CBSE Class 12 Biology Organisms and Populations Assignment Set B

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

Important Questions for NCERT Class 12 Biology Organisms and Populations

Question. Match the following given population interactions
(a) + / + (i) Predation
(b) - / - (ii) Ammensalism
(c) + / - (iii) Competition
(c) - / 0 (iv) Mutualism
(a) a(i), b(ii), c(iii), d(iv)
(b) a(i), b(iii), c(ii), d(iv)
(c) a(iv), b(iii), c(i), d(ii)
(d) a(iv), b(iii), c(ii), d(i)

Answer : C

Question. Phytophagous insects show which of the following interaction :-
(a) Predation
(b) Competetion
(c) Mutualism
(d) Commensalism

Answer : A

Question. Which of the following alternative used by zooplanktons to overcome partial stressful conditions ?
(a) Migration
(b) Diapause
(c) Hibernation
(d) Aestivation

Answer : B

Question. Majority of plants belongs to which of the following category
(a) Regulators
(b) Conformers
(c) Partial regulators
(d) Eurytherms

Answer : B

Question. Temperature is the most ecologically relevant environmental factor. In which of the following habitats temperature can exceed 100° C ?
(a) Tropical desert
(b) Thermal springs
(c) Deep sea hydrothermal vents
(d) Both 2 and 3 

Answer : D

Question. Next to temperature, water is the most important factor influencing the life of organism. Which among the following water characteristics is not an influencing character?
(a) pH
(b) Turbidity
(c) Colour
(d) Salinity

Answer : C

Question. Which of the following cannot be used by prey for defence against predator :-
(a) Cardiac glycosides
(b) Strychnine
(c) Nectar
(d) Quinine

Answer : C

Question. Regarding competition find out the wrong statement.
(a) Unrelated species could compete for same resource
(b) Fitness of one species is lowered in presence of other species
(c) Abingdon tortoise become extinct due to competitor starfish
(d) Balanus leads to exclusion of Chathamalus from rock coasts of scotland

Answer : C

Ques. Niche is
(a) all the biological factors in the organism’s environment
(b) the physical space where an organism lives
(c) the range of temperature that the organism needs to live
(d) the functional role played by the organism where it lives. 

Answer: D

Ques. Pneumatophores occur in
(a) halophytes
(b) free-floating hydrophytes
(c) carnivorous plants
(d) submerged hydrophytes. 

Answer: A

Ques. Plants which produce characteristic pneumatophores and show vivipary belong to
(a) halophytes
(b) psammophytes
(c) hydrophytes
(d) mesophytes.

Answer: A

Ques. Presence of plants arranged into well defined vertical layers depending on their height can be seen best in
(a) tropical rainforest
(b) grassland
(c) temperate forest
(d) tropical savannah.

Answer: A

Ques. It is much easier for a small animal to run uphill than for a large animal, because
(a) small animals have a lower O2 requirement
(b) the efficiency of muscles in large animals is less than in the small animals
(c) it is easier to carry a small body weight
(d) smaller animals have a higher metabolic rate.

Answer: D

Ques. An association of individuals of different species living in the same habitat and having functional interactions is
(a) ecosystem
(b) population
(c) ecological niche
(d) biotic community.

Answer: D

Ques. Most animals are tree dwellers in a
(a) temperate deciduous forest
(b) tropical rainforest
(c) coniferous forest
(d) thorn woodland. 

Answer: B

Ques. Just as a person moving from Delhi to Shimla to escape the heat for the duration of hot summer, thousands of migratory birds from Siberia and other extremely cold northern regions move to
(a) Western Ghat
(b) Meghalaya
(c) Corbett National Park
(d) Keoladeo National Park. 

Answer: D

Ques. Which one of the following is not a parasitic adaptation?
(a) Development of adhesive organs
(b) Loss of digestive organs
(c) Loss of reproductive capacity
(d) Loss of unnecessary sense organs

Answer: C

Ques. Benthic organisms are affected the most by
(a) light reaching the forest floor
(b) surface turbulence of water
(c) sediment characteristics of aquatic ecosystems
(d) water-holding capacity of soil.

Answer: C

Ques. People who have migrated from the planes to an area adjoining Rohtang Pass about six months back
(a) have more RBCs and their haemoglobin has a lower binding affinity to O2
(b) are not physically fit to play games like football
(c) suffer from altitude sickness with symptoms like nausea, fatigue, etc.
(d) have the usual RBC count but their haemoglobin has very high binding affinity to O2. 

Answer: A

Ques. Large woody vines are more commonly found in
(a) temperate forests
(b) mangroves
(c) tropical rainforests
(d) alpine forests. 

Answer: C

Ques. Consider the following four conditions (1 - 4) and select a correct pair of them as adaptations to environment in desert lizards.
Conditions:
1. Burrowing in soil to escape high temperature.
2. Losing heat rapidly from the body during high temperature.
3. Bask in sun when temperature is low.
4. Insulating body due to thick fatty dermis.
(a) 3, 4 (b) 1, 3
(c) 2, 4 (d) 1, 2

Answer: B

Ques. Reduction in vascular tissue, mechanical tissue and cuticle is characteristic of
(a) mesophytes
(b) epiphytes
(c) hydrophytes
(d) xerophytes. 

Answer: C

Ques. Consider the following four statements (1-4) about certain desert animals such as kangaroo, rat.
(1) They have dark colour and high rate of reproduction and excrete solid urine.
(2) They do not drink water, breathe at a slow rate to conserve water and have their body covered with thick hairs.
(3) They feed on dry seeds and do not require drinking water.
(4) They excrete very concentrated urine and do not use water to regulate body temperature.
Which two of the above statements for such animals are true?
(a) 3 and 1 (b) 1 and 2
(c) 3 and 4 (d) 2 and 3 

Answer: C

Ques. Quercus species are the dominant component in
(a) scrub forests
(b) tropical rainforests
(c) temperate deciduous forests
(d) alpine forests. 

Answer: C

Ques. Niche overlap indicates
(a) mutualism between two species
(b) active cooperation between two species
(c) two different parasites on the same host
(d) sharing of one or more resources between the two species. 

Answer: D

Ques. People living at sea level have around 5 million RBC per cubic millimeter of their blood whereas those living at an altitude to 5400 metres have around 8 million. This is because at high altitude
(a) people eat more nutritive food, therefore more RBCs are formed
(b) people get pollution-free air to breath and more oxygen is available
(c) atmospheric O2 level is less and hence more RBCs are needed to absorb the required amount of O2 to survive
(d) there is more UV radiation which enhances RBC production. 

Answer: C

Assertion and Reason Based MCQs

Directions : In the following questions a statement of assertion (A) is followed by a statement of reason (R). Mark the correct choice as :
(A) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(B) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(C) Assertion (A) is true but reason (R) is false.
(D) Assertion (A) is false but reason (R) is true.

Question. Assertion (A) : Clown fish maintains a commensalistic relation with the sea anemone.
Reason (R) : In this interaction, one species benefits and the other is neither harmed nor benefited.
Answer : A

Question. Assertion (A) : Different habitats have different physical and chemical conditions.
Reason (R) : Physicochemical (abiotic) components characterize the habitat of an organism.
Answer : B

Question. Assertion (A) : The term interbreeding implies sexual reproduction.
Reason (R) : Interbreeding result in variation among species.
Answer : A

Question. Assertion (A) : Population has certain attributes which an individual organism does not have.
Reason (R) : Population is a groups of people living in well defined area, at a particular time.
Answer : B

Question. Assertion (A) : Small animals are rarely found in polar regions.
Reason (R) : Heat loss or heat gain is a function of surface area.
Answer : A

Question. Assertion (A) : Ecology is concerned with four levels of biological organisation.
Reason (R) : Organisms, populations, communities and biomes are four levels of biological organisation.
Answer : A

Question. Assertion (A) : A community with more species is more stable than that with less species.
Reason (R) : More the number of species, lesser the variation in the total biomass production year after year.
Answer : A

Question. Assertion (A) : Many freshwater animals cannot live for long in sea water.
Reason (R) : Freshwater animals face osmotic problems in sea water.
Answer : A

Question. Assertion (A) : The human population represents the logistic growth curve.
Reason (R) : When the resources are limited leading to competition between individuals and survival of fittest, the population tends to grow in a logistic manner.
Answer : A

Question. Assertion (A) : Natality contributes to the increase in population density.
Reason (R) : Natality refers to the number of births during a given period in the population which is added to the initial density.
Answer : A

Case-based MCQs

I. Read the following text and answer the following questions on the basis of the same :
Growth of a population with time shows specific and predictable patterns. Two types of growth pattern of population are exponential and logistic growth. When resources in the habitat are unlimited each species has the ability to realize fully its innate potential to grow in number. Then the population
grows in exponential fashion. When the resources are limited, growth curve shows an initial slow rate and then it accelerates and finally slows giving the growth curve which is sigmoid.

Question. The equations correctly represents Verhulst- Pearl logistic growth is :
(a) dN/dt = rN(K-N)/K
(b) dN/dt = rN/K
(c) dN/dt = N(K-N)/K
(d) dN/dt = r(K-N)/K
Answer : A

Question. Identify the correct equation that represents the exponential population growth curve :
(a) dN/dt = rN
(b) dN/dt = rN (K-N/K)
(c) Nt =N0ert
(d) Both (a) and (c)
Answer : D

Question. The population growth is generally described by the following equation : dN/dt = rN(K – N)/K What does ‘r’ represent in the given equation?
(a) Population density at time Y
(b) Intrinsic rate of natural increase
(c) Carrying capacity
(d) The base of natural logarithm
Answer : B

Question. Identify the incorrect statement :
(a) Exponential growth occurs in organisms such as lemmings.
(b) Logistic growth is more realistic.
(c) Exponential growth has two phases lag and log.
(d) In logistic growth, population passes well beyond the carrying capacity of ecosystem.
Answer : D

Question. Study the population growth curves (A and B) in the given graph and select the incorrect statement :

(a) Curve ‘A’ shows exponential growth, represented by equation dN/dt=rN
(b) Curve ‘B’ shows logistic growth, represented by equation dN/dt = r(K-N)/K
(c) Exponential growth curve is considered as more realistic than the logistic growth curve
(d) Curve ‘A’ can also be represented by equation Nt = N0ert
Answer : C

II. Read the following text and answer the following questions on the basis of the same :
Population is a group of individuals of a single species living in a given area. It includes spatial arrangement, genetic variation and the dynamics
including natality, mortality, biotic potential, carrying capacity, etc. Population structure includes features such as density, abundance, spacing, genetic variations. The genetic structure reveals genetic variations and population size.

Question. ______ species inhabit different geographical areas.
(a) Sympatic
(b) Allopatric
(c) Both (a) and (b)
(d) None of these
Answer : B

Question. There is more competition for survival between :
(a) Different animals of same niche.
(b) Same animals of same niche.
(c) Different animals of different niche.
(d) Same animals of different niche.
Answer : B

Question. Increase in population does not increase in :
(a) Health facilities
(b) Employment
(c) Food resources
(d) All of the above
Answer : D

Question. ______ is the study of the ecology of population.
(a) Autecology
(b) Demecology
(c) Endaphology
(d) None of the above
Answer : B

Question. Which Indian state has the least population growth?
(a) Kerala
(b) U.P.
(c) A.P.
(d) Assam.
Answer : A

Very Short Answer Questions

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

Question. Species that tolerate wide range of salinity are called ____________. 
Answer. Euryhaline.

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.

Question. How is diapause different from hibernation?
Answer. 

Question. Give two reasons as to why a weed such a Calotropis flourishes in abandoned fields.
Answer. Calotropis flourishes in abandoned fields because of:
(i) It has dry hairy seeds which help in dissemination (ii) Its have xerophytic adaptations like thick hair on leaves and stems. (iii) It is not grazed by animals as it produces poisonous substances like cardiac glycosides. (Any two)

Question. What is the interaction called between Cuscuta and shoe flower bush?
Answer. Parasitism

Question. If 8 individuals in a laboratory population of 80 fruit flies died in a week, then what would be the death rate of population for the said period? 
Answer. Death rate
Number of individuals dead /Total number of individual =8/80 = 0.1
The death rate will be 0.1 individuals per week.

Question. What are ectotherms?
Answer. Ectotherms are those animals whose body temperature changes and matches with that of the environment in which they are living. They are also called cold-blooded animals.

Question. What does J-shaped growth curve of a population indicate?
Answer. The J-shaped growth curve indicates the minimum or absence of environmental resistance.

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

Question. What does sigmoid growth curve of a population indicate?
Answer. Sigmoid growth curve of a population indicates following characteristics:
(i) Initially the growth is slow.
(ii) The growth becomes rapid and the curve becomes steady due to environmental resistance.

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. Why are some organisms called as eurythermals and some others as stenohaline?
Answer. Eurythermals are organisms that can tolerate and thrive in a wide range of temperature, whereas stenohalines can tolerate a narrow range of salinities.

Question. Which one of the two, stenothermals or eurythermals, shows wide range of distribution on earth and why?
Answer. Eurythermals show a wide range of distribution on earth, as they show tolerance for wide range of temperatures.

Question. What do phytophagous insects feed on? 
Answer. Phytophagous insects feed on plant sap and other parts of plant.

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

Question. Give one function of aerenchyma in aquatic plants. 
Answer. Aerenchyma gives buoyancy to the aquatic plants due to presence of air chambers.

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.

Short Answer Questions

Question. An organic farmer relies on natural predation for controlling plant pests and diseases. Justify giving reasons why this is considered to be a holistic approach.
Answer. Besides acting as ‘conduits’ for energy transfer across trophic levels, predators are used in biological control of plant pests. This ability of the predator is based on its regulating the prey population. The natural predators reduce interspecific competition and do not harm the crop plants. For example, in an area the invasive cactus can be brought under control by cactus-feeding predator (a moth). Using natural predation, the ecosystem is kept stable without harming any of the trophic levels.

Question. Explain the response of all communities to environment over time. 
Answer. Environmental factors like temperature, water, light, soil, etc., may influence the members of communities in varying degrees. Organisms in response to these factors try to adapt according to their capacities, by maintaining a constant internal environment through homeostasis or migration to a less stressful environment or suspending activities till favourable conditions return.

Question.

(a) Label the three tiers 1, 2, 3 given in the above age pyramid.
(b) What type of population growth is represented by the above age pyramid?
Answer. (a) 1 represents Pre-reproductive age group
2 represents Reproductive age group
3 represents Post-reproductive age group
(b) Expanding population.

Question. Bear hibernates whereas some species of zooplanktons enter diapause to avoid stressful external conditions. How are these two ways different from each other? 
Answer. Hibernation is the winter sleep, seen in cold-blooded animals in polar regions, in which theysuspend their metabolic activities when external temperature becomes unfavourable. Whereas,  diapause is the phenomenon seen in insects during their developmental stages, in which metabolic activities are suspended due to unfavourable conditions.

Question. (a) What is “r” in the population equation given: dN/dt = rN?
(b) How does the increase and the decrease in the value of ‘r’ affect the population size?
Answer. (a) ‘r’ is called intrinsic rate of natural increase.
(b) Population size increases with increase in ‘r’ and it decreases with decrease in ‘r’.

Question. How does our body adapt to low oxygen availability at high altitudes? 
Answer. Our body adapts to low oxygen availability by increasing red blood cell production, decreasing the binding capacity of haemoglobin and by increasing breathing rate.

Question. Identify the curves ‘a’ and ‘b’ shown in the graph given below. List the conditions responsible for growth patterns ‘a’ and ‘b’.  

Answer. Curve ‘a’ is exponential growth curve. When the resources (food + space) are unlimited, this type of growth curve appears. Curve ‘b’ is logistic growth curve. When the resources become limited at certain point of time, this type of growth curve appears.

Question. If a marine fish is placed in a freshwater aquarium, will the fish be able to survive? Why or why not? 
Answer. A marine fish if kept in freshwater aquarium will not be able to survive because:
(a) water will enter the body of fish through endosmosis.
(b) it does not have mechanism of salt absorption as in freshwater fishes.
(c) its drinking water habit will cause excess of water to enter the body. So, the marine fish will fail to maintain the osmolarity and hence will die.

Question. Egrets are often seen along with grazing cattle. How do you refer to this interaction? Give a reason for this association.
Answer. The interaction between them can be referred to as commensalism. Egrets always forage close to where the cattle are grazing because the cattle, as they move stir up and flush out insects from the vegetation which otherwise might be difficult for the egrets to find and catch.

Long Answer Questions

Question. Water is very essential for life. Write any three features both for plants and animals which enable them to survive in water scarce environment.
Answer. Plants: Ephemeral mode (complete life cycle in short period); deep tap roots; deciduous leaves; waxy cuticle; sunken stomata; succulence to store water; C4 or CAM pathway of photosynthesis. (Any three)
Animals: No sweating; uricotelic; deposition of fat in sub-epidermal layer; burrowing nature; thick skin; body covered with scales. (Any three)

Question. (a) Following are the responses of different animals to various abiotic factors. Describe each one with the help of an example.
(i) Regulate (ii) Conform
(iii) Migrate (iv) Suspend
(b) If 8 individuals in a population of 80 butterflies die in a week, calculate the death rate of population of butterflies during that period.
Answer. (a) (i) Regulate
• Some organisms maintain homeostasis by physiological and behavioural means, such organisms are called regulators. All birds and mammals and few lower vertebrate and invertebrate species maintain homeostasis by thermoregulation and osmoregulation.
• The success of mammals is largely due to their ability to maintain a constant body temperature.
• In summers, when outside temperature is more than our body temperature, we sweat profusely and the resulting evaporative cooling brings down the body temperature.
• In winters, when temperature is lower we shiver, a kind of exercise that produces heat and raises the body temperature.

(b) Death rate = Number of individuals dead /Total population
                     = 8/80 = 0.1 individuals per butterfly per week

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 suc culents.

Question. Explain mutualism with the help of any two examples. How is it different from commensalism?
Answer. Refer to Basic Concepts Point v1i3).  In commensalism, one species benefits and the other is neither benefitted nor harmed whereas in mutualism both the species are benefitted.

Question. Differentiate between commensalism and mutualism by taking one example each from plants only.
Answer. 

Question. “Analysis of age-pyramids for human population can provide important inputs for long-term planning strategies.” Explain.
Answer. • The shape of the pyramids reflects the growth status of the population and is of three types:
(a) Expanding (Triangular shaped pyramid): Number of prereproductive individuals is very large, reproductive individuals moderate in no. and postreproductive are fewer. Population is
growing and show rapid increases.
(b) Stable (Bell shaped pyramid): Population size remains stable, neither growing nor diminishing i.e., all the age group are evenly balanced.
(c) Declining (Urn shaped pyramid): Population is declining or diminishing population showing negative growth.

Through analysis of the age pyramids of a population proper planing of health, education, transport, infra-structure, finance, food and employment can be done.
Thus, long-term management of resources can be done so that maximum benefits can be provided to the population.

Question. When do you describe the relationship between two organisms as mutualistic, competitive and parasitic? Given one example of each type. 
Answer. Mutualism is referred to as the interspecific interaction in which both the interacting species are benefited from each other. For example, lichens represent close association between fungus and Algae.
Competition is a type of interaction due to limited resources between closely related species where they compete for the same resource and both species suffer. For example, In South American lakes, visiting flamingos and resident fishes compete for zooplanktons.
Parasitism is the mode of interaction between two species, in which one species (parasite) depends on the other species (host) for food and shelter, and in the process damages the host. For example, human liver fluke depends on two hosts, a snail and a fish, to complete its life cycle.

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. Name the type of interaction seen in each of the following examples:
(i) Ascaris worms living in the intestine of humans
(ii) Wasp pollinating fig inflorescence
(iii) Clown fish living among the tentacles of sea-anemone
(iv) Mycorrhizae living on the roots of higher plants
(v) Orchid growing on a branch of a mango tree
(vi) Disappearance of smaller barnacles when Balanus dominated in the coast of Scotlan(d)
Answer. (i) Parasitism (ii) Mutualism
(iii) Commensalism (iv) Mutualism
(v) Commensalism (vi) Competition

Question. Explain co-evolution with reference to parasites and their hosts. Mention any four special adaptive features evolved in parasites for their parasitic mode of life. 
Answer. If the host evolves special mechanism for rejecting or resisting the parasite. The parasite has
to (simultaneously) evolve/co-evolve the mechanism to counter act and neutralise them & i.e.
called co-evolution.
(a) Parasitic adaptation in Animals 
QU It is the mode of interaction between two species in which one species (parasite) depends on the other species (host) for food and shelter, and in this process damages the host. In this process one organism is benefited (parasite) while the other is being harmed (host).
QU Adaptation of parasite:
(b) Loss of unnecessary sense organs as they do not interact with external environment [For example, eyes as they are found in an environment that lacks light.)
(c) Presence of adhesive organs or suckers to cling to host
(a) The parasite have evolved to be host-specific in such a manner that both host and parasite tend to co-evolve.
(b) Parasitic adaptation in plants
(i) Haustoria in Cuscuta
(ii) Loss of chlorophyll
(iii) Loss of leaves/foliage

Question. How do organisms which cannot migrate, tend to overcome adverse environmental conditions?
Explain taking one example each from vertebrates and angiosperms respectively.
Answer. Organisms which 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. Explain with the help of suitable examples the three different ways by which organisms overcome their stressful conditions lasting for short duration. 
Answer. If the stressful conditions remain for short duration, the organism has following alternatives, i.e.,either conform, migration, suspension.
(i) Conform: In some animals called conformers, osmotic concentration of body fluids change with that of the ambient water osmotic concentration. For example, small animals have larger surface area relative to their volume. They lose body heat very fast in low temperature. So, they expand energy to generate body heat through metabolism.
(ii) Migration: The temporary movement of organism from the stressful habitat to a more hospitable area and return when stressful period is over is called migration. For example, migratory birds from Siberia come to Keoladeo National Park (Bharatpur) in every winter.
(iii) Suspend: Those animals who fail to migrate, might avoid the stress by escaping in time.Hibernation of bears during winter or aestivation of snails and fish to avoid summer are examples of this phenomenon. 

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. Water is very essential for life. Write any three features both for plants and animals which enable them to survive in water scarce environment.
Answer. Plants: Ephemeral mode (complete life cycle in short period); deep tap roots; deciduous leaves; waxy cuticle; sunken stomata; succulence to store water; C4 or CAM pathway of photosynthesis.
Animals: No sweating; uricotelic; deposition of fat in sub-epidermal layer; burrowing nature; thick skin; body covered with scales.

Question. How do kangaroo rats and desert plants adapt themselves to survive in their extreme habitat?
Explain. 
Answer. Kangaroo rats are capable of meeting its water requirements through its internal fat oxidation in which water is a by product. It also has the ability to concentrate its urine so that minimal volume of water is used to remove excretory products.
Desert plants have a thick cuticle on their leaf surface and have their stomata arranged in deep pits to minimise water loss. They also have leaves reduced to spines and deep roots to absorb more water. They have a special photosynthetic pathway (CAM).

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. Study the population growth curves in the graph given below and answer the questions that follow:
(i) Identify the growth curves ‘a’ and ‘b’.
(ii) Which one of them is considered a more realistic one and why?
(iii) If dN/dt=rN (K– N)/K is the equation of the logistic growth curve, what does K stand for?
(iv) What is symbolised by N?
Answer. (i) a is exponential growth curve or J-shaped curve.b is logistic growth curve or S-shaped curve.
(ii) Logistic growth curve (b) is considered more realistic because unlimited resources are never available in an ecosystem or in a habitat.
(iii) K stands for carrying capacity.
(iv) N indicates population density, which is the number of species of a population per unit area.

Question. When do you describe the relationship between two organisms as mutualistic, competitive and parasitic? Given one example of each type. 
Answer. Mutualism is referred to as the interspecific interaction in which both the interacting species are benefited from each other. For example, lichens represent close association between fungus and Algae.
Competition is a type of interaction due to limited resources between closely related species where they compete for the same resource and both species suffer. For example, In South American lakes, visiting flamingos and resident fishes compete for zooplanktons.
Parasitism is the mode of interaction between two species, in which one species (parasite) depends on the other species (host) for food and shelter, and in the process damages the host. For example,human liver fluke depends on two hosts, a snail and a fish, to complete its life cycle.

Question. Name the type of interaction seen in each of the following examples:
(i) Ascaris worms living in the intestine of humans
(ii) Wasp pollinating fig inflorescence
(iii) Clown fish living among the tentacles of sea-anemone
(iv) Mycorrhizae living on the roots of higher plants
(v) Orchid growing on a branch of a mango tree
(vi) Disappearance of smaller barnacles when Balanus dominated in the coast of Scotlan(d)
Answer. (i) Parasitism (ii) Mutualism
(iii) Commensalism (iv) Mutualism
(v) Commensalism (vi) Competition

Question. Predation is usually referred to as a detrimental association. State any three positive roles that a predator plays in an ecosystem. 
Answer. (i) They predators act as conduits for energy transfer across trophic levels.
(ii) They keep prey populations under control.
(iii) They help in maintaining species diversity in a community by reducing the intensity of competition among prey species.

Question. Differentiate between commensalism and mutualism by taking one example each from plants only. 
Answer. Differences between commensalism and mutualism

Question. Highlight the differences between the population interactions given below. Given an example of each.
(a) Parasitism (b) Amensalism (c) Mutualism 
Answer. Differences between parasitism, amensalism and mutualism

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. 
(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.

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.