Visualizing Environmental Science

Time

High

Low

Number of paramecia

Carrying capacity of environment (K)

a. Paramecium is a unicellular microorganism. Michael Abbey/Science Source

b. In many laboratory studies, including Gause’s work with Paramecium,

population growth increases exponentially when the population is low

but slows as the carrying capacity of the environment is approached.

This produces a curve with a characteristic S shape.

in a test tube (Figure 7.4a). He supplied a limited amount

of food daily and replenished the media to eliminate the

buildup of wastes. Under these conditions, the population

increased exponentially at first, but then its growth rate de-

clined to zero, and the population size leveled off.

When a population influenced by environmental

resistance is graphed over a long period, the curve has

an S shape (Figure 7.4b). The curve shows the popu-

lation’s initial exponential increase (note the curve’s J

shape at the start, when environmental resistance is low).

Then the population size levels out as it approaches the

carrying capacity of the environment. The rate of popu-

lation growth is proportional to the amount of existing

resources, and competition leads to limited population

growth. Although the S curve is an oversimplification of

how most populations change over time, it fits some pop-

ulations studied in the laboratory, as well as a few studied

in nature.

A population rarely stabilizes at K (carrying capacity),

as shown in Figure 7.4, but its size may temporarily rise

higher than K. It will then drop back to, or below, the

an individual). As the environment deteriorates, the bacte-
ria’s birth rate declines and their death rate increases. The
environmental conditions might worsen to a point where
the death rate exceeds the birth rate, and as a result, the
population decreases. Thus, the environment controls
population size: As the population increases, so does envi-
ronmental resistance, which limits population growth.
Over longer periods, environmental resistance may
eventually reduce the rate of population growth to nearly
zero. This leveling out occurs at or
near the environment’s carrying
capacity (K). In nature, carrying
capacity is dynamic and changes
in response to environmental
changes. An extended drought,
for example, might decrease the
amount of vegetation growing in
an area, and this change, in turn, would lower the carrying
capacity for deer and other herbivores in that environment.
G. F. Gause, a Russian ecologist who conducted
experiments in the 1930s, grew a population of Paramecium

carrying capacity

( K ) The largest

population a particular

environment can

support sustainably

(long term), if there

are no changes in that

environment.

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Is the type of growth exhibited by

Paramecium more likely to be seen in the laboratory or

in nature? Why?

Think Critically