Class 12 Organisms and Populations Mock Test (CBSE)

Topics in Organisms and Populations

• Population attributes
• Population growth
• Exponential and logistic growth
• Life history variation
• Population interactions
• Predation, competition and parasitism
• Commensalism and mutualism

Organisms and Populations practice questions with answers

1. Q1Multiple choice·1 mark
   Which of the following organisms breeds only once in its lifetime?

   1. (a)Birds
   2. (b)Pacific salmon fish
   3. (c)Oysters
   4. (d)Mammals
   Show answer and step marks

   1. Correct option: (b) Pacific salmon fish+1

   Answer: (b)
2. Q2Multiple choice·1 mark
   An interaction between two species where one species is harmed and the other is unaffected is known as:

   1. (a)Mutualism
   2. (b)Competition
   3. (c)Amensalism
   4. (d)Commensalism
   Show answer and step marks

   1. Correct option: (c) Amensalism+1

   Answer: (c)
3. Q3Short answer·2 marks
   State Gause's Competitive Exclusion Principle and provide one suitable example that supports it.
   Show answer and step marks

   1. Statement of Gause's Competitive Exclusion Principle: Two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will eventually be eliminated.+1
   2. Example, any one of: Gause's experiment, in which Paramecium aurelia outcompeted and eliminated Paramecium caudatum when both were grown together on limited food; Connell's field study on the rocky sea coasts of Scotland, in which the larger, competitively superior barnacle Balanus excluded the smaller barnacle Chthamalus from the intertidal zone; NCERT's example of the Abingdon tortoise in the Galapagos Islands, which became extinct within a decade after goats were introduced on the island, apparently due to the greater browsing efficiency of the goats; or any other valid example of one species competitively excluding another that uses the same limited resources.+1

   Answer: Two closely related species competing for the same limited resources cannot coexist indefinitely; the competitively inferior one is eventually eliminated. Example: Paramecium aurelia eliminating P. caudatum in Gause's cultures (or Balanus excluding Chthamalus on Scottish rocky coasts, or the Abingdon tortoise becoming extinct after goats were introduced on the Galapagos Islands).
4. Q4Short answer·2 marks
   State Gause's Competitive Exclusion Principle. Mention one mechanism that competing species might evolve to avoid competition and coexist.
   Show answer and step marks

   1. Statement of Gause's Competitive Exclusion Principle: Two closely related species competing for the same resources cannot coexist indefinitely and the competitively inferior one will eventually be eliminated.+1
   2. Mechanism for coexistence, any one of: resource partitioning: choosing different times for feeding or different foraging patterns, e.g. MacArthur's five warbler species feeding in different parts of the same tree; character displacement: evolving differences (e.g. in beak size) so that the species use different food; or any other valid mechanism.+1

   Answer: Two closely related species competing for the same resources cannot coexist indefinitely; the inferior one is eventually eliminated. Mechanism: resource partitioning, i.e. feeding at different times or foraging differently (e.g. MacArthur's warblers feeding in different parts of a tree).
5. Q5Short answer·3 marks
   Differentiate between mutualism and commensalism. Give one suitable example for each and specify the nature of interaction using signs (+, -, 0) for both participating species.
   Show answer and step marks

   1. Mutualism: an interaction in which both species benefit.+0.5
   2. Sign for mutualism: $+/+$.+0.5
   3. Example of mutualism, any one of: lichens (a fungus and an alga or cyanobacterium); mycorrhizae (a fungus and the roots of a higher plant); a fig tree and its pollinating wasp; or any other valid example.+0.5
   4. Commensalism: an interaction in which one species benefits and the other is neither harmed nor benefited.+0.5
   5. Sign for commensalism: $+/0$.+0.5
   6. Example of commensalism, any one of: an orchid growing as an epiphyte on a mango branch; barnacles growing on the back of a whale; the cattle egret foraging near grazing cattle; a clown fish living among the tentacles of a sea anemone; or any other valid example.+0.5

   Answer: Mutualism: both species benefit ($+/+$), e.g. lichens or mycorrhizae. Commensalism: one species benefits and the other is unaffected ($+/0$), e.g. an orchid growing on a mango branch or barnacles on a whale.
6. Q6Short answer·3 marks
   What is meant by a population in ecology? Explain any three distinct attributes that a population exhibits, which an individual organism does not.
   Show answer and step marks

   1. Definition: a population is a group of individuals of the same species living in a given geographical area.+1
   2. They share or compete for similar resources and can potentially interbreed.+0.5
   3. First attribute, any one of: birth rate (natality): the number of births per individual (per capita) in the population per unit time; death rate (mortality): the number of deaths per individual (per capita) in the population per unit time; sex ratio: the ratio of females to males in the population; age distribution: the proportions of pre-reproductive, reproductive and post-reproductive individuals, shown as an age pyramid; population density: the number of individuals per unit area at a given time; or any other valid population attribute.+0.5
   4. Second attribute, a different one from the same list.+0.5
   5. Third attribute, a different one from the same list.+0.5

   Answer: A population is a group of individuals of the same species in a given area that share or compete for resources and can interbreed. Any three of: birth rate (per capita births per unit time); death rate (per capita deaths per unit time); sex ratio (ratio of females to males); age distribution (proportions of pre-reproductive, reproductive and post-reproductive individuals, shown as an age pyramid); population density (individuals per unit area).

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