Chapter 13 -Organisms and Populations.

02 Jan 2021 7:44 am

Chapter 13: Organisms and Populations, hsc, biology, maharashtra board, 12th std, balbharathi solution,

Chapter 13: Organisms and Populations

Multiple choice question

1.Which factor of an ecosystem includes plants, animals, and microorganisms?


  • Biotic factor
  • Abiotic factor
  • Direct factor
  • Indirect factor

2.An assemblage of individuals of different species living in the same habitat and having functional interactions is _______________.


  • Biotic community
  • Ecological niche
  • Population
  • Ecosystem

3.Association between sea anemone and Hermit crab in gastropod shell is that of _______________.


  • Mutualism
  • Commensalism
  • Parasitism
  • Amensalism

4.Select the statement which explains the best parasitism.


  • One species is benefited.
  • Both the species are benefited.
  • One species is benefited, other is not affected.
  • One species is benefited, other is harmed.

5.Growth of bacteria in a newly innoculated agar plate shows ____________.


  • exponential growth
  • logistic growth
  • Verhulst-Pearl logistic growth
  • zero growth

Define the following term and give one example for each:



Commensalism:Commensalism is an interaction between two species in which one species gets benefited while the other remains unaffected. An orchid growing on the branches of a mango tree and barnacles attached to the body of whales are examples of commensalisms.



Commensalism is the interaction in which one species gets benefited and the other is neither harmed nor benefited.

2. Parasitism


Parasitism is a kind of relationship between two species in which one species which is termed as parasite, derives its food from the other species which is termed as host. Parasitism also involves shelter, in addition to food obtained by a parasite. Parasites may be ectoparasites or endoparasites. Ectoparasites live on the surface of their host while endoparasites live inside the body of the host.

Examples of Parasitism

(i) Cuscuta growing on shoe flower plant: Cucuta grows on the stem of shoe and derive nutrition from the plant.

(ii) Head lice is an ectoparasite and suck human blood

(iii) Ascaris, Taenia, Plasmodium causing diseases in humans

(iv) Koel laying its eggs in crow’s nest is an example of Brood parasitism. Birds lay egg in the nest of its host and host incubate it.



Parasitism is the interaction in which only one species (parasite) is benefited and the interaction is detrimental to other species (host).




Camouflage is the cryptic coloration or patterns adopted by prey species to blend with the surroundings or background so as to escape their predators.




E.g. Lichens represent an intimate, mutualistic relationship between a fungus and photosynthetic algae or cyanobacteria.

3.Give one example for Interspecific competition


Interspecific competition:

E.g. Competition between leopards and lion, resident fish competing with migratory birds Flamingos for common food i.e. zooplankton

4.Name the type of association: Clownfish and sea anemone


Clown fish and sea anemone: Commensalism

5.Name the type of association: Crow feeding the hatchling of Koel


Crow feeding the hatchling of Koel: Brood parasitism

6.Name the type of association: Humming birds and host flowering plants


Humming birds and host flowering plants: Mutualism

7.What is the ecological process behind the biological control method of managing with pest insects?


The ecological process behind the biological control method of managing with pest insects is Predation. Predators regulate the population of prey in a habitat, thus helping in the management of pest insects.

Short answer question

1.How is the dormancy of seeds different from hibernation in animals?


i. Seed dormancy is the inability of viable seeds to germinate even under suitable environmental conditions, whereas hibernation in animals is a state of reduced activities to escape cold winter conditions.

ii. During seed dormancy, growth and development of an embryo are arrested temporarily, whereas in hibernation animals enter a state of inactivity by slowing their metabolism.

2.If a marine fish is placed in a freshwater aquarium, will it be able to survive? Give reason.


i. If a marine fish is placed in a freshwater aquarium, fish would not be able to survive because marine fishes are adapted to high salt concentrations of the marine environment.

ii. Marine fishes have more osmotic concentration (more salt concentration) than marine water which prevents marine water to enter into the body.

iii. When marine fish is placed in a freshwater aquarium, water enters into the body of marine fish due to osmosis, as freshwater creates a hypotonic environment outside the fish’s body.

iv. Entry of water into the body causes its body to swell leading to the death of the marine fish.

3.Name important defense mechanisms in plants against herbivores.


Defense mechanisms in plants against herbivores can be morphological like thorns (in Acacia, Cactus) or chemicals like poisonous cardiac glycosides (produced by Calotropis), secondary metabolites (for e.g.nicotine, caffeine, quinine, strychnine, opium, etc.)

4.An orchid plant is growing on the branch of the mango tree. How do you describe this interaction between the orchid and the mango tree?


An orchid growing on the branch of a mango tree is an epiphyte. Epiphytes are plants growing on other plants which however, do not derive nutrition from them. Therefore, the relationship between a mango tree and an orchid is an example of commensalisms, where one species gets benefited while the other remains unaffected. In the above interaction, the orchid is benefited as it gets support while the mango tree remains unaffected.


i. An orchid plant is growing on the branch of a mango tree represents Commensalism.

ii. In this interaction, one species gets benefited and the other is neither harmed nor benefited.

iii. Orchid is an epiphytic plant. While growing on mango tree it gets support but does not derive any nutrition from the mango tree.

iv. Thus, the orchid plant is benefited, while the mango tree is neither benefited nor harmed.

5.Distinguish between the following:

Hibernation and Aestivation


i.It is also called as winter sleep.It is also called as summer sleep.
ii.It is a state of reduced activities in some organisms to escape cold winter conditions.It is a state of reduced activities in some organisms to escape desiccation due to heat in summer.
iii.Animals rest in warm places.Animals rest in cool, shady, and moist places.
iv.It is shown by bears inhabiting cold regions.It is shown by some fishes and snails.

6.Distinguish between the following:

Ectotherms and Endotherms


i.These are cold-blooded animals.These are warm-blooded animals.
ii.Ectotherms do not possess the ability to generate sufficient heat to keep them warm, thus their body temperature varies with surroundingsEndotherms do possess the ability to generate heat and keep them warm, thus they can maintain constant body temperature.
iii.They are also known as poikilothermic.They are also known as homeothermic.
iv.They are affected by changes in environmental temperatureThey remain unaffected by changes in environmental temperature.
 E.g. Most of the fishes, amphibians, reptilesE.g. Birds, mammals

7.Distinguish between the following:

Parasitism and Mutualism


i.In parasitism, only one species (parasite) is benefited and the interaction is detrimental to other species (host).In mutualism, both species are benefited.
ii.The parasite needs a host, but the host does not need the parasite.Both species need the presence of each other.
E.g.Cuscuta, a parasitic plant commonly found growing on hedge plants.Lichen represents the mutualistic relationship between a fungus and photosynthetic algae or cyanobacteria.

8.Write a short note on Adaptations of desert animals


Adaptations of animals for desert habitats:

1. Desert animal-like Kangaroo rat inhabiting the Arizona deserts has the potential to concentrate its urine to conserve water. This animal never drinks water in its life.

2. Snakes and desert lizards bask in the sun early in the morning and burrow themselves in the sand in the afternoons to escape the heat of the day, to prevent water loss.

3. Camels can store fat in the hump which can be metabolised for energy. A camel can survive for many days without water. Long eyelashes, ears lined with hair, and slit-like nostrils help to keep out sand.

9.Write a short note on the Adaptations of plants to water scarcity.


Adaptations of plants to water scarcity:

Plants found in deserts are well adapted to cope with water scarcity and scorching heat of the desert. Plants have an extensive root system to tap underground water. They bear thick cuticles and sunken stomata on the surface of their leaves to reduce transpiration. In Opuntia, the leaves are modified into spines and the process of photosynthesis is carried out by green stems. Desert plants have special pathways to synthesize food, called CAM (C4 pathway). It enables their stomata to remain closed during the day to reduce water loss by transpiration.


Adaptations of plants for desert habitats:

1. Many desert plants have a thick cuticle on their leaf surfaces and have their stomata in deep pits to minimize loss of water through transpiration.

2. They show a special photosynthetic pathway (CAM – Crassulacean Acid Metabolism) that enables their stomata to remain closed during the daytime.

3. Some desert plants like Opuntia have their leaves reduced (modified) to spines and the photosynthetic function is taken over by the flattened stems.

10.Write a short note on Behavioural adaptations in animals


Behavioural adaptations in animals

Certain organisms are affected by temperature variations. These organisms undergo adaptations such as hibernation, aestivation, migration, etc. to escape environmental stress to suit their natural habitat. These adaptations in the behaviour of an organism are called behavioural adaptations. For example, ectothermal animals and certain endotherms exhibit behavioral adaptations. Ectotherms are cold blooded animals such as fish, amphibians, reptiles, etc. Their temperature varies with their surroundings. For example, the desert lizard basks in the sun during early hours when the temperature is quite low. However, as the temperature begins to rise, the lizard burrows itself inside the sand to escape the scorching sun. Similar burrowing strategies are exhibited by other desert animals. Certain endotherms (warm-blooded animals) such as birds and mammals escape cold and hot weather conditions by hibernating during winters and aestivating during summers. They hide themselves in shelters such as caves, burrows, etc. to protect against temperature variations.


Behavioural adaptations in animals.

a. To cope up with extreme variations in their environment, some organisms respond through behaviourally (like migration, hibernation, and aestivation).

b. For e.g. Desert lizards manage to keep their body temperature fairly constant by behavioural adaptations.

c. They bask in the sun and absorb heat when their body temperature drops below the comfort zone. But when the ambient temperature starts increasing, they move into the shade.

d. Some species burrow into the sand to hide and escape from the heat.

11.Define Population.


Population: Organisms of the same kind inhabiting a geographical area constitute the population.


Individuals live in groups in a well-defined geographical area, share or compete for similar resources, potentially interbreed and thus form a population.


The population is defined as a group of individuals of a species occupying a definite geographic area at a given time.

12.Define Community.


Community: Several populations of different species in a particular area constitute a community that interacts with one another in several ways.

Long answer question

1.With the help of a suitable diagram describe the logistic population growth curve.


Logistic growth curve of population

i. Resources like food and space are not always unlimited. They may be plenty in the beginning; but as the population density increases, competition for those resources starts, resulting in a slowdown in the rate at which the original population was growing. This results in a logistic or sigmoid growth curve.

ii. Competition between individuals for limited resources will weed out the ‘weaker’ ones. Only the ‘fittest’ individuals will survive and reproduce.

iii. A given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. This limit can be called nature’s carrying capacity (K) for that species in that habitat.

iv. A population growing in a habitat with limited resources shows initially a lag phase, followed by phases of acceleration and deceleration and finally an asymptote when the population density reaches the carrying capacity.

v. A plot of population density (N) in relation to time (t) results in a sigmoid curve. This type of population growth is called VerhulstPearl Logistic Growth.

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

2.Enlist and explain the important characteristics of a population.


i. The important characteristics of a population are population size, population density, natality, mortality, sex ratio, immigration, emigration, age pyramids, expanding population, population growth forms, and biotic potential.

ii. Some important characteristics of the population are:

a. Population density: Population density tells us the number of individuals presents per unit space, in a given time.


The density of a population is the total number of individuals in that population present per unit area at a specific time.

b. Natality: Natality is the birth rate of a population.

c. Mortality: Mortality is the death rate of a population.

d. Age distribution and Age pyramids:

1. A population consists of individuals of different ages. The entire population is divided into three age groups – pre-reproductive (0-14 years), reproductive (age 15-44 years), post-reproductive (45-85+years)The relative proportion of individuals of various age groups in the population is referred to as the age structure of the population.

2. If the age distribution (percent individuals of a given age or age group) is plotted for the population, the resulting structure is called as age pyramid.

e. Sex Ratio:

Sex ratio is the ratio of the number of individuals of one sex to that of the other sex.



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