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In natural and suitable conditions land will gradually become colonised by a range of herbaceous plants, then by shrubs and finally by trees as a climax community. There is change in the communities with time, because of the interaction between species and their environment.

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Competition & Predator-Prey Cycles | A-level Biology | OCR, AQA, EDEXCEL

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The factors affecting population growth, and how populations increase in numbers are important concepts in ecology as they are necessary in order to successfully study how ecosystems work.

The number of individuals per unit area of chosen habitat is known as the population density. The population density can be affected by a number of factors:

1. Birth: The number of new individuals born to a population.

2. Immigration: The number of new individuals joining a population.
Both of these factors will serve to increase the population density.

3. Death: The number of individuals within a population that die.

4. Emigration: The number of individuals leaving a population.

Both of these factors will serve to decrease the population density.

Population size can also be affected by the following:

Density dependent factors: These are any factors, dependent on the density of the population in question. Some examples of these are predation, disease and competition.

Density independent factors: These are any factors, not dependent upon the density of the population in question. Some examples of these are climate and catastrophe.
Population changes can be studied using population graphs:
This is an exponential growth curve. This type of curve occurs when a population grows in size under ideal conditions. The population will double in size during a constant period of time. This type of population growth is theoretically possible but is rarely seen in nature. The closest to an exponential growth curve is that of some bacterial colonies which are able to double their numbers with each reproduction, (e.g, 2-4-8-16-32-64-128-256-etc).

Normally populations will be prevented from undergoing uncontrolled exponential growth by limiting factors. Most populations will adhere to a sigmoid growth curve.
The phases that make up a sigmoid growth curve are as follows:
Lag phase: Population growth begins slowly from a few individuals.

Log phase: Exponential growth occurs, the conditions are ideal and maximum growth rate is reached.

S-phase: Growth rate begins to slow down as factors such as food, water and space become limiting.

Stable phase: Carrying capacity for the population has been reached and the population number becomes stable. The carrying capacity is the population size that can be supported by a particular environment.

Decline phase: If there is a sudden change in the environment meaning that the environment can no longer support the population, such as a drought causing food shortage, the population will crash and the whole process begins again.

Competition is often considered to be the most important biotic factor controlling population density.

Competition between organisms may be for a number of different factors, including food, light, territory or reproductive partners.
Intraspecific competition:
This is competition for a resource between individuals from the same population. It causes the population growth rate to slow down, and has a greater effect the less plentiful the resource is.

This form of competition can also be categorised as scramble or contest.
Scramble competition:
This occurs when many members of a population compete for a scarce resource, and each member gains a portion of that resource. This is often seen when populations compete for a food source and each member of the population gains some of that food source.
Contest competition:
This occurs when two or more members of a population compete for a resource but only one member of the population gains that resource. This can be seen when competing for mates or territory for example.
Interspecific competition:
This is competition for a resource between members of different populations in the same community. It will result in the competing populations increasing in size more slowly than normal. This type of competition may result in the extinction of one of the competing populations.

Examples of interspecific reproduction are predator - prey relationships and competition for resources.

In this situation it appears that as the prey population crashes, it is followed by a crash in the predator population due to a reduction in food. Consequently the prey population will then increase since the predation pressure is reduced, this is closely followed by a rise in predator numbers.

The explanation to this appears to be straightforward, however it is not as simple at it at first appears since the individual populations have been shown to follow these patterns independently of one another.

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The factors affecting population growth, and how populations increase in numbers are important concepts in ecology as they are necessary in order to successfully study how ecosystems work. Changes in populations over time The number of individuals per unit area of chosen habitat is known as the population density.