Population Growth Curves
• Exponential Growth (J-shaped curve)
• Logistic Growth (S-shaped curve)
Population ecology
• Population ecology is the study of
populations.
• Population = group of individuals of the same
species occupying a common geographical
area
• Habitat = where a species normally lives
• Density = The number of individuals in a
population per unit area.
Exponential Growth
• Indicated by a J-shaped growth curve
• growth in which the population increase in a
period is a fixed percentage of the size of the
population at the beginning of the period;
• the number of individuals increases over time
logarithmically (e.g. bacterial cultures).
Components of an exponential growth
pattern for a given population
1. Time Zero - time of establishment of a
population; for bacterial culture, time that
the culture was inoculated
2. Lag Phase - time that it takes for the
population to start growing; resources must
be obtained; habitat and microhabitat
establishment; niche definition; population
prepares for reproduction (for bacterial
cultures: time that the cells need to begin
their specific type of metabolism before cell
division can occur).
3. Log Phase - period of exponential
(logarithmic) growth.
4. Stationary Phase - period following
exponential growth where number of deaths
equals the number of new individuals.
5. Death Phase - period following the
stationary phase where the population is
dying due to depletion of resources and or
contamination of habitat with waste
products.
STATIONARY PHASE
DEATH PHASE
EXPONENTIAL/
LOG PHASE
LAG PHASE
LABEL THE PHASES OF
EXPONENTIAL GROWTH
Logistic growth (S-shaped curve)
• Because of limiting factors, populations rarely exhibit Jshaped growth curves.
• When growth encounters environmental resistance,
populations experience an S-shaped or logistic growth curve.
• Early on populations will exhibit very rapid growth, but as
they near the carrying capacity they will level off.
• Logistic growth is density dependent.
• High density and overcrowding put individuals at greater risk
of being killed
– Predators, parasites and pathogens have greater numbers of prey and
hosts in a smaller area to interact with
Components of a Logistic Growth
Curve
•
•
•
•
Lag Phase
Log/ Exponential Growth Phase
Deceleration Phase
Stable, Equilibrium Phase
Growth curve of a laboratory population of
yeast cells.
STABLE EQUILIBRIUM
PHASE
DECELERATION PHASE
EXPONENTIAL/
LOG PHASE
LAG PHASE
Growth curve of the sheep population of Southern Australia.
The smooth curve is the hypothetical logistic curve about
which the real curve seems to fluctuate.
Effects of population density on growth in
open systems:
• Density-independent growth - size of population
is not a factor in determining the resulting
population size overall;
– population size, however, stays about the same as
when it began.
• Most density-independent factors are abiotic.
• Examples: temperature, storms, floods, drought,
habitat destruction
Effects of population density on growth in
open systems:
• Density dependent growth - size of the
population is a factor in determining the
resulting population size overall;
• exponential growth can occur if adequate
resources are available and range expansion
can occur as in an open system.
• Disease is spread more quickly
• Stress can lead to aggression
Population growth
Populations grow, shrink, or remain stable,
depending on rates of birth, death, immigration,
and emigration.
(birth rate + immigration rate) –
(death rate + emigration rate)
= population growth rate
Exponential growth
•Unregulated populations increase by exponential
growth:
•Growth by a fixed
percentage, rather
than a fixed amount.
•Similar to growth
of money in a
savings account
Exponential growth in a growth curve
•Population
growth curves
show change in
population size
over time.
•Scots pine
shows
exponential
growth
Figure 5.10
Limits on growth
•Limiting factors restrain exponential population
growth, slowing the growth rate down.
•Population growth levels off at a carrying capacity—
the maximum population size of a given species an
environment can sustain.
•Initial exponential growth, slowing, and stabilizing at
carrying capacity is shown by a logistic growth curve.
Logistic growth curve
Figure 5.11
Population growth: Logistic growth
•Logistic growth (shown here in yeast from the lab) is
only one type of growth curve, however.
Figure 5.12a
Population growth: Oscillations
•Some populations fluctuate continually above and
below carrying capacity, as with this mite.
Figure 5.12b
Population growth: Dampening
oscillations
•In some populations, oscillations dampen, as population size
settles toward carrying capacity, as with this beetle.
Figure 5.12c
Population growth: Crashes
•Some populations that rise too fast and deplete resources
may then crash, as with reindeer on St. Paul Island.
Figure 5.12d