Last year, in a rat population of 600 rats, 60 new rats were added through reproduction in the current year to make a total population of 660 rats. Find the birth rate of this rat population.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 0.1 Offspring per rat per year
The birth rate is the number of births per member of the population in a given period.
New rats added by reproduction = 60.
Population at the start = 600.
Birth rate = 60 ÷ 600 = 0.1 offspring per rat per year.
Option (1) 0.5 Offspring per month — Incorrect. The data are for a year, and 0.5 does not follow from them.
Option (2) 2 Offsprings per year — Incorrect. A birth rate is per individual, and 2 per rat would need 1200 births.
Option (4) 600/1200 offspring per week — Incorrect. Neither the numbers nor the time unit (a week) match the data.
NCERT Extract:
Population Attributes – “… 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.”