Biology 12

Chapter 14 Population Ecology • MHR 481

14.3 Factors Limiting Natural Population Growth

Density-independent Factors

Many populations that grow exponentially
eventually stop growing quickly, or crash as a result
of a very high death rate (recall Figure 14.13 on
page 478). Such crashes are frequently the result
of abiotic factors such as bad weather. For example,
certain species of bark beetles (which cause serious
damage to forests in various parts of Canada) grow
exponentially until very cold weather in early or
late winter kills many of the individuals that would
produce the next generation. When winters are
mild, or short, enough insects survive to reproduce
during the next summer, creating an exponential
growth curve.
These abiotic limiting factors are referred to as
density-independent factorsbecause they affect
populations regardless of their density. These
factors are effective against small populations as
well as large ones. Figure 14.17 illustrates the effect
of a density-independent factor. Other density-

Figure 14.17Notice that for this hypothetical population,
a decline in population size is linked to temperature, a
density-independent limiting factor. The decline begins
before the population is halfway to its carrying capacity
and well before its own density would inhibit its growth.

Temperature or population density

Time

temperature

population

crash

carrying capacity

independent factors include events like floods or

droughts. For populations located within a small

geographic area, forest fires, hurricanes, or tornadoes

can also limit population growth. Figure 14.18 shows

a memorable example of a density-independent

regulating event.

Density-dependent Factors

As you have seen, the growth of many populations

follows a logistic rather than an exponential growth

curve. A population like this is limited by density-

dependent factors. The strength with which these

factors slow a population’s growth depends on the

density of the population. When a population is

Figure 14.18A severe ice storm hit eastern Ontario and

parts of Québec in January 1998. The devasting ice storm

made roads impassable and knocked down power lines,

leaving hundreds of thousands of people without electricity,

some for several days. In addition, the storm acted as a

density-independent limiting factor, causing declines in the

populations of maple, birch, and cedar trees, among other

species.

EXPECTATIONS

Differentiate between density-independent and density-dependent population

regulating factors, and describe their effect on the growth of populations.

Describe various types of interactions among different species of animals

and plants, and explain how they affect population growth.

Compare and explain the fluctuations of various populations, emphasizing

factors such as carrying capacity, fecundity, and predation.