The size of a population may vary due to:

• abiotic factors
• biotic factors

Abiotic factors

Abiotic factors are the non-living components within an ecosystem.

• Climate: changes in the seasons will have an effect on the temperature, rainfall, humidity and the number of hours of daylight.
• Topographic factors: this includes slope and altitude.
• Edaphic factors: this includes pH, the mineral supply, soil moisture, and soil texture.
• Catastrophes: like fires or floods.
• Human factors: like pollution.

These factors can vary within a habitat which produces microclimates and microhabitats.

They also tend to be density-independent factors. This means that the size of the effect is independent of the population size.

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Biotic factors

Biotic factors look at interactions between organisms.

Intraspecific competition is where organisms of the same species compete for resources like food, territory or a patch of soil from which to grow. This form of competition tends to a have a stabilising effect on a population. For instance, if a population gets too big then intraspecific competition increases and population falls.

It’s also the driving force behind natural selection. This is because it’s the organisms with the best traits or genes which are most likely to win competitions and so survive to pass on their genes. Many species don’t actually fight but, instead, have ritual fights and displays. By avoiding real fights and food shortages an optimum size for a population can be attained.

Interspecific competition is where organisms of different species at the same trophic level compete for resources. For example, plants compete for light sources, herbivores for plants, and carnivores for prey (known as predation).

Biotic factors are usually density-dependent factors. In other words, the size of the effect is dependent on the population size.