¨ A limiting factor is a factor that causes
population growth to decrease.
¨ Some limiting factors that can affect a population are:
Ø Parasitism and disease
Ø Drought and other extreme climates
Ø Human disturbances
¨ A resource base that is limited can also affect the long-term survival
of a species.
v The panda population is one of the most endangered species today
because pandas depend on bamboo for food and the forests are being cleared for timber and farmland.
¨ A density-dependent limiting factor is a limiting
factor that depends on population size.
¨ These factors operate most strongly when a population is large and
¨ They do not affect small, scattered populations as greatly.
¨ Density-dependent limiting factors include:
Ø Parasitism and disease
¨ When populations become crowded, organisms compete with one another
for food, water, space, sunlight, and other essentials.
¨ Competition among members of the same species is a density-dependent
¨ The more individuals living in an area, the sooner they use up the
¨ Competition can also occur between members of different species.
¨ This type of competition is a major force behind evolutionary change.
¨ The species may evolve to occupy separate niches.
¨ No two species can occupy the same niche in the same place at the
¨ A predator-prey relationship is the regulation
of a population by predation.
v The interaction between wolves and moose on Isle
Royal is a classic example. An
increase in the moose population-the prey-is quickly followed by an increase in the wolf population-the predators. As the wolves prey on the moose, the moose population falls. Then a decline in the wolf population because
there is less for the wolves to feed upon. So, the moose have fewer enemies and
that population rises again.
Parasitism and Disease
¨ Like predators, parasites take nourishment at the expense of their
hosts, often weakening them and causing disease or death.
¨ So, parasites can also limit the growth of a population.
¨ Density-independent limiting factors affect all populations in similar ways, regardless of the population size.
¨ Density-independent limiting factors include:
Ø Unusual weather (extreme hot or cold)
Ø Natural disasters (storms or hurricanes)
Ø Season cycles (droughts)
Ø Human activities – damming rivers, clear-cutting forests
¨ Many species show a characteristic crash in population size.
¨ After the crash, the population may soon build up again, or it may
stay low for some time.
¨ Environments are always changing, and most populations can adapt
to a certain amount of change.
¨ Populations often grow and shrink in response to change.
¨ Major upsets can lead to long-term declines in certain populations.