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# Population Ecology and Growth for AP Biology (page 2)

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By McGraw-Hill Professional
Updated on Oct 24, 2011

### Population Growth and Size

Biotic potential is the maximum growth rate of a population given unlimited resources, unlimited space, and lack of competition or predators. This rate varies from species to species. The carrying capacity is defined as the maximum number of individuals that a population can sustain in a given environment.

If biotic potential exists, then why isn't every inch of this planet covered with life? Because of the environment in which we live, numerous limiting factors exist that help control population sizes. A few examples of limiting factors include predators, diseases, food supplies, and waste produced by organisms. There are two broad categories of limiting factors:

Density-dependent factors. These limiting factors rear their ugly heads as the population approaches and/or passes the carrying capacity. Examples of density-dependent limiting factors include food supplies, which run low; waste products, which build up; and population- crowding-related diseases such as the bubonic plague, which just stink.

Density-independent factors. These limiting factors have nothing to do with the population size. Examples of density-independent limiting factors include floods, droughts, earthquakes, and other natural disasters and weather conditions.

There are two main types of population growth:

1. Exponential growth. the population grows at a rate that creates a J-shaped curve. The population grows as if there are no limitations as to how large it can get (biotic potential).
2. Logistic growth. the population grows at a rate that creates an S-shaped curve similar to the initial portion of Figure 18.3. Limiting factors are the culprits responsible for the S shape of the curve, putting a cap on the size to which the population can grow.

Take a look at Figure 18.3. As the population size increases exponentially from point A to point C, there seem to be enough natural resources available to allow the growth rate to be quite high. At some point, however, natural resources, such as food, will start to run out. This will lead to competition between the members of the population for the scarce food. Whenever there is competition, there are winners and losers. Those who win survive; those who lose do not. Notice that the population rises above the carrying capacity. How can this be? This is short-lived, as the complications of being overpopulated (lack of food, disease from increased population density, buildup of waste) will lead to a rise in the death rate that pushes the population back down to the carrying capacity or below. When it drops below the carrying capacity, resources replenish, allowing for an increase in the birth rate and decline in the death rate. What you are looking at in Figure 18.3 is the phenomenon known as a population cycle. Often, as seen in the figure, when the population size dips below the carrying capacity, it will later come back to the capacity and even surpass it.

However, another possibility shown in this figure is that when a population dips below the carrying capacity due to some major change in the environment, when all is said and done, it may equilibrate at a new, lower carrying capacity.

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