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.
Under normal conditions, births and deaths are the most important factors influencing population density. In which of the following situations may immigration contribute more to population growth than the birth rate?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → When a new habitat is just being colonised
Population density changes through births, deaths, immigration and emigration. Under normal conditions, births and deaths matter most. Immigration and emigration become important only under special conditions. One such condition is a new habitat that is just being colonised: few individuals live there yet, so newcomers arriving from elsewhere can add more to the population than births do.
Option (1) When the population has lived in the same habitat for many generations — Incorrect. A population settled in its habitat for many generations is the normal case, in which births and deaths are the main factors.
Option (2) When the sex ratio of the population is nearly equal — Incorrect. The sex ratio affects how many offspring can be produced, but it does not make immigration more important than births.
Option (3) When the death rate of the population is higher than its birth rate — Incorrect. A death rate higher than the birth rate makes the population decline. It says nothing about individuals coming in from elsewhere.
Option (4) When a new habitat is just being colonised — Correct. In a newly colonised habitat the population is just forming, and most of its growth comes from individuals moving in.
NCERT Extract:
Population Growth – “… You can see from the above equation (Fig. 11.2) that population density will increase if the number of births plus the number of immigrants (B + I) is more than the number of deaths plus the number of emigrants (D + E). Under normal conditions, births and deaths are the most important factors influencing population density, the other two factors assuming importance only under special conditions. For instance, if a new habitat is just being colonised, immigration may contribute more significantly to population growth than birth rates.”