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Population Dynamics
J vs. S shaped growth
Principles of Population Growth
• What is a population?
• A healthy or stable population will grow and
die at a steady rate unless it runs out of food
or space, or is attacked in some way by
disease or predators. (it encounters a limiting
factor of some type)
• Scientists study growth in a variety of ways
• One way involves introducing an organism into a
sterile, nutrient rich environment.
– This is going to be for smaller organisms such as bacteria
• Another way is through field studies and radio
tagging organisms
– Larger organisms such as elk
• Populations do not follow linear growth
increase.
• Instead they start out slowly but once they
build up the rate increases dramatically
Population Growth of Houseflies
Population size
1 million
500,000
100
One year
• This type of growth is called exponential
growth.
• Exponential growth means that as a
population size increases the rate of growth
also increases.
• Exponential growth results in a J-shaped graph
curve.
• If left unchecked exponential growth would
continue forever.
• In reality we know that it doesn’t. At some point
growth stops.
• This is where limiting factors come into play.
• They are going to prevent the population from
exceeding an imaginary line called a carrying
capacity.
• A carrying capacity is the number of organisms that
an environment can support indefinitely, without
outside assistance.
• When looking at a graph illustrating population
growth with a carrying capacity the j-shaped
curve becomes an s-shaped curve
Population
Carrying
capacity
0
J curve
S curve
Time
FOOD
PREDATORS
SPACE
Exponential
growth
DISEASE
Characteristics of Population Growth
• Most species follow a set path of population growth
– They start of with slow growth
– Then there is a period of rapid growth as it enters
exponential growth
– Next it reaches its carrying capacity and levels off
– However there are still some fluctuations in the numbers