CBSE Class 12 Biology –Chapter 13 Organisms and Populations- Study Materials


Ecology is the study of interactions among organisms and between the organism and its physical (abiotic) environment.

Ecology is concerned with 4 levels of biological organization: Organisms, Populations, Communities & Biomes.


Physiological ecology (Ecology at the organismic level) is the study of adaptation of an organism to environments in terms of survival and reproduction.

The rotation of earth and the tilt of its axis cause annual variations in temperature & seasons. Major biomes (desert, rain forest, tundra etc.) are formed due to these variations & precipitation (rain & snow).


Regional and local variations within a biome lead to the formation of different habitats.

Life exists even in extreme & harsh habitats. E.g.
  • Rajasthan desert, rain-soaked Meghalaya forests, deep ocean trenches, torrential streams, permafrost (snow laden) polar regions, high mountain tops, thermal springs & compost pits.
  • Our intestine is a habitat for many microbes.

The physico-chemical (abiotic) components (water, light, temperature, soil etc.) & biotic components (pathogens, parasites, predators, competitors etc.) lead to variation of different habitats.

The distinct role and position of an organism in its environment is called its niche. By this, each organism tolerates various conditions, utilises various resources etc.
Abiotic Factors 
a. Temperature 

The most ecologically relevant environmental factor.

Temperature on land varies seasonally. It gradually decreases from equator to the poles and from plains to mountain tops. It ranges from subzero levels (in polar areas & high altitudes) to >500° C (in tropical deserts).

Average temperature in thermal springs & deep-sea hydrothermal vents is above 100° C.

Mango trees cannot grow in temperate countries (Canada, Germany etc.). There is no Snow leopard in Kerala forests. Tuna fishes are rare beyond tropical latitudes in the ocean.

Temperature affects kinetics of enzymes, basal metabolism and other physiological functions of the organism.

Based on range of thermal tolerance, organisms are 2 types:

  • Eurythermal: They can tolerate a wide range of temperatures.
  • Stenothermal: They can tolerate only a narrow range of temperatures.
b. Water 

It is the second most important factor.

Desert organisms have special adaptations to limited water.

Productivity & distribution of plants is dependent on water.

For aquatic organisms, water quality (pH, chemical composition) is important. The salt concentration (salinity in parts per thousand) is less than 5 in inland waters, 30-35 in the sea and > 100 in some hypersaline lagoons.

Based on the tolerance to salinity, organisms are 2 types:

  • Euryhaline: Tolerate a wide range of salinities.
  • Stenohaline: Tolerate only a narrow range of salinity.

Many freshwater animals cannot live for long in sea water and vice versa because of the osmotic problems.

c. Light 

Plants need sunlight for photosynthesis.

Small forest plants (herbs & shrubs) are adapted to photosynthesize optimally under very low light because they are overshadowed by tall, canopied trees.

Many plants depend on sunlight for photoperiodism (e.g. flowering).
Many animals use diurnal and seasonal variations in light intensity and photoperiod for timing their foraging, reproductive & migratory activities.

Sun is the ultimate source for light & temperature on land. Deep (> 500m) in the oceans, the environment is dark and there is no energy available from sun.

The spectral quality of solar radiation is also important for life. The UV spectrum is harmful to many organisms. Not all the colour components of the visible spectrum are available for marine plants.

d. Soil 

Nature & properties of soil is differed due to climate, weathering, sedimentation, method of soil development etc.

Soil composition, grain size & aggregation determine the percolation and water holding capacity of the soils.
These characteristics and parameters like pH, mineral composition & topography determine the vegetation and animals in an area.
In aquatic environment, the sediment-characteristics determine the type of benthic animals. 


Responses to Abiotic Factors 

Organisms maintain a stable internal environment (homeostasis) despite varying external environmental conditions. This is possible by following processes.

a. Regulate 

It is the maintenance of homeostasis by physiological & behavioural means. It ensures constant body temperature (thermoregulation), constant osmotic concentration (osmoregulation) etc. E.g. All birds & mammals, very few lower vertebrates and invertebrates.

Thermoregulation in mammals: The success of mammals is mainly due to their ability to maintain a constant body temperature.
In summer, when outside temperature is more than body temperature (37° C), sweating occurs. This results in evaporative cooling and brings down body temperature.
In winter, when the temperature is below 37° C, shivering occurs. It produces heat and raises the body temperature.

Most of the organisms are not regulators or are partial regulators because thermoregulation is energetically expensive especially for small animals (shrews, humming birds etc.). They have a larger surface area relative to their volume. So they lose body heat very fast when it is cold outside. Then they have to expend much energy to generate body heat. So, very small animals are rare in Polar Regions.

b. Conform 

99% of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature or osmotic concentration change with the surrounding conditions. They are called conformers. 

In aquatic animals, osmotic concentration of body fluids changes with that of the ambient osmotic concentration.
c. Migrate 

Many animals like birds move away temporarily from stressful habitat to a more hospitable area and return when stressful period is over.

E.g. During winter, Keolado National Park (Bhartpur, Rajasthan) hosts migratory birds coming from Siberia and other extremely cold northern regions.

d. Suspend 

In bacteria, fungi & lower plants, thick walled spores help to survive unfavourable conditions. Under suitable conditions, they germinate.

In higher plants, seeds and some vegetative reproductive structures serve to tide over periods of stress by reducing their metabolic activity. They germinate under favourable moisture and temperature.

In animals: Examples are

  • Hibernation of bears during winter.
  • Aestivation of some snails and fishes during summer.
  • Diapause (a stage of suspended development) of many zooplanktons in lakes & ponds.

Adaptation is the morphological, physiological & behavioural attribute that enables an organism to survive and reproduce in its habitat.

Many adaptations have evolved over a long evolutionary time and are genetically fixed.

Adaptations of kangaroo rat in North American deserts:

  • Internal fat oxidation gives water as byproduct if there is no external source of water.
  • Ability to concentrate urine so that minimal volume of water is used to remove excretory products.

Adaptations of desert plants:

  • Presence of thick cuticle on leaf surfaces.
  • Sunken stomata minimise water loss due to transpiration.
  • CAM photosynthetic pathway enables their stomata to remain closed during day time.
  • Desert plants like Opuntia have no leaves (they are reduced to spines). Photosynthesis is done by stems.

Adaptations of mammals:

  • Mammals from colder climates have shorter ears and limbs to reduce heat loss. This is called Allen’s Rule.
  • Aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces loss of body heat.

Physiological and biochemical adaptations:

  • Archaebacteria are found in hot springs & deep-sea hydrothermal vents where temperature is >100° C. Many fish thrive in Antarctic waters (temperature is below 0° C).
  • Many marine invertebrates & fishes live at great depths in the ocean where the pressure is >100 times the normal atmospheric pressure.
  • At a high-altitude place (>3,500 m) we feel altitude sickness. Its symptoms are nausea, heart palpitations & fatigue. This is due to low atmospheric pressure. So the body does not get enough O2. Gradually, we acclimatize the situation and the body compensates low O2 availability by increasing RBC & breathing rate and decreasing the binding capacity of hemoglobin.

Behavioural adaptations:

  • Desert lizards bask in the sun and absorb heat when their body temperature is low, but move into shade when the ambient temperature starts increasing.
  • Some species burrow into the soil to hide and escape from the above-ground heat.


A population is a group of individuals of same species that live in a given geographical area, share or compete for similar resources and potentially reproduce.

E.g. All the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond etc.

Population ecology is an important area of ecology as it links ecology to population genetics & evolution.

Population Attributes
  • Birth rates: Refer to per capita births.
E.g. In a pond, there are 20 lotus plants last year and through reproduction 8 new plants are added.
Hence, the current population = 28
The birth rate = 8/20 = 0.4 offspring per lotus per year.
  • Death rates: Refer to per capita deaths.
E.g. 4 individuals in a laboratory population of 40 fruit flies died during a week.
Hence, the death rate = 4/40 = 0.1 individuals per fruit fly per week.
  • Sex ratio: A population has a sex ratio.
E.g. 60% of the population is females and 40% males.
  • Age pyramid: It is the structure obtained when the age distribution (% individuals of a given age or age group) is plotted for the population.
For human population, age pyramids generally show age distribution of males and females in a combined diagram.
Representation of age pyramids for human population 
  • Population size or population density (N): It is the number of individuals of a species per unit area or volume. E.g. population density of Siberian cranes at Bharatpur wetlands in any year is <10. It is millions for Chlamydomonas in a pond.
Population size is also measured in % cover or biomass. E.g. In an area, 200 Parthenium plants and a huge banyan tree are seen. In such cases, measuring % cover or biomass is meaningful to show importance of banyan tree.
Total number is a difficult measure for a huge population. In such cases, relative population density (without knowing absolute population density) is used. E.g. Number of fish caught per trap indicates its total population density in the lake.
In some cases, indirect estimation of population sizes is performed. E.g. Tiger census in national parks & tiger reserves based on pug marks & fecal pellets.


The population size changes depending on factors like food availability, predation pressure & weather.

Changes in population density give some idea about the population – whether it is flourishing or declining.

4 basic processes that fluctuate the population density:

  1. Natality (B): It is the number of births in a population during a given period.
  2. Mortality (D): It is the number of deaths in a population during a given period.
  3. Immigration (I): It is the number of individuals of the same species that have come into the habitat from elsewhere during a given time period.
  4. Emigration (E): It is the number of individuals of the population who left the habitat and gone elsewhere during a given time period.

Natality & immigration increase the population density and mortality & emigration decrease the population density.


If N is the population density at time t, then its density at time t +1 is

Nt+1 = Nt + [(B + I) – (D + E)] 

Population density increases if B+I is more than D+E. Otherwise it will decrease.

Under normal conditions, births & deaths are important factors influencing population density. Other 2 factors have importance only under special conditions. E.g. for a new colonizing habitat, immigration may be more significant to population growth than birth rates.

Growth Models 

a. Exponential growth

Resources (food & space) are essential for the unimpeded population growth.

If resources are unlimited, each species shows its full innate potential to grow in number. Then the population grows in an exponential or geometric fashion.

If population size = N, birth rates (per capita births) = b and death rates (per capita deaths) = d, then the increase or decrease in N during a unit time period t (dN/dt) will be

dN/dt = (b – d) × N
Let (b–d) = r, then
dN/dt = rN
The r (‘intrinsic rate of natural increase’) is an important parameter for assessing impacts of any biotic or abiotic factor on population growth.
r value for the Norway rat = 0.015
r value for the flour beetle = 0.12
r value for human population in India (1981) = 0.0205
The integral form of the exponential growth equation is
Nt = Population density after time t
N0 = Population density at time zero
r = intrinsic rate of natural increase
e = the base of natural logarithms (2.71828)
Population growth curves 
a = exponential growth (J-shaped curve)
b = logistic growth (Sigmoid curve)

b. Logistic growth

There is no population in nature having unlimited resources for exponential growth. This leads to competition among individuals for limited resources.

Eventually, the ‘fittest’ individuals survive and reproduce.

In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. It is called carrying capacity (K).

A population with limited resources shows initially a lag phase, phases of acceleration & deceleration and finally an asymptote. This type of population growth is called Verhulst-Pearl Logistic Growth. It is described by following equation:
Where N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity
Since resources for growth for most animal populations are limited, the logistic growth model is more realistic.
Life History Variation 

Populations evolve to maximise their reproductive fitness or Darwinian fitness (high r value). Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy.

Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo) while others breed many times (most birds and mammals).

Some produce a large number of small-sized offspring (Oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds, mammals).

These facts indicate that life history traits of organisms have evolved due to limited abiotic and biotic components of the habitat.

Population Interactions

Organisms interact in various ways to form a biological community.
Interaction between two species is called Interspecific interactions. They include

Name of interaction

Species A

Species B

Mutualism: Both species are benefitted (+)



Competition: Both species are harmed (-)

Predation: One (predator) is benefitted. Other (prey) is harmed


Parasitism: One (parasite) is benefitted. Other (host) is harmed


Commensalism: One is benefitted. Other is unaffected (0)



Amensalism: One is harmed. Other is unaffected



In predation, parasitism & commensalisms, the interacting species live closely together.

a. Predation 

In a broad ecological context, all carnivores, herbivores etc. are predators. About 25 % insects are phytophagous.

If a predator overexploits its prey, then the prey might become extinct. It results in the extinction of predator. Therefore, predators in nature are ‘prudent’.

Importance of predators
  • Predators control prey populations.
When certain exotic species are introduced into a geographical area, they spread fast due to the absence its natural predators. E.g. Prickly pear cactus introduced into Australia (1920’s) caused havoc by spreading. Later, it was controlled by introducing a cactus-feeding predator moth.
  • Predators are used in Biological control methods.
  • Predators maintain species diversity in a community by reducing competition among prey species.
E.g. the predator starfish Pisaster in the rocky intertidal communities of American Pacific Coast. In an experiment, all these starfishes were removed from an enclosed intertidal area. It caused extinction of over 10 invertebrate species within a year, due to interspecific competition.
Defenses of prey species to lessen impact of predation
  • Camouflage (cryptic colouration) of some insects & frogs.
  • Some are poisonous and so avoided by the predators.
  • Monarch butterfly is highly distasteful to its predator bird. It is due to a special chemical in its body. It is acquired during its caterpillar stage by feeding on a poisonous weed.
  • Thorns (Acacia, Cactus etc.) are the most common morphological means of defense of plants.
  • Many plants produce chemicals that make the herbivore sick, inhibit feeding or digestion, disrupt its reproduction or kill it. E.g. Calotropis produce highly poisonous cardiac glycosides. Therefore cattle or goats do not eat it. Nicotine, caffeine, quinine, strychnine, opium, etc. are defenses against grazers and browsers.
b. Competition 

It is a process in which fitness of one species (‘r’ value) is significantly lower in presence of another species.

Interspecific competition is a potent force in organic evolution.

Competition occurs when closely related species compete for the same limited resources.

Unrelated species can also compete for the resource. E.g. Flamingoes & fishes in some shallow South American lakes compete for zooplankton.

Competition occurs in abundant resources also. E.g. In interference competition, the feeding efficiency of one species is reduced due to the interfering and inhibitory presence of other species, even if resources are abundant.

Evidences for competition:

  • The Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, due to greater browsing efficiency of the goats.
  • Competitive release: It is the expansion of distributional range of a species when the competing species is removed.
  • Connell’s field experiments: On the rocky sea coasts of Scotland, there are 2 barnacle species: Balanus (larger & competitively superior) & Chthamalus (smaller). Balanus dominates intertidal area and excludes Chthamalus.
When Connell experimentally removed Balanus, Chthamalus colonized the intertidal zone.

Gause’s ‘Competitive Exclusion Principle’:

It states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually. 


This may be true in limited resources, but not otherwise.

Species facing competition may evolve mechanisms for co-existence rather than exclusion. E.g. resource partitioning.

Resource partitioning: 

It is the division of limited resources by species to avoid competition. For this, they choose different feeding times or different foraging patterns. E.g. MacArthur showed that five closely related species of warblers living on a tree could avoid competition and co-exist due to behavioural differences in their foraging activities.

c. Parasitism 

Many parasites are host-specific (they can parasitize only a single host species). They tend to co-evolve. i.e., if the host evolves special mechanisms against the parasite, the parasite also evolves mechanisms to counteract them to remain with the same host species.

Adaptations of parasites: Loss of sense organs, presence of adhesive organs or suckers to cling on to the host, loss of digestive system, high reproductive capacity etc.

Life cycles of parasites are often complex. E.g.

  • Human liver fluke depends on 2 intermediate hosts (a snail & a fish) to complete its life cycle.
  • Malarial parasite needs mosquito to spread to other hosts.

Parasites harm the host. They may reduce the survival, population density, growth and reproduction of the host. They may make the host physically weak and more vulnerable to predation.

Types of parasites:

1. Ectoparasites

Parasites that feed on the external surface of host. E.g.

  • Lice on humans.
  • Ticks on dogs.
  • Ectoparasitic Copepods on many marine fishes.
  • Cuscuta plant on hedge plants.

Cuscuta has no chlorophyll and leaves. It derives its nutrition from the host plant.
Female mosquito is not considered a parasite, because it needs our blood only for reproduction, not as food.

2. Endoparasites

Parasites that live inside the host body at different sites (liver, kidney, lungs, RBC etc).

The life cycles of endoparasites are more complex.

They have simple morphological & anatomical features and high reproductive potential.

Brood parasitism in birds:

Here, the parasitic birds lay eggs in the nest of its host and lets the host incubate them.

During evolution, eggs of the parasitic bird have evolved to resemble the host’s egg in size and colour. So the host bird cannot detect and eject the foreign eggs easily.

E.g. Brood parasitism between cuckoo and crow.

d. Commensalism 


  • Orchid (+) growing as epiphyte on a mango branch (0).
  • Barnacles (+) growing on the back of a whale (0).
  • Cattle egret (+) & grazing cattle (0). The egrets forage close to where the cattle are grazing. As the cattle move, the vegetation insects come out. Otherwise it is difficult for the egrets to find and catch the insects.
  • Sea anemone (0) & clown fish (+). Stinging tentacles of sea anemone gives protection to fish from predators.
e. Mutualism 


  • Lichen: It is a mutualistic relationship between a fungus & photosynthesizing algae or cyanobacteria.
  • Mycorrhizae: Associations between fungi & the roots of higher plants. The fungi help the plant in the absorption of essential nutrients from the soil while the plant provides the fungi with carbohydrates.
  • Mutualism b/w plant & animal through pollination and seed dispersion: 
  1. Fig trees & wasps. The fig species is pollinated only by its ‘partner’ wasp species. Female wasp pollinates the fig inflorescence while searching for suitable egg-laying sites in fruits. The fig offers the wasp some developing seeds, as food for the wasp larvae.
  2. Orchids show diversity of floral patterns. They can attract the right pollinator insect (bees & bumblebees) to ensure pollination. Not all orchids offer rewards.
  3. ‘Sexual deceit’ of Ophrys (Mediterranean orchid). One petal of its flower resembles female bee in size, colour & markings. So male bee ‘pseudocopulates’ with the flower and is dusted with pollen. When this bee ‘pseudocopulates’ with another flower, it transfers pollen to it.
If the female bee’s colour patterns change slightly during evolution, pollination success will be reduced unless the orchid flower co-evolves to maintain the resemblance of its petal to the female bee.

CBSE Class 12 Biology Important Questions
Chapter 13 – Organisms and Populations

1 Mark Questions

Chapter 13
Organisms and Populations

1 Marks Questions
1. Which are the factor responsible for the wide variety of habitat formed within each biome?
Ans.Regional and local variations

2. Fresh water animals are unable to survive for long in sea water. Give reason.
Ans,.Due to osmotic problems.

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

4. Define diapause. Which organisms exhibit it?
Ans.A stage of suspended development, zooplanktons.

5. Calculate the death rate if 6 individuals in a laboratory population of 60 fruit flies died during a particular week.
Ans. 6/60 =0.1 individuals per fruitfly per 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?
Ans. Predation.

7. An organism has to overcome stressful condition for a limited period of time. Which strategies can it adopt to do so?
Ans.(i) Migration
(ii) Suspension of active life by hibernation/aestivation/spore formation.

8. Write what do phytophagous insects feed on?
Ans.Plant sap and other parts of plant.

9.Why do leaves contains Sunken stomata?
Ans.Leaves contains sunken stomata i.e. Stomata arranged in deep pits to minimizes water loss by transpiration.

10.Name the type of interaction that is detrimental to both the interaction.

11.What type of interaction is shown by sparrows eating the seeds?

12.Define homeostasis?
Ans.Homeostasis refers to the maintenance of a steady internal environment by organisms.

13.Give an example of suspension?
Ans.Hibernation is frogs, reptiles or polar bear.

14.What is Allen’s rule?
Ans.Mammals living in colder regions have short ears & limbs to minimise heat loss.

15.“Cuckoo bird lays eggs in the nest of crow” which type of interaction is shown in this relation?
Ans.Brood parasitism.

16.Give one function of aerenchyma in aquatic plants?
Ans.Aerenchyma in aquatic plants provides buoyancy & helps them in floating.

17.What does J-shaped curve indicates?
Ans.J-shaped curve indicates that the resources are unlimited in a habitat.

18.Name the type of interaction in which one species is harmed while other is neither benefitted nor harmed?

19.Why are calotropis plants not browsed by herbivores?
Ans.Because calotropis plant produces a highly poisonous glycoside that is a cardiac poison & thus, directly kills the predator.

20.What are the two primary requirements of a parasite from host?
Ans.Food & shelter.

21.What is the ecological principle behind biological control method of managing pest insects.
Ans.Predation, where predators prey upon pests & control their number.

22.Write the equation for verhulst – poarl logistic growth of population.
Ans.ΔNΔt = rN(KN)KΔNΔt = rN(K−N)K

23.Name the mechanism employed by ophrys to get its flowers pollinated?

24.List any two factors which determine the nature of soil?
Ans.Climate & weathering process.

2 Mark Questions

Chapter 13
Organisms and Populations

2 Marks Questions
1. What are the four levels of biological organisation with which ecology basically deals?
Ans.Organisms, population, communities and biomes.

2. Differentiate between stenohaline and euryhaline organisms.
Ans.Euryhaline :Organisms tolerant in wide range of salinities.
Stenohaline :Organisms tolerant to narrow range of salinities.

3. List four features which enable the Xeric plants to survive in the desert conditions.
Ans. (i) thick cuticle
(ii) Stomata in deep pits
(iii) Stomata closed during day time
(iv) leaves reduced to spines (CAM photosynthetic pathway).

4. Mention the attributes which a population has but not an individual organism.
Ans.Birth rate, Death rate, Sex ratio, age groups.

5. Differentiate between stenothermal and eurythermal organisms.
Ans.Eurythermal :Organisms that can tolerate and thrive in wide range of temperaturesStenothermal :Organisms restricted to a narrow range of temperature.

6. What are the four ways through which the living organisms respond to abiotic factors?
Ans. (i) Regulate (ii) Conform (iii) migrate (iv) Suspend

7. Why do clown fish and sea anemone pair up? What is this relationship called?
Ans.Clown fish lives in tentacles of sea Anemone and gets protection from predators.
Interaction – commeasalisn.

8.Distinguish between ectotherms& Endotherms?
Ans.Ectotherms are those animals whose body temperature changes & matches with that of environment in which they are living whereas Endotherms are those animal whose body temperature is maintained relatively constant by physiological regulation.

9.“Lichens are considered good examples of obligate mutualisms”. Comment?
Ans.Lichens show an intimate mutualistic relationship between a fungus & an algae or cynobacterium where the fungus helps in absorption of nutrients & provides it to bacteria while the algae or cyanobacterium prepares the food.

10.Give any two examples of defense mechanism in plants against herbivory?
Ans. i) plants develops certain morphological means of defense e.g. thorns in bougainvillea & spinesin cactus.
ii) plants produce & store certain chemicals which functions with by directly killing them or by inhibiting them from feeding .

11.What is Brood parasitism? Give an example. What adaptation has evolved in this phenomenon?
Ans.Brood parasitism refers to the phenomenon in which one bird species by its eggs in the nest ofanother bird species Evolution has occurred in such a way the eggs of the parasitic birds resemblethose of the host bird in size, colouretc to avoid host bird detecting the foreign eggs & ejecting themfrom the nest e.g. cuckoo bird lays eggs in the nest of crow. It is considered as a parasitic type ofinterspecific interaction because in this relationship the parasite i.e. eggs of cuckoo birds dependson crow’s nest for its food & shelter but the crow is harmed because there is competition for limitedfood and shelter amongst the crow’s egg & cuckoo’s egg thus, in parasitic interspecific interactionthe parasite is benefited while the host is harmed.

12.An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid & the mango tree?
Ans.Orchids grows as epiphytes on mango tree. This is an example of commensalism in which orchids are benefited by getting a shelter while the tree is neither benefited nor harmed.

13.State Gauss’s competitive exclusion principle?
Ans.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 resources are limiting & not otherwise.

14.What is migration? Why do animals show this phenomenon?
Ans.Migration is a phenomenon in which organisms can move away temporarily from the stressful conditions in the habitat with hospitable conditions e.g. birds undertake long distance migration during winter.

15.How do desert lizards maintain a fairly constant body temperature?
Ans.Desert lizards manage to deal with high temperature by keeping their body temperature fairlyconstant by behavioral means. They bask in the sun & absorb heat when their body temperature isbelow the comfort level & move into shade when it is higher.

16.Differentiate between Hibernation & aestivation?
Ans.Hibernation is the phenomenon of spending cold period in inactive stage by an animal whereasaestivation is the phenomenon of spending dry & hot conditions in an inactive stage by animal.

17.Name the bind of interaction present between the following :-
i) Indian Nightingale & crow
ii) Nodulated roots & rhizobium
iii) Plasmoduim& man
iv) Orchids & Mongo tree
Ans. i) Indian Nightingale &crow :- Brood parasitism
ii) Nodulated roots & rhizobium :- Mutualism
iii) Plasmoduim& man :- Perasitism
iv) Orchids & Mongo tree :- Commensalism.

18.Define carrying capacity?
Ans.The maximum number of individuals of a population that can be sustained by a given habitat is called its carrying capacity.

19.If a marine fish is placed in fresh water aquarium, will the fish be able to survive. Why or why not?
Ans.No, marine fish is unable to survive in a fresh water aquarium because they are adapted to live insaline sea water. They are unable to cope with outside hypotonic environment because ofOsmoregulation problem.

20.Out of the two population growth models, which one is more realistic & Why?
Ans. Logistic or S-shaped growth curve is more realistic because no population can continue to grow exponentially, as the resource availability becomes limiting at certain point of time.

21.What role do predators play in an ecosystem?
Ans.Predators plays an important role in ecosystem :-
i)They act as conduct for energy transfer to higher trophic level.
ii)Theykeep the prey population under control which otherwise can reach very high population density.
iii)They help in maintaining species diversity in a community.

22.Most living organisms cannot survive at temperature above 450c. How are some microbes able to live in habitat with temperature exceeding 1000c.
Ans.Some microbes are able to live in habitats with temperate exceeding 1000 c because theypossess minimum amount of free water in their body. Removal of water provide resistance to hightemperature.

23.Give below is a graph depicting organismic response to changing external condition. Name the type of organisms which will show:-
i) pattern A
ii) pattern B

Ans. i) Conformers shows pattern A where body temperature changes with the ambienttemperature.
ii) Regulators shows pattern B where body temperature remains constant.

24.Mention any two ways in which organisms tide over unfavourable conditions by suspending their activities.
Ans. i) Hibernation – phenomenon of spending cold period in inactive stage by an animal e.g. frog,reptiles, polar bear.
ii) Aestivation – phenomenon of spending dry & hot conditions in an inactive stage by an animal e.g. snail, fishes.

25.Why are predators “prudent in nature?
Ans.Predators are said to be prudent in nature because if a predator is too efficient & over exploiti ts prey, then the prey might become extinct & following it the predator will also become extinct for lacking of food

3 Mark Questions

Chapter 13
Organisms and Populations

3 Marks Questions
1. How does the shape of age pyramid reflect the growth status of a population?
Ans.Shape of pyramids reflects growth statusof the population (a) growing (b) Stable (c) declining.

2. Darwin showed that even a slow growing animal like elephant could reach enormous number in absence of checks. With the help of your understanding of growth models, explain when is this possible? Why is this notion unrealistic?
Ans. Possible if the growth model is Exponential, i.e., having unlimited resources. Its an unrealistic situation because resources are limited. Hence, it follows logistic growth model.

3. How will you measure population density in following cases?
(i) fish in a lake
(ii) tiger census in a national park
(iii) single huge banyan tree with large canopy .
Ans. (i) fish caught per trap.
(ii) number per unit area
(iii) percentage cover in biomass.

4. Species facing competition might evolve mechanism that promotes coexistence rather than exclusion. Justify this statement in light of Gause’s competitive exclusion principle, citing suitable examples.
Ans.State Gause’s competitive exclusion principle.Mechanisms is resource partitioning. E.g., experiment of Mac Arthur on Warblers (Refer page 325, NCERT book, Biology – XII).

5.Describe the specific adaptation of xerophytes with respect to root system, stem & leaves.
Ans. i) ADAPTATIONS INROOTS :- Xerophytes have well developed & extensively branched long rootsystem. While some perennial xerophytes of succulent nature possess extensive but shallow root system.They can absorb water from dew drops & small rain droplets.
ii) ADAPTATION IN STEM:- stems of woody xerophytes are comparatively stunted hard & rigid. Theymay be covered with thick e.g. Acacia main stem & branches may occur as think, fleshy, flattened & greenmodified structure called phylloclade.
iii) ADAPTATION IN LEAVES:- Leaves are usually short sized which decreases the chances of gettingover – heated when exposed to solar radiation & thus by reducing rate of transpiration. Leaves ofssxerophytes are usually thick, fleshy green & leathery which are known to store water.

6.List the important characteristics of a population & Explain?
Ans.A population has following four major characteristics :-
i) Population Density :- The size of a population in relation to a definite unit of space is termed aspopulation density. The maximum limit of density depends upon energy flow in an ecosystem, nutritionalstatus of trophic level & metabolic equilibrium. Population density can be mathematically calculatedas : D=N/atD=N/at
ii) Birth Rate / Natality :- The birth rate or natality denotes the produced number of new individuals byany natural method in per unit time. The birth can be expressed lay formula B=NntB=Nnt
iii) Death Rate / Mortality :- It refers to death rate of individuals in the population. It is expressed in asnumber of individual dying in a given period.Death Rate = no of death in population Timeno of death in population Time
iv) Carrying Capacity:- Each habitat or ecosystem has a certain space which can accommodate a finitenumber of organisms depending on its size & productivity. This is called carrying capacity

7.Describe the specific adaptations of hydrophytes with respect to roots, stem & leaves?
Ans. i) ADAPTATIONS IN ROOTS:- Root system is feebly developed &unbranched some floating plants orsubmerged plants lack roots Root hairs are absent except rooted floating hydrophyte. True root caps areabsent.
ii) ADAPTATIONS IN STEM :- In submerged hydrophytes, stems are long slender & flexible whereas inthe free – floating hydrophytes stem are modified as thick, stout, stoloniferous& occur horizontal onwater surface.
iii) ADAPTATIONS IN LEAVES:- Leaves are thin, long, ribbon shapes submerged forms. In free floatingplants, the petioles of leaves show indefinite power of growth.

8.Name& explain the bind of interaction in the following.
i) Algae & fungi in
ii) Head louse & humans
iii) Hermit crab & sea anemone.
Ans. i) Algae & fungi in lichens :- Lichens shows an intimate mutualistic interaction in with both fungushelps in absorption of nutrients & provides protection, while algae or cyanobacterium prepares the food.
ii) Head louse & humans:- Head louse shows ectoparasitism on humans in which head louse is gettingnutrition from human body & is thus benefited while human beings are harmed.
iii) Hermit crab & sea anemone:- Hermit crab & sea anemone shows commensalism as hermit crab isbenefited because it gets protection from predators which stays away from stinging tentacles of the seaanemone.

9.Mention the different defense mechanism to reduce the impact of predation?
Ans.plant species evolved various defense mechanism to reduce impact of predation :-
i) Certain insect species & frogs have camouflage or cryptic colouration to avoid detection by their predators.
ii)Some animals like monarch butterfly are highly distasteful to their predators because they accumulate a certain chemical by feeding on poisonous weeds during its caterpillar stage.
iii)Some prey are poisonous & hence are avoided by predators .
iv)Plants have evolved certain morphological, or chemical defense mechanism against herbivores e.g. thorns in bougainvillea.
v)plants also produce certain chemicals which functions as :-

  • They make animal feel sick.
  • They may inhibit them from feeding.
  • They may interfere with digestion.
  • They may directly kill them.

10.Mutualism often involves co-evolution of mutualists. Describe taking the example of animal plant (wasp-fig) relationship.
Ans.Plants need the help of animals for pollination their flowers & dispersing their seeds. Animals obviously have to be paid fees for the services that plants expect from them. Plants offer rewards or fees in the form of seed dispersers “plant – animal interactions often involve co-evolution of the mutualists that is, the evolution of the flower & its pollinator species can be pollinated only by its partner wasp species & no other species. The female wasp uses the fruit not only as an oviposition site but uses the developing seeds within the fruit for nourishing its larvae. The wasp pollinates the fig inflorescence while searching for suitable egg-laying sites. In return for the favors of pollination the fig offers the wasp some of its developing seeds as food for the developing wasp larvae.

11.How do kangaroo rats live in the absence of water in North American deserts?
Ans.The kangaroo rat in North American deserts is capable of meeting all its water requirement through its internal oxidation of fat, where water is by-product, it can also concentrate its urine to a minimal volume.

12.How is diapause different from Hibernation?
Ans.Diapauses is the phenomenon of spending unfavourable climatic conditions by insects during their development whereas. Hibernation is a phenomenon of spending the winter in a resting or dormant conditions by cold – blooded animals to escape cold by hiding them in hollow tree trunk or burro or caves etc, revealing minimum physiological activity.

13. Study the three representative figures of age of pyramid relating to human population & answer the following question.

i) Mention the given to the three binds of age profile (a), (b) and (c)
ii) Which one of them is ideal for a population & why.
iii) How do such age – profile helps policy making concerned about our growing population & prepare for future generation.
Ans. i) (a) is called young population
(b) is called stable population
(c) is called declining population
ii) Among the three, stable population is ideal because it has identical birth death rate.
iii) Age profile helps policy makers get concerned about our wing population & to make on idea for future population growth so that they make future plans.

14. In the adjacent population growth curve :-

i) What is the name given to curve (a) & (b).
ii) What is the status of food & space in thecurve (a) & (b).
iii) In absence of predators, which curve “a” or “b” would appropriately depict the prey population?
iv) When does curve ‘b’ changes into curve ‘a’.
Ans. i) Curve (a) is known as exponential growth curve & curve (b) is known as logistic growth curve.
ii) Food & space is less in curve ‘a’ whereas plenty of food & space is available in curve ‘b’.
iii) Curve “a”.
iv) When the food resources in a given place become unlimited the curve (b) assumes a J – shape & changes into curve (a).

5 Marks Questions

Chapter 13
Organisms and Populations

5 Marks Questions
1. What is altitude sickness? What its causes and symptoms? How does human body try to overcome altitude sickness?
Ans. Breathlessness at high attitudes.Cause :Low atmospheric pressure at high altitudes due to which body does not get enough oxygen. Symptoms :Nausea, fatigue and heart palpitations.
Body adapts by :
(a) increasing red blood cell production
(b) decreasing binding affinity of haemoglobin
(c) by increasing breathing

2. Orchid flower, Ophrys co-evolves to maintain resembelance of its petal to female bee. Explain how and why does it do so?

  • employs ‘Sexual deceit’
  • one petal bears uncanny resemblance to female of the bee.
  • Male bee is attracted to what it perceives as a female ‘pseudocopulates,’during which pollen dusted on male been is body .
  • Male bee transfers pollen to another flower when the same bee pseudocopulates with another flower.
  • Ophrysdoes so because pollination success will be reduced unless it co-evolves with female bee.

3.Describe the exponential growth model of a diagram along with a curve?
Ans.This kind of curve is observed in the case of under population of reindeer growing in apredator free natural environment having plenty of food. In this case, the curve formed is J-curvethe small population first takes time to adjust into new environment so there is no increase in thepopulation. Once they get adapted they multiply exponentially. This growth & multiplicationcontinues so far the food is available. After sometime the food supply becomes less as compared tothe population increases. This causes mass starvation & mortality & results in the formation of Jshaped curve.
The J-shaped growth form is described by equation
ΔNΔt= rN or ΔNΔtNΔNΔt= rN or ΔNΔtN

4.Describe the logistic growth model of population along with a suitable curve. Why is this curve more realistic?
Ans.The logistic growth curve shows a sigmoid or a S-shaped curve. It has three phases:-
(i) Lag-phase :- It is the early phase of little or no growth. Lag phase is one in which under population of cells adapt to or stablises with the growth conditions before embarking up their multiplication.
(ii) Log phase or Exponential phase :- It is the middle phase of rapid or geometric rise, Once stabilized cells starts to multiply rapidly when the small population is stablised, the multiply becomes faster because of the plenty amount of food & other requirements of life.
(iii) Stationary phase or steady phase:- Soon after the amount of food decreases in proportion to the number of cells & this results in the onset of stationary phase. During this phase, the number of new cells produced is roughly equal to the number of cells dead & so there is no net increase in the number of cells.
Sigmoid growth curve is demonstrated by foΔNΔt = rN (KN)NΔNΔt = rN (K−N)N

ΔN rate of change inpopulation ΔN− rate of change inpopulation Δt – change in timeΔt – change in time. 
K – carrying capacity
R – biotic potential

5.Give an example to show that completely unrelated species can also compete for same resources?
Ans.Completely unrelated species can also compete for same resources for e.g. In certain shallow lakes of South America the visiting flamingoes & the native fishes compete for the same zooplanktons as their food.

6.What is Age pyramid? What are the different types of age pyramid?
Ans.The geometrical diagrammatic representation of different age groups in a population of any organism is called Age of pyramids. These are of three types:-
i) Expanding pyramid:- It is a broad base, triangular pyramid which represents a population containing large number of young people. It is rapidly expanding population with high birth rate.
ii) Stable pyramid:- It represents a moderate proportion of young to old. As the rate of growth becomes slow & stable i.e.- pre-reproductive & reproductive age groups becomes more or less equal in size.
iii) Declining Pyramid:- The type of pyramid of population decreasing in size is characterised by a narrow base because there are fewer pre-reproductive individuals than in the other two age categories.

7. Differentiate between regulators & conformers? Why do small animals do not show regulations?
Ans.The organisms which maintain homeostasis by physiological or behavioral means & ensures aconstant body temperature & constant osmotic concentration etc. are called regulators e.g. all birds,mammals some lower vertebrates & invertebrates, for example in summer, when outside temp is morethan our body temperature we sweat profusely evaporative cooling brings the body temp – down.Whereas those organisms which cannot maintain a constant internal environment. Their bodytemperature changes with ambient temperature e.g. majority of animals & nearly all plants.

Small organisms does not show regulationbecause thermoregulation is an energy –expensive process. Since small animals havelarge surface area relative to volume, they tendto lose body heat very fast when it is coldoutside they have to expend much energy togenerate body heat through metabolism.

Organisms and Populations Class 12 Biology MCQs

1. Animals from colder climates generally have shorter limbs. This is called
(a) Allen’s rule
(b) Johnson’s rule
(c) Arber’s rule
(d) Niche rule


Answer: a

2. Niche is defined as
(a) a component of an ecosystem
(b) an ecologically adapted zone of a species
(c) the physical position and functional role of a species within the community
(d) all plants and animals living at the bottom of a water body.


Answer: c

3. It natality is balanced by mortality in a population at a given time, there will be a/an
(a) decrease in the population size
(b) increase in the population size
(c) zero population growth
(d) population explosion


Answer: c

4. Mycorrhiza is an example of
(a) ectoparasitism
(b) mutualism
(c) endoparasitism
(d) predation


Answer: b

5. The interspecific interaction in which one partner is benefitted and the other is unaffected (neutral), is called
(a) amensalism
(b) mutualism
(c) competition
(d) commensalism


Answer: d

6. Individuals of one kind, i.e., one species occupying a particular geographic area, at a given time form a/an
(a) community
(b) biome
(c) population
(d) deme


Answer: c

7. The formula of exponential population growth curve, is
(a) dN/dt = rN
(b) dt/dN rN
(c) dN/rN = dt
(d) rN/dN = dt


Answer: a

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


Answer: c

9. Amensalism is an association between two species where [NCERT Exemplar]
(a) one species is harmed and other is benefitted
(b) one species is harmed and other is unaffected
(c) one species is benefitted and other is unaffected
(d) both the species are harmed.


Answer: b

10. A population has more young individuals compared to the older individuals. What would be the status of the population after some years? ]NCERT Exemplar]
(a) It will decline
(b) It will stabilise
(c) It will increase
(d) It will first decline and then stabilise


Answer: c

11. Which of the following would necessarily decrease the density of a population in a given habitat? [NCERT Exemplar]
(a) Natality > mortality
(b) Immigration > emigration
(c) Mortality and emigration
(d) Natality and immigration


Answer: c

12. What parameters are used for tiger census in our country’s national parks and sanctuaries? [NCERT Exemplar]
(a) Pug marks only
(b) Pug marks and faecal pellets
(c) Faecal pellets only
(d) Actual head counts


Answer: b

13. The organisms which can tolerate and thrive in a wide range of temperature, are called _______ .


Explaination: Eury thermal

14. The salinity (measured in parts per thousand) in the sea is ______ .


Explaination: 30-35

15. _______ is any attribute of an organism (morphological, physiological and behavioural) that enables it to live and reproduce in the given area. 24 Match the terms in Column I with their


Explaination: Adaptation

16. _______ refers to the number of births during a given period of time that are added to the initial density.


Explaination: Natality

17. In a logistic growth curve, the final phase is an _______ .


Explaination: Asymptote

18. _______ fish breed only once in their life time.


Explaination: Pacific salmon

19. An orchid growing as an epiphyte on a mango tree, is an example of _______ .


Explaination: Commensalism

20. _______ is an important process as it facilitates energy transfer through various organisms.


Explaination: Predation

21. _______ showed that five closely related species of warblers living on the same were able to avoid competitions and co-exist.


Explaination: MacArthur

22. Zooplanktons enter, a state of suspended development under unfavourable conditions.


Explaination: Diapause

23. Match the terms in Column 1 with those in Column II.

Column IColumn II
A. Amensalism1. The interspecific interaction, where both are equally benefitted.
B. Parasitism2. The interspecific interaction, where one is benefitted and one is neutral.
C. Mutualism3. The interspecific interaction, where one is harmed and the other is neutral.
D. Commen­salism4. The interspecific interaction, where one is benefitted and one is harmed.
E. Competition 

Explaination: A – 3, B – 4, C – 1, D – 2

24 Match the terms in Column I with their descriptions in Column II.

Column IColumn II
A. Home­ostasis1. Animal which can tolerate a wide range of temperature.
B. Confor­mers2. The number of births in a given population at a given time.
C. Natality3. Per capita births in a given population.
D. Eury- thermal4. A Maintenance of a relatively constant internal environment.
 5. Animals which change their body temperature according to the ambient temperature.

Explaination: A – 4, B – 5, C – 2, D – 1

25. Zooplanktons enter a state of suspended development, called diapause, under unfavourable conditions. [True/False]


Explaination: True.

26. The success of mammals is due to their ability to change their body temperature according to their surroundings. [True/False]


Explaination: True.

27. Small animals like shrews and humming birds are rarely found in polar regions. [True/False]


Explaination: True.

28. Organisms living in water bodies (lake, sea, river) do not face any water related problems. [True/False]


Explaination: False.

29. dSfdt=rN is the equation describing logistic growth. [True/False]


Explaination: False.

Directions (Q30 to Q32): Mark the odd one in each of the following groups.
30. Aestivation, Migration, Hibernation, Diapause.


Explaination: Migration.

31. Parasitism, Predation, Commensalism, Amensalism.


Explaination: Amensalism.

32. Ticks, Lice, Copepods, Tapeworm.


Explaination: Tapeworm.

33. Who is considered as the ‘Father of Ecology ’ in India?


Explaination: Ramdeo Misra.

34. What is ecology at the organismic level?


Explaination: Ecology at the organismic level is essentially physiological ecology, which tries to understand how different organisms are adapted to their environments in terms of not only survival but also reproduction.

35. What causes the annual variation in the intensity and duration of temperature?


– Rotation of earth around the Sim.
– Tilt of the earth on its axis.

36. Name the two factors that cause the formation of major biomes.


– Variation in the intensity and duration of temperature.
– Variation in precipitation.

37. What does the ecological niche of an organism represent?


Explaination: Ecological niche of an organism represents the range of conditions it can tolerate, the resources it utilises and its functional role in the ecosystem.

38. Why are mango trees unable to grow in temperate climate? [AI 2016C]


– The levels of thermal tolerance of species determine their geographical distribution, because temperature affects the physiological functions by affecting the kinetics of enzymes.
– Stenothermal organisms (like mango) can tolerate and survive only in a narrow range of temperature, say tropics.

39. Mention the effect of global warming on the geographical distribution of stenothermals like amphibians. [Foreign 2012]


Explaination: Stenothermal animals have tolerance to a narrow range of temperatures and hence their geographical distribution would be much affected.

40. Between amphibians and birds, which will be able to cope with global warming? Give reason. [HOTS]


Explaination: Birds will be able to cope with global warming; they are eurythermals and can tolerate a wide range of temperatures.

41. How do herbs and shrubs survive under the shadow of big canopied trees in forests?


Explaination: The herbs and shrubs growing in the forests are adapted to photosynthesise optimally under very low light conditions.

42. Name a ‘photoperiod’-dependent process, one each in plants and in animals. [Foreign 2013]


– Flowering in plants.
– Timing of foraging or migration in plants.

43. Mention any two activities of animals which get cues from diurnal and seasonal variations in light intensity. [Delhi 2011C]


(i) Timing their foraging.
(ii) Migratory activities.
(iii) Reproduction. (any two)

44. Why are green algae not likely to be found in the deepest strata of the ocean? [AI 2013] [HOTS]
Why are green plants not found beyond a certain depth in the ocean? [Delhi 2011] [HOTS]
Why are green algae not found at lower depths of a sea? [Delhi 2011C]


Explaination: Green algae or plants are not found beyond a certain depth, as light (not all colour components of visible spectrum) is not available.

45. What is the advantage of homeostasis to organisms that exhibit it?


Explaination: Homeostasis enhances the overall fitness of organisms because all biochemical reactions and physiological functions proceed with maximal efficiency.

46. Which feature of mammals, is the success rate of them, attributed to?


Explaination: Their ability to maintain a constant body temperature irrespective of the environmental temperature.

47. Why have many animals not evolved thermo¬regulation? [HOTS]


Explaination: Thermoregulation is an energy-expensive phenomenon; considering the cost and benefit, many animals have not evolved thermoregulation.

48. What are partial regulaters?


Explaination: Partial regulaters are those species which have the ability to regulate, but only over a limited range of environmental conditions, beyond which they simply conform.

49. What are osmoconformers?


Explaination: Osmoconformers are those organisms which regulate the osmolarity of their body fluids according to their surrounding medium.

50. What is migration?


Explaination: Migration refers to the movement of animals from the stressful habitat to a more hospitable area and return to the original place once the stressful period is over.

51. Name the National Park in India where migratory birds arrive in winter from Siberia.


Explaination: Keolado National Park, Bharatpur.

52. Mention how bears escape from stressful time in winter. [Delhi 2013C]


Explaination: Bears go into hibernation in winter.

53. How do snails escape from the stressful time in summer? [AI2013C]
How do animals like fish and snails avoid summer-related unfavourable conditions? [Delhi 2010]


Explaination: Snails and fish go into aestivation in summer.

54. When and why do animals like frog/bear hibernate?


Explaination: If the unfavourable (stressful) conditions are for a short duration and if the animals are not able to migrate, they hibernate during extreme winter to avoid the stress by escaping in time.

55. When and why do animals like snails go into aestivation?


Explaination: When the snails are not able to migrate from the stressful conditions in the habitat, they undergo aestivation during hot summers to avoid stress by escaping in time.

56. Give an example of an organism that enters ‘diapause’ and why? [Delhi 2014]


– Zooplankton.
– It is to tide over the temporary unfavourable conditions in the habitat.

57. How do seed-bearing plants tide over dry and hot weather conditions? [AI 2013C]


Explaination: In seed-bearing plants, the seeds and other vegetative reproductive structures serve to tide over the unfavourable conditions; they germinate to form new plants under favourable conditions.

58. Define adaptation.


Explaination: Adaptation is defined as any morphological, physiological or behavioural attribute of an organism that enables it to survive and reproduce in its habitat.

59. How do spines help the cactus plants survive in the desert?


(i) Leaves are reduced to spines to check transpiration.
(ii) Spines keep away the browsing animals.

60. What is meant by Allen’s Rule?


Explaination: Allen’s rule refers to the reduction of heat loss in animals by having shorter ears and limbs.

61. Why is population ecology considered an important area of ecology?


Explaination: Population ecology is an important area of ecology, because it links ecology with population genetics and evolution; natural selection operates at the level of population.

62. If 8 individuals in a laboratory population of 80 fruit flies died in a week, then what would be the death rate of the population for the said period? [Delhi 2010]


Explaination: The death rate of fruit flies is 8/80, i.e. 100 per thousand or 10 per cent or 0.1 per individual.

63. In a pond, there were 20 Hydrilla plants. Through reproduction, 10 new Hydrilla plants were added in a year. Calculate the birth rate of the population. [Delhi 2010]


Explaination: The birth rate of Hydrilla is 10/20, i.e. 500 per thousand or 50 per cent or 0.5 per individual.

64. Define population density.


Explaination: Population density refers to the total number of individuals of a species present per unit area or volume at a given time.

65. Provide an instance where the population size of a species can be estimated indirectly, without actually counting them or seeing them. [Delhi 2016C]


Explaination: Tiger census is carried out by counting the pug marks or faecal pellets.

66. Define natality.


Explaination: Natality is defined as the number of births that are added to the initial density in a population during a given period of time.

67. Define mortality.


Explaination: Mortality refers to the number of deaths in the population during a given period.

68. What does nature’s carrying capacity for a species indicate? [Foreign 2016]


Explaination: It refers to the maximum number of individuals of a population that the given environment can sustain with its resources.

69. Name two organisms (one plant and one animal) which breed only once in their lifetime.


Explaination: Bamboo; Pacific Salmon Fish.

70. Why have life history variations evolved?


Explaination: Life history variations have evolved in organisms to maximise their reproductive fitness or Darwinian fitness in their natural habitats.

71. Mention any two reasons why plants depend on other organisms for their survival, even though they make their own organic food.


(i) Plants depend on animals (mainly insects) for pollination.
(ii) They also depend on animals for dispersal of seeds and/or fruits.
(iii) They depend on the soil microbes, which can carry out decomposition of organic matter and return the inorganic nutrients to the soil for absorption by plants. (any two)

72. Why are cattle and goats not browsing the Calotropis growing in the fields? [Foreign 2011]


Explaination: Calotropis plants produce highly toxic cardiac glycosides; hence, the cattle avoid browsing them.

73. Write one common feature among predation, parasitism and commensalism.


Explaination: In all these, the interacting species live closely together and one of the species is benefitted.

74. What term is given to the predators of plants.


Explaination: Herbivores.

75. What type of interaction is shown by a sparrow eating the seeds?


Explaination: Predation.

76. An exotic variety of prickly pear introduced into Australia turned out to be invasive. How was it brought under control? [Delhi 2013C]


Explaination: By introducing a cactus-feeding moth.

77. Why are predators ‘prudent’ in nature? [HOTS]


Explaination: Predators are prudent in nature because if a predator is too efficient and over-exploits its prey, then the prey might become extinct and following it, the predator will also become extinct.

78. Why do predators avoid eating monarch butterfly? How does the butterfly develop this protective feature? [Foreign 2010]


– The monarch butterfly is highly distasteful to the predators.
– It is due to a chemical present in its body; the butterfly acquires this chemical during its caterpillar stage, by feeding on a poisonous weed.

79. Write what the phytophagous insects feed on. [Delhi 2012]


Explaination: Phytophagous insects feed on the sap and other parts of the plants.

80. Why is the problem of predation in plants more severe than that in animals? [HOTS]


Explaination: Problem of predation is severe for plants because they cannot move away from their predators.

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