Transcript Factors that affect populations

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:
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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
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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:
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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 Activities
Overexploitation of cod fishery- grand banks