Population size

Limiting factors such as food availability will prevent the population growth continuing at its rapid rate. It is predicted that by the end of this century, the global population will have reached 11 billion. A limiting factor is an environmental resource or constraint that limits population growth. 

Population growth curve

Most natural populations will show the same characteristics in their population structure over time. 

There are three main phases to this graph:

• Phase 1 - Occurs where there is a period of slow growth called lag. The small population that is initially present reproduce to increase the total population. Here the birth rate is higher than the death rate meaning the population increases in size. 
• Phase 2 - A period of rapid growth, this is called log. As the number of individuals increases, the total population increases exponentially. There are no limiting factors at this stage.
• Phase 3 - A stable state. This is called the stationary phase. Any population growth is prevented by external constraints. Although it remains relatively stable, the population size can fluctuate. Here, birth and death rates are approximately equal. Any increase or decrease accounts for the fluctuations. 

Carrying capacity = It is the maximum number of species an environment can support.

Limiting factors

A limiting factor prevents further growth in the population. 
Limiting factors can be divided into abiotic and biotic factors:

• Abiotic factors = non-living factors include temperature, light, pH and water availability. 
• Biotic factors = Living factors include predators, disease, and competition.

Migration

Immigration - Movement of individual organisms into an area. This increases the population. Such as Christmas Island Red Crabs which migrate each year to reproduce. 

Emigration - Movement of individuals organisms away from a particular area decreasing the population size. Such as the Norway Lemming. They emigrate away from areas of high population density or poor habitat.

Density Independent Factors

These are factors that have an effect on the whole population regardless of its size. These include earthquakes, fires, and storms. They can often remove a whole population.

Density Dependent Factors

A factor which affects the size or growth of a population due to its density. Such as food availability, water, shelter, and predators.