Factors that affect populations

• What aspects of the ecosystem can limit the population of a species?

• Anything that changes the relationship between births, deaths, immigrations and emigrations will change the overall population size.

• A limiting factor = a factor that causes population growth to decrease

• Some limiting factors include:– Access to resources– Competition– Predation– Parasitism and disease– Drought and other climate extremes – Human disturbances

Limiting Factors

• There are 2 main categories of limiting factors:– Density-dependent factors- factor that depends

on the size of the population to act– Density-independent factors- affect all

populations in similar ways, regardless of population size

Density-Dependent Factors• A limiting factor that depends on population size including:

– Intra and interspecific Competition – organisms struggle for resources– Predation – predator-prey relationship controls population of both the

predator and the prey– Parasitism – control population by weakening/killing host. Less hosts=

less parasites as well.– Disease - control population by weakening/killing host

• Density dependent factors become limiting only when the population density (number of organisms per unit area) reaches a certain level

• These factors operate most strongly when a population is large and dense

Competition

• As resources become more scarce competition for that resources increases

• With greater competition, there is a larger struggle for survival

• It becomes more likely that a given individual will not succeed.

Intraspecific Competition

Predator-Prey Relationship• The number of predators and prey in a system are directly related

– Predators increase- Prey decreases– Prey decreases- predator decreases– Predator decreases- prey increases– Prey increases- predator increases

Disease

Density-Independent Factors• Affect all populations in similar ways, regardless of the

population size including:– Unusual weather– Natural disasters– Seasonal cycles– Human activities (damming rivers/clearing forests)

• Many species will show a characteristic “crash,” then the population will build back up right away, or stay low for some time

• Environments are always changing, and most populations can adapt to a certain amount of change by growing or shrinking in size.

• Major upsets in the ecosystem can lead to long-term declines in certain populations (human activities)

Density Independent- Extreme Weather

Seasonal

Human ActivitiesOverexploitation of cod fishery- grand banks