[Explained] Types of Population Pyramids

Age Distribution


In most populations, individuals are of different ages.
The proportions of individuals in each age group are called the age structure of that population.
The age distribution is important, as it influences both, natality and mortality of the population.
The ratio of the various age groups in a population determines the current reproductive status of the population, thus anticipating its future.  

The model representing geometrically the proportions of different age groups in the population of any organism is called the age pyramid.


Different types of Population Pyramids 

There are 3 kinds of population pyramids namely:
  • Expansive Pyramid
  • Stable Pyramid
  • Declining Pyramid

(a) Pyramid with a broad base (Expanding). aka Population explosion, Expansive Pyramid

It indicates a high percentage of young individuals.
 In rapidly growing young populations, the birth rate is high and population growth may be exponential as in yeast, house fly, etc. Under such conditions, each successive generation would be more numerous than the preceding one, and thus a pyramid with a broad base would result.

Pyramid for expanding population also known as Expansive pyramid
Pyramid for expanding population also known as Expansive pyramid

(b) A bell-shaped polygon (Stable), Stationary Pyramid

It indicates a moderate proportion of young to old. As the rate of growth becomes slow and stable, i.e., the pre-reproductive and reproductive age groups become more or less equal in size, post-reproductive group remaining as the smallest. it results in a bell-shaped structure.

Pyramid for stable population also known as Stationary Pyramid
Pyramid for stable population also known as Stationary Pyramid

(c) An urn-shaped figure (Declining). aka Negative population growth, Constrictive Pyramid

It indicates a low percentage of young individuals. If the birth rate is drastically reduced the pre-reproductive group dwindles in proportion to the reproductive and post-reproductive groups and it results in an urn-shaped figure, which indicates that the population is dying off. ·

Pyramid for declining population also known as Constrictive Pyramid
Pyramid for declining population also known as Constrictive Pyramid

Sex ratio

The distinction between male and female is called sex. An individual is either a male or a female but
a population has a sex ratio. The ratio between the number of males and females is known as the sex ratio. The sex ratio of some species in a population may change according to
a change in population density. The sex is also controlled by temperature, photoperiod, minerals and growth substances.

What does Emigration, Immigration, and Migration mean?

This is the movement of individuals (or seeds, spores, larvae, etc., as means of propagation) into or out of the population. This plays an important role in the geographic distribution of plants and animals to areas that were not previously occupied by them. Population dispersal may take place in search of food to avoid predators presenting overcrowding due to wind, water, light or temperature, breeding behavior, and due to some physical reasons. Population dispersal may occur in three ways viz., 

Regulation of Population size
Regulation of Population Size

a. Emigration 

It involves the outward movement of an organism from one place or country to another for permanent residence.


(b) Immigration.

 It is the inward movement of the organism to any place or country. Thus both emigration and immigration are closely related, involving one-way movement.


(c) Migration. 

It involves the mass two-way movement of the entire population, where the organisms return again to the area from which they had moved. Such movements generally take place during unfavorable conditions in the original area. Such movements are generally seasonal and periodical. Migratory movements are generally found among mammals, birds, reptiles, fishes, and some insects.

What is Bioenergetics?

Energy is defined as the ability to do work. And the energy transformations that occur within the population are known as bioenergetics. Energy flow in a population provides the most reliable basis for (i) evaluating observed fluctuations in density, and (ii) determining the role of a population within its Community.

Life History Variations

Populations evolve to maximize their reproductive fitness, in the habitat in which they live. Some organisms breed only once in their life rime like Pacific Salmon fish, bamboo. while others breed several times during their lifetime( most birds and mammals). Some produce a large number of small-sized offspring (oysters, pelagic fishes), while others produce a small number of large-sized offspring ( birds, mammals) Life-history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which they live.

You can also check Learn about Carrying Capacity and Environmental Stochasticity and Population Growth Curve which are highly relevant to the study of population.

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