VBQs Class 12 Biology Organisms and Populations
Very Short Answer Type Questions
Question. Provide an instance where the population size of a species can be estimated indirectly, without actually counting them or seeing them.
Answer : Tiger census in National parks and Tiger reserves was done on the basis of counting pug marks / faecal pellets.
Question. Name the interaction that exists between sucker fish and shark.
Answer : Commensalism.
Question. State the type of interaction that exists between ticks and dogs.
Answer : Ecto (Parasitism).
Question. Name the type of association that the genus Glomus exhibits with higher plants.
Answer : Glomus exhibits arbuscular mycorrhizal (AM) association.
Question. Name two intermediate hosts which the human liver fluke depends on to complete its life cycle so as to facilitate parasitisation of its primary host.
Answer : Snail and Fish.
Question. What does nature’s carrying capacity for a species indicate ?
Answer : (In nature) a given habitat has enough or limited resources to support a maximum possible number and nature’s carrying capacity indicates that no further growth in population is possible.
Question. Name two inter specific interactions where one partner is neutral i.e. not affected.
Answer : Amensalism and commensalism.
Question. Why are cattle and goats not seen browsing on Calotropis growing in the field ?
Answer : Cattle and goats are not seen browsing on Calotropis because Calotropis produces a highly poisonous chemical called cardiac glycosides.
Short Answer Type Questions
Question. What is mutualism ? Mention any two examples where the organisms involved are commercially exploited in agriculture.
Answer : Interaction between two species in which both are benefitted
(i) Rhizobium in the roots (nodules) of legumes
(ii) Mycorrhiza / Glomus with the roots of higher plants.
Question. Differentiate between commensalism and mutualism by taking one example each from plants only.
Answer : Commensalism : In this interaction, one species is benefitted and the other species is neither benefitted nor harmed.
e.g. an orchid growing as an epiphyte on the branch of a mango.
Mutualism : In this interaction, both the interacting species are benefitted.
e.g. Lichens exhibit mutualistic relationship between a fungus that absorbs water and nutrients from soil and photosynthesizing algae / cyanobacteria.
Question. Describe the mutual relationship between fig tree and wasp and comment on the phenomenon that operates in their relationship.
Answer : Wasp helps in pollination while fig tree helps in oviposition and nourishment of larva.
The phenomenon seen here is mutualism (two organisms existing symbiotically or in mutual cooperation and benefits to each other) and Coevolution (the evolution of fig tree and wasp in accordance with the changes in each other) and also coextinction.
Question. How does Monarch butterfly defend itself from predators ? Explain.
Answer : Monarch butterflies are highly distasteful to their predators. This butterfly species accumulates a chemical by feeding on a poisonous weed during its caterpillar stage.
Question. Why do clown fish and sea anemone pair up ?
What is this relationship called ?
Answer : Clown fish gets protection from its predators by moving around the stinging tentacl es of the sea anemone. The sea anemone is neither helped nor harmed by the interaction with the fish. This relationship is called Commensalism.
Detailed Answer :
Clown fish maintains a commensalistic relation with the sea anemone. In this interaction, one species benefits and the other is neither harmed nor benefited. Sea anemone has stinging tentacles that provide protection to clown fish from predators.
The anemone does not appear to derive any benefit from the clown fish.
Question. Besides acting as ‘conduits’ for energy transfer across trophic levels, predators play other important roles. Justify.
Answer : Besides acting as ‘conduits’ of energy transfer across trophic levels, predator play other important roles like :
(i) They keep prey population under control.
(ii) Predators also help in maintaining species diversity in a community by reducing the intensity of competition among competing prey species.
Question. Name and explain the type of interaction that exists in mycorrhizae and between cattle egret and cattle.
Answer : Mycorrhiza is an association between fungi and the roots of higher plants. It is an example of mutualism in which both fungi and plants are dependent on each other for nutritional needs. The fungi help the plant in the absorption of essential nutrients from the soil while the plant provides the fungi with carbohydrates.
The interaction that exists between cattle egret and cattle is known as commensalism. In this type of interaction, one species is benefitted whereas the other is neither benefitted nor harmed. The cattle egret (bird) usually forages in close proximity to the grazing cattle. As cattle moves in the grass they stir up the grass and flush out the insects which then become an easy target for the egret. In this way, the cattle is neither benefitted nor harmed but the egret is benefitted.
Question. When do you describe the relationship between two organisms as mutualistic, competitive and parasitic ? Give one example of each type.
Answer : Mutualistic : Both the interacting organisms are benefitted from each other e.g., Lichens – Algae and fungi mutually help each other. (any other appropriate example)
Competition : When two organisms belonging to closely related species / unrelated groups compete for the same resources that are limited, both are losers e.g. superior barnacle dominates and excludes the small barnacles / in some South American lakes visiting flamingoes and resident fishes compete for their common food (zooplankton) in the lake / any other appropriate example.
Parasitic : One of the two organisms is dependent on the other (host) for nutrition and support / the host is harmed and the parasite is benefitted e. g. Malarial parasite and human / Cuscuta on host plant / or any other appropriate example.
Question. Highlight the differences and a similarity between the following population interactions, competition, predation and commensalism.
Question. (i) Explain any two defence mechanisms plants have evolved against their predators.
(ii) How does predation differ from parasitism ?
Answer : Plants develop following defence mechanisms :
(i) (a) Thorns are (morphological) means of defence.
(b) Plants may produce / store chemicals such as nicotine, strychnine etc. for defence which inhibit digestion / disrupts reproduction / kill the predator / calotropis produces highly
poisonous cardiac glycosides / plants may produce chemicals such as nicotine/ caffeine/ quinine/ strychnine/ opium are produced as defence.
|(a)||Lives and feed on the host.||Only feeds on prey.|
|(b)||host specific.||prudent / not prey specific.|
|(c)||Co-evolve with the host.||Control / check prey population.|
Question. (i) Write the parasitic adaptations the parasites have evolved in accordance with their lifestyles.
(ii) Hosts and parasites tend to co–evolve. Explain.
Answer : (i) Parasites have evolved the following adaptation :
(a) Loss of unnecessary sense organs.
(b) Presence of hook / adhesive organs and suckers.
(c) Loss of digestive system.
(d) High reproductive capacity.
(ii) In some cases, a parasitic species may evolve with its host taxa. Long–term co-evolution sometimes leads to a relatively stable relationship, tending to commensalism or mutualism, as it is in the evolutionary interest of the parasite that its host thrives. A parasite may evolve to become less harmful for its host or a host may evolve to cope with the unavoidable presence of a parasite, to the point that the parasite’s absence causes the host harm.
Question. (i) Explain the birth rate and death rate in population with the help of an example.
(ii) What is age pyramid ?
Answer : (i) In a population, the birth rate and death rate refer to per capita births and deaths, respectively.
Examples of birth rate and death rate are :
Birth rate : If in a pond, there were 20 lotus plants last year and through reproduction
8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year.
Death rate : If 4 individuals in a laboratory population of 40 fruit flies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruit fly per week.
(ii) A population at any given time is composed of individuals ages. If the age distribution (per cent individuals of a given age or age group) is plotted for these population, the resulting structure is called an age pyramid.
Question. (i) A parasite has to adapt to be able to live in a host. Write the various parasite adaptations?
(ii) Mention an adaptive feature exhibited in brood parasitism in Koel and Crow.
Answer : (i) The various parasitic adaptation enabling it to live in host are as follows :
(a) Loss of unnecessary sense organs.
(b) Adhesive organs or suckers to cling on to the host.
(c) Loss of digestive system.
(d) High reproductive capacity.
(e) Loss of chlorophyll and leaves.
(ii) The eggs of the parasitic bird (Koel) resemble the host’s egg (Crow) in size and colour to reduce the chances of the host bird detecting the foreign eggs and ejecting them out from the nest.
Question. Explain co-evolution with reference to parasites and their hosts. Mention any four special adaptive features evolved in parasites for their parasitic mode of life.
Answer : Co-evolution can be defined as reciprocal adaptations in two interacting organisms that brings about the evolutionary change in both of them. In terms of the relation of host and parasite, it can be explained as follows :
Parasite is an organism that is totally dependent on the host organism for its survival but in doing so, it also harms the host. The host evolved over a long period of time to protect itself from parasite, while parasite evolved so that it can find other way to derive nutrition from the host and hence, the cycle continues.
The various parasitic adaptation enabling it to live in host are as follows :
(i) Loss of unnecessary sense organs.
(ii) Adhesive organs or suckers to cling on to the host.
(iii) Loss of digestive system.
(iv) High reproductive capacity.
(v) Loss of chlorophyll and leaves.
Question. a) In a pond there were 200 frogs. 40 more were born in the year. Calculate the birth rate of the population.
b) Population in terms of number is not always a necessary parameter to measure population density. Justify with two examples.
Answer : a) Birth rate: No of individuals/ Total no. of individuals = 40 /200 = 0.2 frogs per year.
b) Number is not always a necessary parameter to measure population density.
1. If there are 200 Parthenium plants but only a single huge banyan tree with a large canopy, the population density of banyan is low relative to that of Parthenium which amounts to underestimating the enormous role of the Banyan in that community. In such cases, the per cent cover or biomass is a more meaningful measure of the population size.
2. In a dense laboratory culture of a microbial population in a petri dish, the total number of microbes is again not an easily adoptable measure because population is huge, counting is impossible and time-consuming.
Long Answer Type Questions
Question. (i) Name the two growth models that represent population growth and draw the respective growth curves they represent.
(ii) State the basis for the difference in the shape of these curves.
(iii) Which one of the curves represents the human population growth at present ? Do you think such a curve is sustainable ? Give reason in support of your answer.
Answer : (i) The two types of growth models are as follows :
(a) Exponential growth : When the resources are unlimited, population tends to grow in an exponential pattern.
If the population size is N and the birth and death rates (not per capita) are b and d respectively, then increase or decrease of N at t (time period) is given by dN/dt = (b – d) * N
If (b – d) = r , then
dN/dt = rN
where r = Intrinsic rate of natural increase
or Nt = N0 ert
Nt = Population density at time t
N0 = Population density at time 0
r = Intrinsic rate of natural increase
e = Base of natural algorithm (2.71828)
(b) Logistic Growth : When the resources are limited leading to competition between individuals and survival of fittest, the population tends to grow in a logistic manner.
A population 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. This type of population growth is called Verhulst-Pearl Logistic Growth.
(ii) The difference in the shape of these curves is the amount of resources available for the given population. For exponential growth, the amount of resources is considered infinite, while, logistic growth happens when there are limited resources.
(iii) The human population represents the logistic growth curve as the numbers of human beings are increasing rapidly but available resources are not increasing at the same pace. Such a growth pattern is not sustainable because at one point the human population would reach a place where there would not be enough resources for everyone. For sustainable growth, we must find out ways to develop and use already present resources more intelligently.
Question. (i) What is an age-pyramid ?
(ii) Name three representative kinds of age-pyramids for human population and list the characteristics for each one of them.
(i) What is an age-pyramid ?
(ii) Explain with the help of figures the three different types of age pyramids represented by a human population.
Answer : (i) If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called the age pyramid.
(ii) Expanding : Pre reproductive population is greater than reproductive or post reproductive population / growing with maximum no. of individuals in pre reproductive phase and least no. in post reproductive phase.
Stable : Pre-reproductive and reproductive population are almost similar / ideal for population / maintains balanced continuity / no. of individuals in reproductive phase is almost same, less no. of individuals in post reproductive phase.
Declining : Pre-reproductive population is less than reproductive population / less no. of individuals in pre reproductive phase than reproductive phase.
In lieu of the above explanation the following diagram can be considered.
Question. “Analysis of age pyramids for human population can provide important inputs for long-term planning strategies.” Explain.
Answer : Analysis of age-pyramids for human population can provide important inputs for longterm planning, strategies : The different age groups present in a population determines its reproductive status. Distribution of age groups highly influences the growth of the population.
Each population displays following three ecological ages or age groups :
Population having large number of young members grows rapidly, while the population bearing more number of post-reproduction members tends to be declining. There are basically three types of age-pyramids found to be present in human population.
Therefore, through the analysis of the agepyramids of a particular population, the distribution of resources can be done more efficiently. A better planning strategy can be adopted considering the demand of the resource, thus, long term management of resources can be done in such a way that the population can derive maximum benefit with minimum effects on nature, leading the population to flourish efficiently.
(i) Which of the above represents the increase or decrease of population ?
(ii) If N is the population density at time t, then what would be its density at time (t + 1) ? Give the formula.
(iii) In a barn there were 30 rats. 5 more rats enter the barn and 6 out of the total rats were eaten by the cats. If 8 rats were born during the time period under consideration and 7 rats left the barn, find out the resultant population at time (t + 1).
(iv) If a new habitat is just being colonized, out of the four factors affecting the population growth which factor contributes the most ?
Answer : (i) Natality (B) and Immigration (I) represents increase of population and Emigration (E) and Mortality (D) represent decrease of population.
(ii) Nt + 1 = Nt + [(B + I) – (D + E)]
(iii) Here Nt = 30; I = 5; E = 7; D = 6; B = 8
Putting the value in Nt + 1
= Nt + [(B + I) – (D + E)]
Nt + 1 = 30 + [(8 + 5) – (6 + 7)]
= 30 + [13 – 13]
= 30 + 0 = 30 rats
(iv) Immigration contributes the most.
Question. (i) Compare, giving reason, the J-shaped and S-shaped models of population growth, of a species.
(ii) Explain ”fitness of a species” as mentioned by Darwin.
Answer : (i)
|J shaped – growth curve||S-shaped – growth curve|
|Resources are unlimited||Resources are limited|
|Growth is exponential||Logistic Growth|
|As resources are unlimited all individuals survive and reproduce||Fittest individual will survive and reproduce|
|Growth Equation : dN/dt|
= rn (If explained)
|Growth Equation : dN/|
dt = rN (K-N/K) (If explained)
Note : Marks to be awarded only if the corresponding difference is written.
(ii) When resources are limited, competition occurs between individuals, fittest will survive, which will reproduce to leave more progeny.