Biology 2013-2014

Section 5-2: Limits to Growth
SWS Science I
SWS Science II

Limiting Factors

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

       Some limiting factors that can affect a population are:



       Parasitism and disease

       Drought and other extreme climates

       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:



       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 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.



       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:

       Unusual weather (extreme hot or cold)

       Natural disasters (storms or hurricanes)

       Season cycles (droughts)

       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.