Logistic Growth
Obviously, neither bacteria nor elephants cover the planet. This means that exponential growth does not continue in natural populations for very long. What might cause population growth to stop or to slow down?
Growth Slows Down Suppose that a few animals are introduced into a new environment. At first, as the animals begin to reproduce, the population increases slowly. Then, because resources are unlimited, the population grows exponentially. In time, however, the rate of population growth begins to slow down. This does not mean that the size of the population has dropped. The population is still growing, but at a much slower rate.
As resources become less available, the growth of a population slows or stops. The general, S-shaped curve of this growth pattern, called logistic growth, is shown below in the graph of a yeast population. Logistic growth occurs when a population's growth slows or stops following a period of exponential growth. How might this happen?
Logistic Growth This graph shows the S-shaped curve of logistic growth. As resources become less available, the population growth rate slows or stops. The growth of this yeast population has leveled off at its carrying capacity.
Population growth may slow down when the birthrate decreases, when the death rate increases, or when both events occur at the same rate. Similarly, population growth may slow down when the rate of immigration decreases, the rate of emigration increases, or both. When the birthrate and death rate are the same, or when the rate of immigration is equal to the rate of emigration, then population growth will slow down or even stop for a time. Note that even when the population growth is said to stop, the population is still rising and falling somewhat, but the ups and downs average out around a certain population size.
Carrying Capacity If you look again at the graph above, you will see a horizontal line through the region of the graph where the growth of the yeast population has leveled off. The point at which that line intersects the y-axis tells you the size of the population when the average growth rate reaches zero. That number, in turn, represents the largest number of individuals—in this case, yeast cells—that a given environment can support. Ecologists call this number the carrying capacity of the environment for a particular species.
If you examine natural populations of familiar plant and animal species, you will find that many of them follow a logistic growth curve. In the natural world there are many factors that can slow the growth of a population. The factors that limit population growth are discussed in the next section. Click on the activity at right to see how controlling the resources of a population of bacteria affects its growth.
Population Growth
Do fruit flies and rabbits show similar trends in population growth?
Using Tables and Graphs Make a graph using the data in each data table. One graph will show the growth rate of a fruit fly population. The other graph will show the growth rate of a population of rabbits.
Using Tables and Graphs What type of growth pattern is exhibited by the fruit fly population? Is it the same type of growth as in the rabbit population? Explain.
Drawing Conclusions Does either graph indicate that there is a carrying capacity for the population? If so, when does the population reach its carrying capacity? What is the maximum number of individuals that can be supported at that time?
Predicting Animals such as foxes and cats often prey on rabbits. Based on the growth curve of the rabbit population, what might happen if a group of predators move into the rabbits' habitat during the tenth generation and begin eating the rabbits?