A population pyramid is a diagram of population distribution based on age and sex. The illustrated bar graph shows females on one side and males on the other. Along the vertical line are age groupings, such as 5 to 9 and 10 to 14, and along the horizontal line are population percentages by age.
Also known as an age picture diagram or an age pyramid, these graphs are used by demographers to understand the population makeup of a city, country or other area. Demographers not only learn current population profiles, but can also discover past population trends. Because these pyramids can be used to understand reproductive patterns, demographers can make predictions about future increases and decreases in a population group, as well as forecast the probability of a species continued existence.
Each age group on the pyramid is called a cohort and represents a percentage of the population. As the shape of the pyramid changes, it can indicate shifts in population. For example, when the graph becomes more rectangular, population growth is slowing down and the rate of new generations is nearly equal to older generations which are dying off. When the population pyramid actually resembles a pyramid, it indicates faster population growth.