Population Dynamics:
How Populations Change
Biotic Potential and Environmental Resistance
• Biotic potential – maximum reproductive rate of a
population under ideal conditions
– Assumes all young live long enough to reproduce
– Populations must have all the resources they need to
survive
– Factors influencing biotic potential include, but are not
limited to, the age at which an individual is able to
reproduce and the number of young born at any one
time
• Is it biologically better to have offspring at a younger age?
Does this depend on the age of the mother or father?
• Examples of biotic potential
– If a pair of houseflies produce one generation
every two weeks, they could have 391,000,000
descendents at the end of one year.
– Some bacteria reproduce about every 20
minutes. At this rate, 72 generations could be
produced in 24 hours.
• The number of individual bacteria cells produced
would be enough to cover the entire surface of the
earth to a depth of over 20 centimeters!
• What factors control the unlimited growth of
bacterial colonies?
Biotic Potential and Environmental Resistance
• Environmental resistance – name given to a
collection of factors that reduce the growth
rate of a population
– The advantage of a high biotic potential, such
as the housefly, is to counteract the effects of
environmental resistance.
– This is the have more than you need moto!
Density Dependent Factors
• Definition: Factors that influence a
population differently if the population is
crowded than if it is not crowded.
– A disease spreads more rapidly through a
crowded population that through a sparse one.
– A loss of a food source affects a crowded
population severely since there is less food to
go around..
Density Independent Factors
• Definition: Factors that influence all
populations regardless of their density.
– Forest fires
– Drought
– Lack of sunshine
Carrying Capacity
• Definition: The number of individuals a
population in a particular are can support in terms
of space, food, and shelter.
– Interactions between biotic potential (max. reprod. rate)
and environmental resistance (reduces growth rate) tend
to hold most populations at a fairly stable level that
matches the carrying capacity of the area.
– How do humans affect the carrying capacity of an area?
• Remove habitats for housing
• Destroy the food in an area
• Change the flow of water in an area