Limiting Factors

¨       A limiting factor is a factor that causes population growth to decrease.

¨       Some limiting factors that can affect a population are:

Ø       Competition

Ø       Predation

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.

Density-Dependent Factors

¨       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 dense.

¨       They do not affect small, scattered populations as greatly.

¨       Density-dependent limiting factors include:

Ø       Competition

Ø       Predation

Ø       Parasitism and disease

Competition

¨       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 limiting factor.

¨       The more individuals living in an area, the sooner they use up the available resources.

¨       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 same time.

Predation

¨       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 Factors

¨       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.