Read the given passage and answer the following questions:
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. A population has certain attributes whereas, an individual organism does not. An individual may have births and deaths, but a population has birth rates and death rates. In a population these rates refer to per capita births and deaths. The rates, hence, expressed are change in numbers (increase or decrease) with respect to members of the population. Here is an example. 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. If 4 individuals in a laboratory population of 40 fruitflies 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 fruitfly per week. Another attribute characteristic of a population is sex ratio. An individual is either a male or a female but a population has a sex ratio (e.g., 60 per cent of the population are females and 40 per cent males). A population at any given time is composed of individuals of different ages. If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid. For human population, the age pyramids generally show age distribution of males and females in a diagram. The shape of the pyramids reflects the growth status of the population - (a) whether it is growing, (b) stable or (c) declining.
Choose the correct statements about measuring population size:
A. In an area with many carrot grass plants but a single huge banyan tree, per cent cover or biomass is a more meaningful measure of the banyan population.
B. The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets.
C. Population size, technically called population density, need not necessarily be measured in numbers only.
D. We are mostly obliged to estimate population sizes directly, by actually counting them or seeing them.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, B and C only
Population size, technically called population density (N), need not be a simple count. Total number is usually the best measure, but sometimes it is meaningless, and sometimes counting is impossible.
A. In an area with many carrot grass plants but a single huge banyan tree, per cent cover or biomass is a more meaningful measure of the banyan population. — True. Counting would make the banyan look unimportant next to the many carrot grass plants. Its large canopy is better shown by per cent cover or biomass.
B. The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets. — True. Tigers are hard to see and count, so their numbers are estimated from signs they leave behind.
C. Population size, technically called population density, need not necessarily be measured in numbers only. — True. Biomass and per cent cover are also used.
D. We are mostly obliged to estimate population sizes directly, by actually counting them or seeing them. — False. The book says the opposite: we are mostly obliged to estimate population sizes indirectly, without actually counting them or seeing them. The tiger census in statement B is one such indirect method.
So A, B and C are correct, which is Option (2).
NCERT Extract:
Population Attributes – “… Population size, technically called population density (designated as N), need not necessarily be measured in numbers only. Although total number is generally the most appropriate measure of population density, it is in some cases either meaningless or difficult to determine. In an area, if there are 200 carrot grass (Parthenium hysterophorus) plants but only a single huge banyan tree with a large canopy, stating that the population density of banyan is low relative to that of carrot grass 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.”
Population Attributes – “… We are mostly obliged to estimate population sizes indirectly, without actually counting them or seeing them. The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets.”