Organisms and Populations

Key idea Age pyramid is the diagrammatic representation of proportions of different age groups (pre-reproductive, reproductive and post-reproductive) in a population.

The age pyramids are useful for predicting the attributes of a population and on the basis of varying proportions of pre-reproductive age group in a population, these are of three types

Expanding/Triangular age pyramid This age pyramid designates the large proportion of pre-reproductive age group in comparison with other two age groups. Thus, this age pyramid indicates a growing or expanding population.

Stable/Bell-shaped age pyramid This age pyramid symbolise that the pre-reproductive age group is proportionate with the reproductive age group, therefore in the future the population size of species will remain constant or same. Hence, this pyramid signifies a stable population.

Declining/Spindle-shaped age pyramid This age pyramid shows that the proportion of pre-reproductive age group individuals is quite less than that of the reproductive age group, thus this age pyramids signifies a declining population.

Both the assertion and reason are true and the reason is not correct explanation of assertion. In commensalism type of community interaction, one organism is benefitted and other is neither benefitted nor harmed, such as in the case of cattle egret bird and the cattle. The egrets are benefitted by obtaining insect as food, which gets exposed due to the grasing action of cattle, but the cattle remains unaffected i.e. it is neither benefitted from the presence of egret nor harmed.

A bell-shaped polygon indicates a moderate proportion of young to old. As the rate of growth becomes slow and stable, the pre-reproductive and reproductive age group become more or less equal in size and post-reproductive group remains the smallest.