) Mention how have organisms optimised their survival and reproduction in a habitat.
Answer. (a) Plants, animals and microorganisms.
(b) (i) Some organisms regulate to maintain homeostasis by physiological and behavioural means.
(ii) In some animals and plants the osmotic concentration of the body fluids change with that of the ambient water osmotic concentration (Conform).
(iii) Some animals migrate to avoid unfavourable conditions.
(iv) Some bacteria, fungi and lower plants, under unfavourable conditions slow down metabolic rate and form thick-walled spores to overcome stressful conditions (Suspend).

Question. Comment on the following diagrams:
A, B, C, D, G, P, Q, R, S are species.

Answer. Fig. I: It is a single population and all individuals are of the same species, i.e., A individuals interact among themselves and their environment.
Fig. II: It is a community and it contains three populations of species A, B and C. They interact with each other and their environment.
Fig. III: It is a biome. It contains three communities of which one is in climax and other two are in different stage of development. All three communities are in the same environment and they interact with each other and their environment.

Question. (a) Explain giving reasons why the tourists visiting Rohtang Pass or Mansarovar are advised to resume normal ‘active life’ only after a few days of reaching there.
(b) It is impossible to find small animals in the polar regions. Give reasons. 
Answer. (a) Initially the person suffers from altitude sickness/nausea, fatigue and heart palpitation because of low oxygen availability and low atmospheric pressure. Gradually the body increases RBC production, decreasing binding capacity of Hb and increases the breathing rate to get acclimatised.
(b) Small animals have larger surface area relative to their volume, so they lose heat much faster, & have to spend more energy to generate body heat.

Please refer to attached file for CBSE Class 12 Biology Organisms and Populations Assignment Set A