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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