Biology

Organisms and Populations- Competency-Based Questions

Class 12 Biology Competency-Based Questions Chapter 13

1 Mark Questions

1. In the image shown below, the dark grey part depicts the geographical distribution of the killer whale.

Which of these statements describes the killer whale and its habitat CORRECTLY?

A. Low thermal tolerance and narrow geographical distribution

B. High thermal tolerance and narrow geographical distribution

C. Low thermal tolerance and widespread geographical distribution

D. High thermal tolerance and widespread geographical distribution

Ans: D. High thermal tolerance and widespread geographical distribution

2. Two statements are given below – one labelled Assertion (A) and the other labelled Reasoning (R).

Assertion (A): Only stenohaline fish can survive in freshwater.
Reasoning (R): Stenohaline fish can tolerate a narrow range of salinity.

Which of the following is correct?

A. Both A and R are true, and R is a correct explanation of A.

B. Both A and R are true, but R is not a correct explanation of A.

C. A is true, but R is false.

D. A is false, but R is true.

Ans: D. A is false, but R is true.

3. The Atlantic salmon is a fish that can tolerate a salinity of 0 to 33 ppt (parts per thousand).

Which of the following is it likely to be classified as, and why?

A. Stenohaline, because it can survive only in freshwater.

B. Euryhaline, because it can survive only in marine water.

C. Euryhaline, because it can survive major variations in salt concentration.

D. Stenohaline, because it can survive only a narrow variation in salt concentration.

Ans: C. Euryhaline, because it can survive major variations in salt concentration.

4. Every year, millions of monarch butterflies fly from the United States and Canada to Mexico to escape the cold weather.

Which response to abiotic stress does the statement depict?

A. Migration

B. Regulation

C. Suspension

D. Conformation

Ans: A. Migration

5. Tilapia is a fish found in a variety of freshwater environments and is capable of adjusting its internal salt concentrations to match the salinity of the water.

Which response to abiotic stress does the statement depict?

A. Migration

B. Regulation

C. Suspension

D. Conformation

Ans: D. Conformation

6. The adaptations in which of the following animals enable it to satisfy the following conditions:

1. Allen’s rule

2. Loses body heat slowly

A. A

B. B

C. C

D. D

Ans: C. C

7. The population of sparrows inhabiting a garden decreased dramatically. It was found that the mortality was equal to natality for the given population.

Which of the following is TRUE for the population of sparrows?

A. The number of emigrants was equal to immigrants.

B. The number of emigrants was less than immigrants.

C. The number of emigrants was more than immigrants.

D. The population of sparrows was unaffected by emigrants and immigrants.

Ans: C. The number of emigrants was more than immigrants.

8. The black walnut plant secretes juglone, a chemical substance that destroys other plants like pepper growing within its root zone.

Which type of population interaction does the above statement represent?

A. Predation

B. Competition

C. Mutualism

D. Amensalism

Ans: D. Amensalism

9. The larvae of insects such as the African midge inhabit temporary pools in hollows of rocks when water evaporates in their arid environment. These larvae revive when immersed in water even after a few years.

Which response to abiotic stress does the statement depict?

A. regulation

B. migration

C. suspension

D. conformation

Ans: C. suspension

10. Trees in some regions have very thin bark, broad leaves placed at different angles to allow them to capture adequate sunlight, and leaves with pointed tips that allow water to run off easily.

In which biome are such trees likely to be found?

A. arctic

B. desert

C. coastal

D. rainforests

Ans: D. rainforests

2 Marks Questions

11. A forest can accommodate a maximum of 500 deer (i.e., its carrying capacity). If the initial population size is 100 deer and the maximum per capita growth rate is 0.05 per year, calculate the rate of change of the population size of deer for the given time period.

Ans:

12. A patch of grassland was inhabited by 100 goats in August 2022. Between August 2022 and December 2022, it was found that the average natality was 10, the average mortality was 8, the average immigration was 5, and the average emigration was 3.

At the end of December 2022,

(a) What was the population density of goats in the same patch of grassland?

(b) Was there a net increase or net decrease in the population of goats?

(c) What was the value by which there was a net increase or decrease in (b)?

Ans:

(a) Population density is calculated by the following formula:

Nt+1 = Nt + [(B+I)-(D+E)]

= 100 + [(10+5)-(8+3)]

= 104

(b) net increase

13. Population density is calculated by the following formula:

Nt+1 = Nt + [(B+I)-(D+E)]

Imagine a population of fish in an artificial lake that acts as a closed system.

(a) Which quantity or quantities of the above formula will NOT be used in calculating the population of fish?

(b) Give reason for (a).

Ans:

(a)
- I (immigration)
- E (emigration)
(b) A closed system will not have any individual entering it from inside or leaving it by going outside.

14. Read the scenario below and answer the questions that follow.

In the Caribbean, there are several species of Anolis lizards that live in the same habitats and feed on similar prey. However, research has shown that when two species have nearly identical body size and feeding habits, one species will outcompete the other for resources, leading to the exclusion of the other species.

(a) For the above example of competition to satisfy Gause’s Competitive Exclusion Principle, name two conditions that MUST be met.

(b) State two ways by which the two species could co-exist in the same habitat.

Ans:

(a)
- Resources are limited
- Competition is between closely related species
(b)
- Feeding at different times of the day
- Feeding on different parts of the same resource

15. Read the scenario below about interference competition and answer the questions that follow.

Red squirrels and gray squirrels both live in forests and compete for food and nesting sites. Red squirrels are known to be more aggressive than gray squirrels, and they will often chase gray squirrels away from food sources and nesting sites.

(a) The resources present in the habitat above must be limiting. Is this statement true or false? Justify your answer.

(b) Which species’ r-value is likely to drop and why?

Ans:

The statement is FALSE. The feeding efficiency of one species could be reduced due to the interfering and inhibitory presence of the other species, even if resources are abundant.
Gray squirrel. Because of the red squirrels’ aggression, the gray squirrels have limited access to the resources.

16. Which one of the following examples can be classified as mutualism? Justify.

(i) Male fireflies use flashing lights to attract females. However, some females will mimic the flash pattern of another species to attract and consume males of that species.

(ii) The pitcher plant produces a scent that mimics the scent of ripe fruit, which attracts insects that feed on fruit. When the insects land on the plant, they slip on the slippery surface of the pitcher and fall into the digestive fluid inside.

(iii) The night-blooming flowers of senita cacti are visited by tiny senita moths that transfer pollen. The female moth lays eggs on a flower petal, and eventually, the larva feeds on the seeds and fruit tissue.

Ans:

Option (iii) can be classified as mutualism as both species benefit.

In (i) and (ii), one species is being killed.

3 Marks Questions

17. Read the news clipping shown below and answer the questions that follow.

(a) Draw an age pyramid for the above data, clearly labelling the post-reproductive, reproductive, and pre-reproductive parts.

(b) Name the type of the age pyramid drawn in (a).

Ans:

(a)

(b) Declining

18. The surface area and volume of the bodies of two organisms (X and Y) are compiled in the table below.

Organism	Surface Area (cm²)	Volume (cm³)
X	3800	1900
Y	6.3	0.03

Based on the above table, answer the questions that follow:

(a) Calculate the surface area to volume ratio of organisms X and Y.

(b) Which of the two organisms is more suited to survive in a cold environment?

(c) State your reason for (b) in terms of heat loss.

Ans:

(a)

Organism X = 3800/1900 = 2:1

Organism Y = 6.3/0.03 = 210:1

(b) Organism X

(c) A low surface area to volume ratio means that the organism will lose less heat/retain heat better in a cold environment.

19. A country introduced the one-child policy in order to curb the population growth. Shown below are two age pyramids of the country, one from 1980 and the other from 2020.

(a) Identify the pyramids from 1980 and 2020.

(b) State one characteristic each of pyramid 1 and pyramid 2 that helped you in answering (a).

Ans:

(a)

Pyramid 1 – 2020
Pyramid 2 – 1980

(b) Pyramid1

More population is in the post-reproductive stage
Pyramid 1 is in the declining phase.

Pyramid 2

More population is in the pre-reproductive and reproductive stages
Pyramid 2 is in the expanding phase.

20. The following table contains values of the population of bacteria growing over time.

Time (in days)	Population (×106 cells/mL)
0	0.5
2	0.6
4	1.0
7	3.2
9	5.2
11	5.3
14	5.3

Based on the values in the table above,

(a) Construct a population growth curve.

(b) Indicate the carrying capacity in the graph.

(c) Give reason for the position of the carrying capacity.

Ans:

(a) and (b)

21. Mistletoe is a plant that grows on the branches of oak trees. It takes nutrients from the oak tree and can weaken or kill it over time.

(a) Identify the host and the parasite.

(b) Give one point of similarity and difference between the above scenario and predation.

Ans:

(a)
- Host – oak tree
- Parasite – mistletoe
(b) Similarities
- The parasite and predator benefit from the relationship.
- The host and prey are harmed in the relationship.
- One species always benefits, and the other is always harmed.
Differences:
- The parasite lives inside or on the host, whereas the predator does not.
- The parasite may or may not kill the hos,t but the predator always kills the host.

22. The following table contains values of the population of seals inhabiting Alaska. 3

Year	Number of seals
1975	1500
1980	3000
1985	5500
1990	7400
1995	7200
2000	7100
2005	7050
2010	7100

(a) Based on the values in the table above, construct a population growth curve.

(b) Name and describe the type of growth as seen in the curve plotted.

Ans:

(a)

(b)

Logistic growth
Population size increases rapidly at first, then slows down as it approaches the carrying capacity of the environment

23. Mark the following statements as TRUE or FALSE and give a reason to support your answer.

(a) Competition happens only between closely related species.

(b) Competition always leads to the elimination of one species.

(c) Two species compete only when the resources are limited.

Ans:

(a) False. Unrelated species can also compete for the same resource.
(b) False. Some species in a competitive relationship might co-exist through resource partitioning.
(c) False. Species can also compete when resources are abundant, where one species interferes with the feeding patterns of another species.

5 Marks Questions

24. A small town has a population of 10,000 people. In the past year, there were 250 births and 970 deaths in the town.

(a) Calculate the birth rate per thousand for the given population.

(b) Calculate the death rate per thousand for the given population.

(c) Complete the following statement.

The state of population in the town is ___________ (stable/declining/increasing) because the birth rate is _________ (equal to/greater than/lesser than) the death rate.

(d) Name two other factors that would influence the density of the given population.

Ans:

(a)

Birth rate = (Number of births / Total population) x 1000

= (250/10000) x 1000

= 0.025 x 1000

= 25 births per 1000 people

(b)

Death rate = (Number of deaths / Total population) x 1000

= (970/10000) x 1000

= 0.097 x 1000

= 97 deaths per 1000 people

(c)

Declining
Lesser than

(d)

Immigration/entry of individuals of the same species into the habitat
Emigration/exit of individuals of the same species from the habitat

25. Show below is a graph that represents the predator-prey relationship. Answer the questions that follow:

(a) Give one evidence that the ecosystem is stable.

(b) What will be the effect of the migration of rabbits on the population of wolves and why?

(c) Name and describe one adaptation that helps prey escape predation.

(d) In a hypothetical scenario, all wolves vanish from the ecosystem.

(i) What will be the consequence on the vegetation present in the ecosystem and why?

(ii) It is found that the population of rabbits does not increase dramatically. State the ecological phenomenon responsible for keeping the population of rabbits in check.

Ans:

(a) The number of predators and prey cycles over generations.
(b)
- The number of wolves will increase.
- As the population of rabbits increases, there will be plenty of food for the wolves to eat and consequently reproduce.
(c)
- Camouflage
- Some organisms blend in with their surroundings because of their colour.

Poisonous
Some organisms are highly distasteful to their predators because of special chemicals present in their bodies.

(d)
- The vegetation will decline.
- In the absence of predators, the population of rabbits will no longer be in check.
- As the population of rabbits increases, the vegetation will decrease.
(e) Competition with another species