Visualizing Environmental Science

162 CHAPTER 7 Human Population Change and the Environment

population growth for a short period. How-

ever, organisms don’t reproduce indefinitely

at their biotic potentials because the envi-

ronment sets limits, which are collectively

called environmental resistance. Examples of

environmental resis tance include such unfavor-

able environmental conditions as limited food,

water, shelter, and other essential resources (re-

sulting in increased competition), as well as increased dis-

ease and predation.

Using the earlier example, we find that bacteria never

reproduce unchecked for an indefinite period because

they run out of food and living space, and poisonous body

wastes accumulate in their vicinity. With crowding, bacte-

ria become more susceptible to parasites (high population

densities facilitate the spread of infectious organisms such

as viruses among individuals) and predators (high popula-

tion densities increase the likelihood of a predator catching

exponential population growth (Figure 7.3).

When a population grows exponentially, the

larger the population gets, the faster it grows.

Regardless of species, whenever a population

grows at its biotic potential, population size

plotted versus time gives the same J-shaped

curve. The only variable is time. It may take

longer for a dolphin population than for a

bacterial population to reach a certain size (because

dolphins do not reproduce as rapidly as bacteria), but

both populations will always increase exponentially as

long as their growth rates remain constant.

Environmental Resistance

and Carrying Capacity

Certain populations—particularly those of bacteria, pro-

tists, and certain insects—may exhibit exponential

Ý«œ˜i˜Ìˆ>Ê«œ«Õ>̈œ˜Ê}ÀœÜ̅ÊUʈ}ÕÀiÊÇ°ÎÊ

exponential

population growth

The accelerating

population growth

that occurs when

optimal conditions

allow a constant

reproductive rate.

Time (hours)

0 1 2 3 4 5 6 7 8 9

10

1

4

16

64

256

1,024

4,096

16,384

65,536

262,144

1,048,576

Number of bacteria

600

500

400

300

200

100

0234 10

700

800

900

1,000

1,100

0

156789

Time (hours)

Number of bacteria

(in thousands)

CNRI/Science Photo Library/Science Source

a. Streptococcus bacterium in the process of dividing.

c. When bacterial numbers are graphed, the curve of

exponential population growth has a characteristic J

shape.

b. When bacteria divide at a constant rate, their

number increases exponentially.