POPULATION GROWTH
CHAPTER 13
Chapter 13
Population Growth
HOW FAST CAN A POPULATION
GROW?
• Most organisms produce more offspring than
can survive
• The maximum growth rate (biotic potential) is
the rate at which a population would grow if
every individual survived and reproduced.
• The population would grow exponentially.
Populations Can Grow, Shrink, or
Remain Stable
• Population calculated by
– Births
– Deaths
– Immigration
– Emigration
• Population change =
(births + immigration) – (deaths + emigration)
Exponential Population Growth
(when all conditions are perfect)
• J – shaped curve.
• Exponential growth – as a
population gets larger, it also
grows faster.
• Occurs when there are no
factors to limit or slow growth
• Look how fast the population
grows if all conditions are
ideal!
However, no population can grow forever
• Limiting factors control the size of a
population
• For animals, limited food, water, shelter and
other resources limit population sizes
• For plants, limited water, sunlight, space and
nutrients
Population Density
• The number of organisms in an area. The
more organisms within a small space = a
more dense population. 18 deer/acre is
more dense than 8 deer/acre
Population Density Effects
• Density Dependent– Factors that limit growth as the density increases.
Ex. Resources, predation, parasitism, disease
• Density Independent– Affect the population regardless of density
– Mostly abiotic Ex. Natural disasters, habitat
destruction, climate changes
Environmental Resistance
(all factors that limit the population growth)
• CROWDING (density)
– Predators, parasites and diseases
• LESS RESOURCES
– Water, space, food, shelter, etc,…
• A population will grow until the environment
cannot support any more organisms. This is
the carrying capacity.
Carrying Capacity
• Is the maximum population size an
environment can support for a long period of
time.
• A population may exceed its carrying capacity,
but not for long! Most populations fluctuate
around the carrying capacity.
Logistic Growth
(conditions are never perfect)
Carrying capacity
Time (hours)
• When limiting factors
control population
growth
• S-shaped curve
(logistic growth)
The resulting curve is an S curve
(logistic curve)
Logistic Growth of a Sheep Population on the
island of Tasmania, 1800–1925
Exponential Growth, Overshoot, and
Population Crash of a Reindeer
Factors affecting population growth
1. Predator/prey – balance one another; ex: wolves
2. Introduced species – impact native species,
decreases biodiversity
3. Plant/herbivore – herbivores control plant
populations;
4. Overgrazing –plants can’t reproduce fast enough
5. Plant competition – plants release chemicals to
inhibit growth of other plants
Population Cycles for the Snowshoe
Hare and Canada Lynx
Why do we care?
• Predators and prey
influence one another's
evolution and control
population numbers
• Traits that enhance a
predator's ability to find
and capture prey will be
selected for
• Traits that enhance the
prey's ability to avoid
being eaten will be
selected for.
• Coevolution (Caribbean
snail and crab)
Case Study: Exploding White-Tailed
Deer Population in the U.S.
• 1900: deer habitat destruction and
uncontrolled hunting
• 1920s–1930s: laws to protect the deer
• population explosions for deer can creates
problems:
– Lyme disease
– Deer-vehicle accidents
– Eating garden plants and shrubs
• Ways to control the deer population
hunting, introduce predators, relocation, reduce
food resources, fencing
Kaibab Deer in Arizona
how humans affect populations
• In 1905, the deer population on the Kaibab
Plateau in Arizona was estimated to be about
4,000 on 300,000 hectares of range.
• The carrying capacity was estimated to be
about 30,000 deer.
• The Kaibab forest area was severely
overgrazed by sheep, cattle, and horses and
most of the tall, perennial grasses had been
eliminated in the area.
• In 1906 President T. Roosevelt created the
Grand Canyon National Park
To increase the number of deer, the government
banned the hunting of the deer and killed their
predators (800 mountain lions, 20 wolves, 7400
coyotes, and 500 bobcats between 1907 and
1939)
In 1924, the deer population
had
increased to 100,000! Way
beyond the carrying capacity
of
the land
• The deer did not have enough resources and
were starving! In order to restore the herd to
a healthy number of deer, hunting was again
allowed and by 1939, the population was
down to 10,000
• Starvation killed more deer than the predators
ever had!
• Should humans interfere with the predatorprey relationship?