5 Steps to a 5 AP Biology, 2014-2015 Edition

224 ❯ STEP 4. Review the Knowledge You Need to Score High

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 fac-

tors 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, earth-

quakes, 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 simi-

lar 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 Ato

pointC, 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.

Carrying capacity C

A New carrying capacity

Figure 18.3 Carrying capacity.

KEY IDEA