17. Population Ecology


17.6. Carrying Capacity


Many populations reach a maximum size when they reach the stable equilibrium phase. This suggests that the environment sets an upper limit on population size. The carrying capacity is the maximum sustainable population for an area. The carrying capacity is determined by a combination of limiting factors. But keep in mind that some limiting factors are more important than others (figure 17.12).



FIGURE 17.12. Carrying Capacity

A number of factors in the environment, such as food, oxygen supply, diseases, predators, and space, determine the maximum number of organisms that can be sustained in a given area—the carrying capacity of that area. The environmental factors that limit populations are collectively known as environmental resistance.


Carrying capacity is not an inflexible number, however. Often, such environmental differences as successional changes, climatic variations, disease epidemics, forest fires, and floods can change the carrying capacity of an area for specific species. In aquatic ecosystems, one of the major factors that determine the carrying capacity is the quantity of nutrients in the water.

Where nutrients are abundant, the numbers of various kinds of organisms are high. Often, nutrient levels fluctuate with changes in current or runoff from the land, and plant and animal populations fluctuate as well. In addition, a change that negatively affects the carrying capacity for one species may increase the carrying capacity for another. For example, the cutting down of a mature forest followed by the growth of young trees increases the carrying capacity for deer and rabbits, which use the new growth for food, but decreases the carrying capacity for squirrels, which need mature, fruit-producing trees as a source of food and old, hollow trees for shelter.

Wildlife management practices often encourage modifications to the environment that will increase the carrying capacity for the designated game species. The goal of wildlife managers is to have the highest sustainable population available for harvest by hunters. Typical habitat modifications include creating water holes, cutting forests to provide young growth, planting food plots, and building artificial nesting sites.

In some cases, the size of the organisms in a population also affects the carrying capacity. For example, an aquarium of a certain size can support only a limited number of fish, but the size of the fish makes a difference. If all the fish are tiny, a large number can be supported, and the carrying capacity is high; however, the same aquarium may be able to support only one, large fish. In other words, the biomass of the population makes a difference (figure 17.13). Similarly, when an area is planted with small trees, the population size is high; however, as the trees get larger, competition for nutrients and sunlight becomes more intense, and the number of trees declines as the biomass increases.



FIGURE 17.13. The Effect of Biomass on Carrying Capacity

Each aquarium can support a biomass of 2 kilograms of fish. The size of the population is influenced by the body size of the fish in the population.



15. How is the carrying capacity of an environment related to the stable equilibrium phase of a population growth curve?

16. How are the concepts of carrying capacity and limiting factors related?

17. Describe an event that could change the carrying capacity for an organism and explain why the event would alter carrying capacity.