BIOLOGY CHAPTER 5
POPULATIONS
5-1 How Populations Grow

(Otter introduction page 119- Read)
Sea otters are important to
populations of kelp, sea
urchins, & killer whales.
CHARACTERISTICS OF
POPULATIONS:
1-Geographic distribution or
range- the area inhabited by
a population.
EX: A few cubic cm (bacteria
in a rotting apple)…to millions
of square km (Migrating
whales)
2-Population Density- # of
individuals per unit area.
Density refers to HOW
MANY…not where!!
3-Growth Rate
4-Age Structure-# of people
in different age groups in the
population.
AGE STRUCTURE
POPULATION GROWTH
3 Factors That Affect
Population Size:
1-# of births
2-# of deaths
3-# of individuals that enter or
leave a population
A population will increase or
decrease depending on how
many individuals are added to it
or removed from it.
When birth rate = death
rate…population stays the same
size
When birthrate is GREATER
than death rate…the population
grows.
When DEATHRATE is greater
than birthrate…the population
shrinks.
IMMIGRATION-(INTO a
population)… movement of
individuals into an area occupied
by an existing population.
Immigration can cause a
population to grow.
EX: Animals come into a
territory in search of mates or
food.
EMIGRATION-(EXIT from a
population)… movement of
individuals out of a population.
Emigration can cause a
population to decrease in size.
EX: Young animals leave the
area where they were born to
find mates & establish
territories. EX: shortage of
food
EXPONENTIAL GROWTHgrowth pattern in which the
individuals in a population
reproduce at a constant rate.
At first, the # of individuals
increases slowly. Over time the
population becomes larger &
larger at a very rapid rate!
Under IDEAL conditions…plenty
of food, water, space;
protected from predators &
disease…a population will
continue to grow…
*Under ideal conditions with
unlimited resources, a
population will grow
exponentially.
See Exponential Growth
Curves on page 121…J-Shaped!
How are these graphs alike?
Besides showing different types
of organisms, how do they differ?
What do these differences
indicate?
What is another major difference
between the reproduction of
bacteria & that of elephants?
How are these graphs alike?Both
plot time on the x axis & # of
organisms on the y axis…curve on
graph is “j” shaped.
Besides showing different types of
organisms, how do they differ?
Population size is given in hundreds
of thousands for bacteria & in
millions for elephants. The elapsed
time is in hours for bacteria & in
hundreds of years for elephants.
What do these differences
indicate?Bacteria reproduce very
rapidly in a short period of time,
but elephants reproduce much more
slowly over a longer period of time.
What is another major difference
between the reproduction of
bacteria & that of
elephants?Bacteria reproduce
asexually, every bacterium is
capable of producing 2 offspring.
Elephants reproduce sexually- 2
parents- male & female- so two
parents are needed to produce one
offspring.
Bacteria double every 20
minutes under ideal conditions!
?Do ideal conditions usually
exist in nature??
BIOTIC POTENTIAL- the size a
population would reach if all
offspring were to survive and
produce young.
In order for this to happen- ideal
conditions would have to exist!
2 Elephants under ideal conditions
would produce more than 20 million
descendents after 750 years.
In reality- no population ever
reaches its biotic potential.
The factors that prevent this
ideal growth are called
Limiting Factors or
Environmental Resistance
LOGISTIC GROWTH &
THE LOGISTIC GROWTH CURVE…
As resources become less
available, the growth of a
population slows or stops.
Logistic Growth- growth pattern
in which a population’s growth
rate slows or stops following a
period of exponential growth.
A logistic growth curve has a
general “s” shape.
Growth starts out slow, then
hits a period of exponential
growth. Exponential growth
continues until the population
reaches the “carrying capacity”.
Carrying Capacity- largest # of
individuals of a population that
a given environment can
support.
A horizontal line is drawn
through the part of the graph
where the population growth
has leveled off…this is how
carrying capacity is indicated on
a logistic growth curve.
The average growth rate is =
to zero at the carrying
capacity.
**Complete population trends
on page 123….graphing &
analyzing data.
5-2 Limits to Growth Page 124
LIMITING FACTOR- a factor
that causes the growth of a
population to decrease.
2 Types Of Limiting Factors:
1- Density-Dependent Factorsa limiting factor that depends on
population size.
EX: competition, predation,
parasitism, & disease
2-Density-Independent Factorslimiting factors that affect all
populations regardless of population
size. EX: unusual weather, natural
disasters, seasonal cycles, & certain
human activities.
COMPETITION- when
organisms compete/struggle
with one another for food,
water, space, sunlight, or other
essentials of life.
Competition usually occurs when
populations become crowded.
Competition is the major force
behind evolutionary change!
PREDATION- interaction in
which one organism captures &
feeds on another organism.
PREDATOR-PREY RELATIONSHIP
mechanism of population control
in which a population is
regulated by predation.
(See figure 5-7 page 126)
Periodic increases in the moose
population are quickly followed
by increases in the wolf
population (predators!).
Predator-Prey Graphs
PARASITISM & DISEASE
Parasites are similar to
predators in many ways….they
take nourishment from hosts
and can cause them to become
sick or die.
HEARTWORMS
Nematods- Roundworms
DENSITY INDEPENDENT
FACTORS tend to be abiotic.
They affect all populations the
same way regardless of size.
5-3 HUMAN POPULATION
GROWTH…page 129
The human population tends to
increase with time.
A long time ago, when life was
harsh…limiting factors kept
the human population sizes low.
Many years ago childhood
diseases kept half of the
children from surviving to
adulthood.
People would have several
children because death rates
were so high…this was to assure
that some would survive!
Agriculture & industry made life
safer & easier…
-more reliable food sources
-better sanitation
-better healthcare
-better medicine
*Birth rates remained high & we
experienced exponential growth.
See Figure 5-10 page 129
What would the graph look like
if we were to extend the graph
to thousands of years in the
future? Why?
PATTERNS OF POPULATION
GROWTH….
Demography-the scientific
study of human populations.
Demography examines
characteristics of human
populations & attempts to
explain how these populations
change over time.
Birth rates, death rates, & age
structure of a population help
predict why some countries have
high growth rates & why others
grow more slowly.
DEMOGRAPHIC TRANSITIONchange in a population from high
birth rate & death rates to low
birth & death rates.
The demographic transition is
complete when the birthrate
falls to meet the death rate…&
population growth stops.
This has happened only in a
few countries.
AGE STRUCTURE
Population growth depends on
how many people of different
ages makes up a population.
AGE-STRUCTURE DIAGRAMS-
population profiles which
graph the #s of people in
different age groups in the
population.
See figure 5-13….Page 131
Future Population Growth
Page 132
*Independent student
reading!
END OF CHAPTER 5 NOTES!