Populations and Population Growth


Populations are groups of individuals of the same species that occupy a given area at the same time. Each population has unique attributes. The important attributes associated with population are population growth and population regulation.

Population Growth

In animals, the population shows birth, death, and dispersal. Therefore, animal populations change over time. The death of an organism is characterized by survivorship curves. In the survivorship curve graph, the numbers of survivors are plotted on Y-axis, and age is plotted on X-axis.

Types of Survivorship curves

There are three types of survivorship curves.

Survivorship curve type I

This gives a convex curve on the graph. In type I, populations survive to old age. Then they die rapidly. Environmental factors are not important. These factors do not influence mortality. Thus, most individuals live their potential lifespan.


Human populations show a type I survivorship curve.

Survivorship curve type II

This gives a diagonal line. In type II, populations have a constant chance of death throughout their lives. In type II, the environment has an important influence on mortality. It has a harsh effect on the young ones than the old.


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Populations of rodents and birds show a type II survivorship curve.

Survivorship curve type III

This gives a concave curve in the graph. In type III, populations show a very high juvenile mortality rate. There is a much lower mortality rate in adulthood.


Fishes and many invertebrates show type III survivorship curves.

Types of Population Growth

There are two types of growth.

  • Exponential growth
  • Logistic growth
Exponential Population growth

The increase of the population by the same ratio per unit time is called exponential growth. Different populations have different potentials to increase their numbers. Not all populations display the same capacity for growth.


Many factors influence reproductive potential. Some of them are given below:

  • Number of offspring produced
  • The likelihood of survival to reproductive age
  • The duration of the reproductive period
  • The length of the time it takes to reach maturity.
Logistic Population growth

The growth in which population reaches the carrying capacity and does increase further is called logistic population growth. The population size that a particular environment can support is called carrying capacity.


Logistic population growth is symbolized by K. exponential growth cannot occur indefinitely. There are many environmental resistances. These resistances are climate, food, space, etc.

These resistances check the population growth rate. The population reaches the carrying capacity, the growth curves become a sigmoid or flattened S shape.

Population Regulation

Every species has different conditions for survival. But population density and competition effects are common in all the species.

Population Density

The number of individuals per unit space is called population density. There are two types of factors.

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Density Independent Factors

The factors which are not influenced by the density of population are called density-independent factors. They influence the number of organisms in the population. These include the factors such as weather conditions.

A cold winter with little snow cover can destroy a population of lizards. These lizards live beneath the litter of the forest floor. A certain percentage of individuals will freeze to death. Their death is not affected by the size of the population.

Similarly, human activities like construction and deforestation affect animal populations.

Density Dependent Factors

The factors which are influenced by the density of population are called density-dependent factors. These factors are more severe when population density is large.

Animals often use territorial behavior, song, and scent to attract partners for reproduction. These actions become more prominent with the increase of population density. Thus, they are density-dependent factors.

Some other density-dependent factors are competing for resources, disease, predation, and parasitism.