CBSE 2025 · Region 1 · Set 1 · Q30 · 4 marks
Read the following passage and answer the questions that follow : In nature, we rarely find isolated, single individuals of any species; majority of them live in groups in a well-defined geographical area, share or compete for similar resources, potentially interbreed and thus constitute a population. The population has certain attributes whereas, an individual organism does not. A population at a given time is composed of individuals of different ages. The size of the population tells us a lot about its status in the habitat. Whatever ecological processes we wish to investigate in a population, be it the outcome of competition with another species, the impact of the predator or the effect of pesticide application, we always evaluate in terms of any change in the population size. The size, in nature, could be low or go into millions. Population size, technically called population density (N) need not necessarily be measured in numbers only. The size of a population for any species is not a static parameter. It keeps on changing with time depending on various factors including food availability, predation pressure and adverse weather.(a)The Monarch butterfly is highly distasteful to its predator because of a special chemical present in its body. How does the butterfly acquire this chemical ?(b)If population density at a time t + $\displaystyle 1$ is $\displaystyle 800$, Emigration = $\displaystyle 100$, Immigration = $\displaystyle 200$, Natality = $\displaystyle 200$ and Mortality = $\displaystyle 150$, calculate the population density at time t and comment upon the type of age pyramid that will be formed in this case. Student to attempt either sub-part (c) or (d) :What is the difference in a method of measuring population density in an area if there are $\displaystyle 200$ carrot grass plants to only single huge banyan tree ?Name two methods to measure the population density of tigers.
Read the following passage and answer the questions that follow : In nature, we rarely find isolated, single individuals of any species; majority of them live in groups in a well-defined geographical area, share or compete for similar resources, potentially interbreed and thus constitute a population. The population has certain attributes whereas, an individual organism does not. A population at a given time is composed of individuals of different ages. The size of the population tells us a lot about its status in the habitat. Whatever ecological processes we wish to investigate in a population, be it the outcome of competition with another species, the impact of the predator or the effect of pesticide application, we always evaluate in terms of any change in the population size. The size, in nature, could be low or go into millions. Population size, technically called population density (N) need not necessarily be measured in numbers only. The size of a population for any species is not a static parameter. It keeps on changing with time depending on various factors including food availability, predation pressure and adverse weather.
(a)
The Monarch butterfly is highly distasteful to its predator because of a special chemical present in its body. How does the butterfly acquire this chemical ?
(b)
If population density at a time t + $\displaystyle 1$ is $\displaystyle 800$, Emigration = $\displaystyle 100$, Immigration = $\displaystyle 200$, Natality = $\displaystyle 200$ and Mortality = $\displaystyle 150$, calculate the population density at time t and comment upon the type of age pyramid that will be formed in this case.
Student to attempt either sub-part (c) or (d) :
What is the difference in a method of measuring population density in an area if there are $\displaystyle 200$ carrot grass plants to only single huge banyan tree ?
Name two methods to measure the population density of tigers.
Marking-scheme solution
(a)
The butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.
(b)
Nt+$\displaystyle 1$ = Nt + [(B + I) – (D + E)],
$\displaystyle \Rightarrow$ $\displaystyle 800$ = Nt + [($\displaystyle 200$ + $\displaystyle 200$) – ($\displaystyle 150$ + $\displaystyle 100$)]
$\displaystyle \Rightarrow$ $\displaystyle 800$ = Nt + ($\displaystyle 400$ – $\displaystyle 250$)
$\displaystyle \Rightarrow$ $\displaystyle 800$ = Nt + $\displaystyle 150$
$\displaystyle \Rightarrow$ Nt = $\displaystyle 800$ – $\displaystyle 150$ = $\displaystyle 650$
Comment
$\displaystyle \star$ As the population density is increasing with time so age pyramid would be of expanding population.
$\displaystyle \star$ Single huge banyan tree - measured in terms of biomass or percent cover ,
$\displaystyle \star$ carrot grass- measured in terms of percent cover
Pug marks, faecal pellets.
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CBSE Class 12 Biology past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.