Chapter 14 Population Ecology • MHR 483In addition to its role in evolution, intraspecific
competition is an important density-dependent
factor limiting the growth of many populations.
Members of a population may compete for a
variety of needed resources, including food, water,
sunlight, soil nutrients, shelter, or breeding sites
(see Figure 14.20). The effect is always the same —
a reduction in the population’s growth rate.
Figure 14.20For gannets, which nest on rocky islands such
as this one off Cape St. Mary’s in Newfoundland, the limited
number of nesting sites determines the carrying capacity of
the environment. As a result, only a certain number of pairs
can nest and reproduce at any given time. When Nis low, all
birds can find a suitable nest site and reproduce, so the
population grows. Above a certain N, however, many pairs
fail to breed successfully and, as a result, population growth
slows and eventually levels off.
Density-dependent limiting factors can involve
interactions between two or more populations as
well as interactions within a single population.
For instance, two species with similar habitat
requirements (for example, see Figure 14.21) may
compete with each other for soil nutrients, food, or
other resources found in that habitat. This type of
interaction between two or more populations is
referred to as interspecific competition. In some
cases, one species may eventually out-compete the
other or others, and the “losing” species disappears
from the area (see Figure 14.22 on the next page). A
result like this often indicates that the interacting
species had very similar ecological niches. Ecologists
explain the effects of interspecific competition by
referring to the competitive exclusion principle.
Essentially, this theory states that species with
niches that are exactly the same cannot co-exist; if
two species have completely overlapping niches,
one will always exclude the other, as shown by the
dashed line in Figure 14.22B. However, if the niches
of competing species are sufficiently different, they
can both live in a particular area, although the
density of one or more of the populations may be
lowered by the presence of the other (as shown by
the dashed line in Figure 14.22A).Figure 14.21In Ontario, garlic mustard (Alliaria petiolata,
a plant introduced from Europe) invades moist woodland
habitats and crowds out many native species. It is a
particular threat to some endangered plant species,
such as the wood poppy (Stylophorum diphyllum).For some species, internal rather than external cues (such
as a reduced food supply) seem to limit population size.
Small rodents kept in experimental conditions with
abundant food and shelter will reproduce quickly until their
population reaches a certain density. At this point, even
though the resources they need most are still unlimited,
reproduction decreases. High density seems to produce a
stress response in which hormonal changes delay sexual
maturation, cause shrinkage of reproductive organs, reduce
the effectiveness of the immune system, and often produce
aggressive behaviour (sometimes including cannibalism).
Exactly what cues trigger these hormonal changes is unclear,
but similar effects of crowding have been noticed in some
animal populations in nature.