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POPULATION DYNAMICS
Mrs.
Cummings
CP Biology
POPULATION BASICS
A population is a group of organisms of the
same species living within a certain area.
(Members of the same species are able to
reproduce and have fertile offspring)
The population size is the number of
organisms within the group.
The population density is the number of
organisms per unit area.
POPULATION GROWTH
Population growth is the increase in the size
of a population over time.
As the organisms within a population reproduce, the
population size increases.
To graph population growth:
X-axis = time
Y-axis = population size
POPULATION GROWTH
POPULATION GROWTH
Under ideal conditions, a population would
grow rapidly and keep increasing toward infinity.
Ideal conditions include favorable temperature,
unlimited food/water, & lack of disease or predators.
Biotic potential is the highest possible rate of
reproduction for a population under ideal
conditions.
UNLIMITED POPULATION GROWTH:
J-SHAPED CURVES
Under ideal conditions, a population that starts
out with just 1 or 2 individuals will increase in
size slowly at first, then more quickly as more
individuals become capable of reproducing.
When graphed, this type of population growth
resembles the letter J and is therefore called a
J-shaped curve or exponential growth
curve.
UNLIMITED POPULATION GROWTH:
J-SHAPED CURVES
The size of the
population is always
increasing.
The rate at which the
population is growing is
also increasing.
You can tell how fast
the population is
growing by how steep
the line is.
LIMITING FACTORS
In reality, most populations do not exhibit Jshaped growth curves.
Limiting factors are circumstances that slow
down population growth, preventing
populations from reaching their biotic
potential.
Examples: Food, water, and space availability;
predator species; unfavorable temperatures, etc .
Limiting factors can be either density-
DENSITY-DEPENDENT LIMITING FACTORS:
S-SHAPED CURVES
When the population size is small, densitydependent limiting factors have little effect, and
the population can grow at a fast rate.
As the population increases in size and density,
density-dependent limiting factors begin to have
more of an impact, and the growth rate of the
population slows down.
Results in S-shaped curve, also called a population
growth curve or logistical growth curve.
Density-dependent limiting factors include things
like food, water, shelter, and predators.
DENSITY-DEPENDENT LIMITING FACTORS:
S-SHAPED GROWTH CURVES
DENSITY-DEPENDENT LIMITING FACTORS:
S-SHAPED CURVES
The carrying capacity of a population is the
largest sustainable size that it can reach in its
given environment (such as a pond or woodland)
under a particular set of conditions.
The population can no longer grow because it has
reached the maximum size that its environment can
support.
DENSITY-DEPENDENT LIMITING FACTORS:
S-SHAPED CURVES
At carrying capacity,
the size of the
population levels off.
This is only possible
when the birthrate
equals the death
rate of the
population.
DENSITY-INDEPENDENT LIMITING
FACTORS & THEIR GROWTH CURVES
Density-independent limiting factors will
inhibit the growth of a population whether it
is large or small.
When a density-independent limiting factor
affects a population, the population size may
drop suddenly.
The growth curve resembles a J-shaped or Sshaped curve until the drop-off occurs.
These populations never reach their carrying
capacity.
DENSITY-INDEPENDENT LIMITING
FACTORS & THEIR GROWTH CURVES
The most
common densityindependent
limiting factors
are seasonal
changes in
weather,
especially
temperature.
POPULATION GROWTH RATE CURVES
Growth rate is a measure of how fast a
population is growing.
Recall that in a population growth curve, the growth
rate is indicated by the slope (steepness) of the line.
A population growth rate curve shows how
the rate of population growth changes over time.
In a population growth rate curve, time goes on
the x-axis, and growth rate goes on the y-axis.
POPULATION GROWTH RATE CURVES
Growth Rate Curve for a Population of
Yeast
Growth Rate (cells/hour)
200
180
160
140
120
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80
60
40
20
0
1
2
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Time (hours)
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POPULATION INTERACTIONS
The limiting factors that control population
sizes often involve interactions among
organisms.
These include interactions between members
of the same species and interactions between
different species.
PREDATOR-PREY RELATIONSHIPS
Predation is the feeding of one organism on
another.
By eating prey, predators act as a limiting factor on
prey populations.
Predation is a density-dependent limiting factor. The
greater the density of the prey population, the greater
the likelihood of a predator finding one and eating it.
But, since they are a food source for predators,
prey species are also a limiting factor on predator
populations.
Also a density-dependent limiting factor!
PREDATOR-PREY RELATIONSHIPS
TIME
PREDATOR-PREY RELATIONSHIPS
Predation can prevent overpopulation of prey
species.
Predators help keep prey populations at sizes
that the environment can sustain (carrying
capacity!!)
PARASITE-HOST RELATIONSHIPS
Parasitism is when a small organism lives on or
inside of a larger organism and causes it harm.
The smaller organism is the parasite, and the larger
organism is called the host.
Parasitism can be a limiting factor on host
populations.
It is density-dependent because the parasite is more
likely to be passed from one organism to another in
a dense host population than in a more spread-out
host population.
SYMBIOTIC RELATIONSHIPS
Parasitism is one type of symbiotic relationship.
A symbiotic relationship or symbiosis is a
close interaction between organisms of two
different species.
There are three types of symbioses:
Parasitism- One organism benefits, the other is
harmed
Commensalism- One organism benefits, the other is
not affected
Mutualism- Both organisms benefit
SYMBIOTIC RELATIONSHIPS
Parasitism – One organism (the parasite)
benefits while the other (the host) is harmed.
Even though the parasite harms the host, killing the
host is not its goal. The host is its home; if the host
dies, it will need to find another host, which is risky!
Caterpillar & wasp larvae
Human tapeworm
SYMBIOTIC RELATIONSHIPS
Commensalism – One organism benefits;
the other is neither harmed nor helped.
Barnacles and sea turtle
Sucker fish and shark
SYMBIOTIC RELATIONSHIPS
Mutualism – Both organisms benefit.
Flower and butterfly
Bird and crocodile
DISEASE AND POPULATIONS
Disease is a density-dependent limiting factor.
The denser the population, the greater the
chance that disease will spread.
Humans have used diseases to purposely
control populations of certain species.
DISEASE AND POPULATIONS
Studying how diseases spread through populations
(epidemiology) can be complicated!
For example, the bubonic plague spread rapidly
through Europe in the 14th century and by some
estimates killed over half the population!
The plague is caused by a bacterium, which lives on
fleas, which are carried by rodents & other animals…
So the spread of the plague through human
populations also depends on the population densities
of fleas, rats, and other animals.
COMPETITION FOR
RESOURCES
Usually many different species live in the same
area at the same time.
Often several species living in a given area share
the same food source.
For example, mice are food for owls & other birds,
snakes, foxes, and many other animals.
Interspecific competition is competition
between members of different species in an area.
Intraspecific competition is competition
between members of the same species in an area.
INTERSPECIFIC COMPETITION
Competition between populations of
different species.
Interspecific competition is a density-dependent
limiting factor.
It may result in one of three possible outcomes:
Extinction of one species in the area
Movement of one population to another area
Adaptation of one species due to natural
selection (this occurs over many generations!)
INTERSPECIFIC COMPETITION:
EXTINCTION
When 2 or more species living in the same
environment compete for the same resources,
one possibility is that one of the species will
die out and become extinct in that
environment.
Paramecia (singular: paramecium) are single-celled
organisms that live in large numbers in aquatic
environments. In a classic lab experiment, two
different species of paramecia are placed in a
culture dish together with a limited supply of food.
The results are shown on the following slide:
INTERSPECIFIC COMPETITION:
EXTINCTION
When grown separately,
populations of P. aurelia
and P. caudatum both live.
Growth of
P. aurelia
When grown together,
P. caudatum dies out.
P. aurelia &
P. caudatum
grown together
Growth of
P. caudatum
300
600
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Number per 0.5 mL
500
Number per 0.5 mL
Number per 0.5 mL
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0 2 4 6 8 10 12 14 16
Day
Day
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8 10 12 14 16
Day
INTERSPECIFIC COMPETITION:
EXTINCTION
Interspecific competition between P. aurelia and P.
caudatum results in extinction of the P. caudatum
P. aurelia &
population.
Growth of
P. aurelia
P. caudatum
grown together
Growth of
P. caudatum
300
600
600
500
Number per 0.5 mL
500
Number per 0.5 mL
Number per 0.5 mL
250
400
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150
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50
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100
0
0
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0 2 4 6 8 10 12 14 16
0 2 4 6 8 10 12 14 16
Day
Day
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Day
INTERSPECIFIC COMPETITION:
MOVEMENT
A second possibility is that one species will
move into another, usually nearby
environment.
• For example, several different species
of Maine warblers make their nests in
the same kind of spruce tree, but each
species lives only within a certain
zone of the tree (such as the crown,
lower branches, or outer branches).
INTERSPECIFIC COMPETITION:
ADAPTATION
The third possible outcome of interspecific
competition is that rapid evolution will occur in one
of the species, causing it to acquire different traits
so it no longer has to compete with the other
species.
The finches that Darwin studied had developed
different-shaped beaks suited for different types of
seeds.
Even though they live in the same area, they do not
compete for the same food source because of these
adaptations.
This occurs because of random mutations and takes
many generations!
INTRASPECIFIC COMPETITION
Competition between members of the
same species.
Intraspecific competition is a density-dependent
limiting factor.
Species have several ways of avoiding
competition within their own populations:
•
•
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Life Cycles & Life Spans
Dominance
Role Separation
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Behavioral &
Physiological Changes
Emigration
Territoriality
LIFE CYCLES & LIFE SPANS
The life cycle of a species can help reduce
competition between members of that species.
Frogs and tadpoles do not compete because their food
and habitats are different.
In many plants and fungi, dispersal of seeds or spores to
distant locations helps prevent competition among
members of the same species.
Life spans may also help avoid intraspecific
competition.
In many insect species, adults die shortly after the young
are produced, so old and young do not have to compete
with each other.
DOMINANCE
A social hierarchy or pecking order is a
chain of command that develops within a
population and helps prevent conflict between
members of the same species.
The term “pecking order” comes from chickens
pecking at each other to establish dominance.
Male hierarchies also form in some populations,
in which males establish dominance by fighting,
especially with horns or antlers.
Often the dominant male is the only one who
mates, which helps prevent overbreeding.
ROLE SEPARATION
Competition is reduced in societies in which
each member has a definite role.
Occurs in many insect populations such as bees,
ants, and termites
BEHAVIORAL &
PHYSIOLOGICAL CHANGES
Overcrowding can cause changes in both behavior
and physiology in members of a population.
For example, overcrowded rats become increasingly
aggressive to the point that more deaths occur than
normal (behavior).
Stress from overcrowding can also cause hormonal
changes that reduce litter size (physiology).
As a result of these changes, birthrate
decreases while death rate increases.
This reduces the population size to a number the
environment can support.
EMIGRATION
Emigration is when some members of an
overcrowded population relocate to a new area.
Some members remain in the original environment.
Results in 2 low-density populations.
Bee example from Chapter 27 Intro
TERRITORIALITY
Territoriality is when different members of the
same species occupy and defend different
territories.
By staying separated, competition for food and
other resources is avoided.
Males that control a territory are often the only
ones to mate, which helps limit population growth.
THE HUMAN POPULATION
 Human population growth resembles a J-shaped curve.
THE HUMAN POPULATION
THE HUMAN POPULATION
Growth rate is declining.
The growth rate is much higher in developing
countries than in industrialized countries.
Population size is still increasing.
THE HUMAN POPULATION
AGE STRUCTURE DIAGRAMS
(POPULATION PYRAMIDS)
To predict population growth, it is useful to
know the age of everyone in the population.
People are often categorized by age groups.
An age structure diagram (also called a
population pyramid) is a tool used to visually
display how many people within a given
population are in each age group.
The proportion of females of reproductive age
is especially important.
AGE STRUCTURE DIAGRAMS
(POPULATION PYRAMIDS)