Population growth curves

Populations are dynamic, which means that they change over time, both in size and composition. The way these changes occur can be illustrated using a population growth curve, which represents the number of organisms in a population over time.

The population growth curve typically shows three distinct phases:

1. Period of slow growth - Initially, as the birth rate exceeds the death rate, the population starts to grow.
2. Period of rapid growth - The population expands exponentially when there are no significant restrictions on its growth.
3. Stable state - Eventually, external factors, known as limiting factors, stop further growth, leading to a stable population size.

This pattern, resembling an ‘S’ (sigmoid) shape, is common across many natural populations. The graph's exponential and stable phases are often depicted using different scales. For example, a logarithmic scale might be used when the population grows rapidly over a short period of time, so that all the points on the graph can be represented.

Carrying capacity

Populations can't expand infinitely. The ecosystem's carrying capacity is the maximum population size it can sustainably support.

When a population reaches this point:

• Birth and death rates tend to equalise.
• Population growth slows down, and the population size stabilises, though it may fluctuate slightly.
• If environmental conditions degrade, the population size might decrease, sometimes drastically.

Limiting factors

Limiting factors are conditions that prevent populations from increasing indefinitely by restricting access to essential resources.

Examples of limiting factors on population size include:

• Biotic factors, such as competition, predation, and disease.
• Abiotic factors, including the availability of light, water, and space.

Limiting factors fall into two categories:

• Density-independent - Their impact remains constant regardless of population size and can lead to extinction, such as in the case of natural disasters.
• Density-dependent - Their impact increases as the population size increases, for example, competition for resources.

The effect of migration on population sizes

Migration also plays a significant role in altering population sizes.

There are two types of migration:

• Immigration - This leads to an increase in population size as organisms move into an area.
• Emigration - This causes a decrease in population size as organisms leave an area.