From the video in the previous section, you learned the four factors affecting population growth: births, deaths, immigration, and emigration. As populations grow, they begin to exhibit what is called exponential growth. When conditions are ideal and organisms have unlimited amounts of resources, birth rates increase. There are many more births than deaths and the population increases rapidly over time.

Click on the following graphs to see an illustration of exponential growth over time.

With exponential growth, the size of the population grows geometrically. The amount of offspring produced will always be greater than the generation before. This is what makes the population increase quickly to a large number. You may notice that the population stays small for a period of time, and then it rapidly increases to large numbers of individuals.

In nature, populations may experience periods of exponential growth, but they always reach a final tipping point where the amount of limited resources is not enough to support the population. This is called the carrying capacity. For example, when a population of zebras exceeds the amount of water available, zebras will begin to die. These deaths will bring the population numbers below the carrying capacity. As the water is replenished, the zebras will be able to reproduce again, allowing the population to grow. This cycle is repeated over and over again throughout the long-term survival of the species.

Click on the following graph to see carrying capacity illustrated.

The red line will continue to weave above and below the carrying capacity line over time. This shows that the population is in equilibrium. The population growth shown in this graph is also called logistic growth. The population initially shows a period of exponential growth, followed by a slowing of growth and ending at carrying capacity and equilibrium.