The Logistic Growth Curve The curve in Figure 5–5 has an S-shape that represents what is called logistic growth. Logistic growth occurs when a population's growth slows and then stops, following a period of exponential growth. Many familiar plant and animal populations follow a logistic growth curve.

What kinds of changes in a population's characteristics can produce logistic growth? Remember that a population grows when more organisms are born (or added to it) than die (or leave it). Thus, population growth may slow for several reasons. Growth may slow because the population's birthrate decreases. Growth may also slow if the death rate increases—or if births fall and deaths rise together. Similarly, population growth may slow if the rate of immigration decreases, the rate of emigration increases, or both. There are several reasons why these rates might change in a population, as you will see in the next lesson.

Carrying Capacity When the birthrate and the death rate are the same, and when immigration equals emigration, population growth stops. The population may still rise and fall somewhat, but the ups and downs average out around a certain population size. If you look again at Figure 5–5, you will see a broken, horizontal line through the region of the graph where population growth levels off. The point at which that line intersects the y-axis represents what ecologists call the carrying capacity. Carrying capacity is the maximum number of individuals of a particular species that a particular environment can support. Once a population reaches the carrying capacity of its environment, a variety of factors act to stabilize it at that size.

5.1 Assessment

Review Key Concepts

1. 1. Review List four characteristics that are used to describe a population.

   2. Infer On your travels through eastern Canada and the United States, you notice gray squirrels everywhere. What can you infer about the squirrels' geographic range?

2. 1. Review What natural factors can change a population's size?

   2. Relate Cause and Effect More dandelion seedlings develop in a lawn than dandelion plants are removed. What is likely to happen to the lawn's dandelion population?

3. 1. Review When do populations grow exponentially?

   2. Apply Concepts Why does exponential growth show a characteristic J-shaped curve?

4. 1. Review What is the characteristic shape of a logistic growth curve?

   2. Explain Describe when logistic growth occurs.

   3. Form a Hypothesis What factors might cause the carrying capacity of a population to change?

PRACTICE PROBLEM

5. Suppose you are studying a population of sunflowers growing in a small field. How would you determine the population density of sunflowers in a square meter of the field and in the entire field? Describe your procedure.

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