Lesson 2 of 13 · 7 min
Population attributes
NCERT §11.1.1
Rohan's first job is a pond of lotus plants at the lake's edge. Last year's survey counted them; this year he counts again.
The lesson in notes
In short
A population has features that no single individual has. An individual is born and dies; a population has a birth rate and a death rate.
These rates are per capita: the change in number divided by the number of members.
Birth rate example: a pond had 20 lotus plants last year and 8 new ones were added, making 28. Birth rate = 8/20 = 0.4 offspring per lotus per year.
Death rate example: 4 of 40 laboratory fruit flies die in a week. Death rate = 4/40 = 0.1 individuals per fruit fly per week.
An individual is male or female, but a population has a sex ratio, for example 60 per cent females and 40 per cent males.
A population contains individuals of different ages. Plotting the per cent of individuals in each age group gives an age pyramid.
For humans the age pyramid usually shows males and females side by side.
The shape of the pyramid tells whether the population is growing, stable or declining. A broad base of young age groups marks a growing population; a narrow base marks a declining one.