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Population Biology
Population size: the number of
organisms in a population (N)
Biotic Potential: highest rate of
reproduction under ideal conditions.
-populations very rarely reach
their biotic potential, because conditions
are not perfect all of the time.
Under perfect conditions, a population
will continue to increase indefinitely.
-unlimited resources, such as food and
water, unlimited space, all waste is
removed so that it does not build up,
etc.
The human population shows this
pattern (so far) . Why?
Population Numbers
Unlimited Population
600
500
400
300
200
100
0
1
2
3
4
5
6
7
8
Time
This is an example of an
EXPONENTIAL growth curve.
9
Carrying Capacity: the number of
individuals that a given environment
can support.
Ex: Number of lions in the African
plains, or the number of zebras
at carrying capacity, birth rate is
equal to the death rate.
Population Numbers
Normal Population Growth
300
250
200
150
100
50
0
Carrying Capacity
1
2
3
4
5
6
7
8
Time
This is an example of a
LOGISTIC growth curve.
9
Environmental Resistance:
the sum of limiting factors facing a
population.
Ex: water requirements, space,
need for food, competition, lack of
mates.
They all add up to the Environmental
Resistance, which holds numbers down
Limiting Factors:
-control population size and growth
-can change depending on
environmental conditions.
Ex: rain after a long drought
Population Density: the number of
organisms per unit area, or volume.
Limiting Factors can be Density
Dependent (related to the density of
the population), or Density
Independent (does not matter what
the density of the population is)
Ex: flood/storm (DI) vs levels of disease
(DD).
Density Dependent Factors
1)Predation: size of prey population is
held down by predators, size of predator
population is dependent on size of prey
population.
Number of
Individuals
Population Relationships
30
20
Foxes
10
Rabbits
0
1
2
3
4
5
Time in Years
6
7
2) Disease: Disease spreads more
rapidly through a dense population,
populations can be reduced by disease
3) Food/water resources: the higher
the density of organisms, the faster the
food and water supply will be used up.
4) Space for nesting: High density
populations will have a high level of
competition for the best areas to raise
young.
Interspecific and Intraspecific
Competition:
Interspecific: competition between
different populations.
Ex: lions vs. hyenas
Intraspecific: within a population.
Ex: lions vs. lions
Both types are Density Dependent
Dispersion and Competition
Dispersion is the pattern of distribution the
individuals within a population take.
Random: Individuals are placed by chance,
or by self determination.
Ex: trees in a forest, a coral reef.
Even Distribution: Individuals are evenly
spaced, located at regular intervals. Usually
occurs because of intraspecific competition.
Ex: Competition between pine trees for sunlight
Clumped Distribution: Individuals are
bunched together in clusters, for protection,
reproduction, or because of narrow habitat
tolerances.
Ex: schools of fish, flocks of birds, clumps of
one type of plant within a forest.
Population Growth Patterns
1) Rapidly growing populations: Many species,
such as insects, plants, and fungi are found in
rapidly changing environments.
Such species are called r-strategists
r-strategists populations grow exponentially
when environmental conditions allow them to
reproduce.
When conditions worsen, the population size
drops quickly.
R-strategists use the strategy of creating many
offspring, but spending little energy and time on
each individual.
Many offspring, little input
EX: Dandelions: Lots of seeds, blown by the wind,
no care at all.
Blue Crabs: Eggs are released by the thousands,
but no energy is spent on the young.
Population
R-Strategist Population Growth
1
2
3
Years
4
5
2) Slowly Growing Populations
Organisms that grow slowly often have small
population sizes.
These organisms are called k-strategists, because
their population is usually below the carrying
capacity (K).
k-strategists have fewer young, but spend more
time and energy on each individual.
K-strategists grow slowly, have a longer life span,
and are less susceptible to environmental change.
Many endangered k-strategists are in trouble
because they are being hunted in numbers that
cannot be supported.
Examples: Sharks
Gorillas
Tigers
Lions
Population
K-strategist Population Growth
1
2
3
Years
4
5
Summary of R vs. K strategists
R:
Mature rapidly
Short-lived
K:
Mature Slowly
Long-lived
Have many offspring
Few offspring at a time
Invest little energy in young Care for their young
Boom or Bust
population
More stable population