Download Populations

Lesson Overview
How Populations Grow
5.1 How
Populations Grow
BENCHMARK
SC.912.L.17.5 Analyze
how population size is
determined by births,
deaths, immigration,
emigration, and limiting
factors (biotic and
abiotic) that determine
carrying capacity.
Lesson Overview
How Populations Grow
THINK ABOUT IT
In the 1950s, a fish farmer in
Florida tossed a few plants called
hydrilla into a canal. Hydrilla was
imported from Asia for use in
home aquariums because it is
hardy and adaptable. The few
plants he tossed in reproduced
quickly and kept on reproducing.
Today, their tangled stems snag
boats in rivers and overtake
habitats; native water plants and
animals are disappearing. Why
did these plants get so out of
control? Is there any way to get rid
of them?
Lesson Overview
How Populations Grow
THINK ABOUT IT
Meanwhile, people in New England
who fish for a living face a different
problem. Their catch has dropped
dramatically, despite hard work and
new equipment. The cod catch in one
recent year was 3,048 metric tons.
Back in 1982, it was 57,200 metric
tons—almost 19 times higher! Where
did all the fish go? Can anything be
done to increase their numbers?
Lesson Overview
How Populations Grow
Describing Populations: How do
ecologists study populations?
• population: group of
organisms of a single species
that lives in a given area
e.g. hydrilla population
(see map)
• Researchers study
populations’ geographic
range, density and
distribution, growth rate, and
age structure
Lesson Overview
How Populations Grow
Geographic Range
• geographic range:
area inhabited by a
population
• varies enormously in
size, depending on
the species
Lesson Overview
How Populations Grow
Geographic Range : EXAMPLES
A bacterial population in a
rotting pumpkin may have a
range smaller than a cubic
meter, whereas the population
of cod in the western Atlantic
covers a range that stretches
from Greenland down to North
Carolina.
Humans have carried hydrilla
to so many places that its
range now includes every
continent except Antarctica,
and it is found in many places
in the United States.
Lesson Overview
How Populations Grow
Density and Distribution
• Population density: number of individuals per unit
area
• Populations of different species often have very
different densities, even in the same environment
• EXAMPLE: A population of ducks in a pond may
have a low density, while fish and other animals in
the same pond community may have higher
densities.
Density and Distribution
RANDOM
DISTRIBUTION
An example of a
population that shows
random distribution is the
purple lupine. These wild
flowers grow randomly in
a field among other
wildflowers. The dots in
the illustration represent
individual members of a
population with random
distribution.
UNIFORM
DISTRIBUTION
An example of a
population that
shows uniform
distribution is the
king penguin. The
dots in the
illustration represent
individual members
of a population with
uniform distribution.
CLUMPED
DISTRIBUTION
An example of a
population that shows
clumped distribution
is the striped catfish.
These fish organize
into tight groups. The
dots in the illustration
represent individual
members of a
population with
clumped distribution.
distribution: how individuals in a population are spaced out across the range
of the population
Lesson Overview
How Populations Grow
Growth Rate
• How populations change over time
• determines whether the population size increases,
decreases, or stays the same
EXAMPLE: Hydrilla populations
• in their native habitats tend to stay more or less the
same size over time
• have a growth rate of around zero; (neither increase nor
decrease in size)
• in Florida, by contrast, has a high growth rate—which
means that it increases in size
EXAMPLE: Cod
• decrease in size
• has a negative growth rate
Lesson Overview
How Populations Grow
Age Structure
• age structure—the number of males and females of each age a
population contains
• Most plants and animals cannot reproduce until they reach a certain
age
• among animals, only females can produce offspring
Lesson Overview
How Populations Grow
Population Growth
Factors Affecting Population Growth
• increase or decrease in size
depending on how many individuals
are added to or removed from it
• ADDED: birthrate, immigration
• REMOVED: death rate, emigration
Lesson Overview
How Populations Grow
Population Trends:
Birthrate and Death Rate
• A population can grow when its
birthrate is higher than its death
rate.
• If the birthrate equals the death rate,
the population may stay the same
size.
• If the death rate is greater than the
birthrate, the population is likely to
shrink.
Lesson Overview
How Populations Grow
Immigration and Emigration
• immigration: population
may grow if individuals
move into its range from
elsewhere
• emigration: population may
decrease in size if
individuals move out of the
population’s range
Lesson Overview
How Populations Grow
Exponential Growth: What Happens?
What does a population need to grow?
• food and space it
• protection from predators and disease
• remove its waste products
Why does the population grow?
• members of the population will be able to produce offspring
• those offspring will produce their own offspring
What is exponential growth?
• under ideal conditions with unlimited resources, a population
will grow exponentially
• the larger a population gets, the faster it grows
• size of each generation of offspring will be larger than the
generation before it
Lesson Overview
How Populations Grow
Exponential Growth:
Organisms That Reproduce Rapidly
•
•
•
•
plot the size of this population
J-shaped curve
rises slowly at first, and then rises faster and faster
if no limits leads to an infinitely large size population
• a single bacterium divides to
produce two cells every 20 minutes.
• after 20 minutes, under ideal
conditions, the bacterium divides to
produce two bacteria. After another
20 minutes, those two bacteria
divide to produce four cells. After
three 20-minute periods, we have
2×2×2, or 8 cells.
Lesson Overview
How Populations Grow
Organisms That Reproduce Slowly
NOT ALL ORGANISMS REPRODUCE FAST
EX: a female elephant
• produce a single offspring only every 2 to 4 years
• newborn elephants take about 10 years to mature
After 750 years there would be nearly 20 million elephants!
Lesson Overview
How Populations Grow
NON-NATIVE SPECIES/INVASIVE SPECIES:
Organisms in New Environments
• an organism is moved to a new
environment, its population grows
exponentially for a time
EXAMPLE”: European gypsy moths
(plant-eating pest)
• accidentally released from a
laboratory near Boston
• spread across the northeastern
United States within a few years.
• Ate the leaves of thousands of acres
of forest
• formed a living blanket that covered
the ground, sidewalks, and cars.
Lesson Overview
How Populations Grow
Logistic Growth
• occurs when a population’s growth slows and then stops
• follows a period of exponential growth
Natural populations don’t grow exponentially for long.
Sooner or later, something stops exponential growth. What
happens?
Lesson Overview
How Populations Grow
Phases of Growth
This graph traces the phases of growth that
the population goes through.
Lesson Overview
How Populations Grow
Phase 1: Exponential Growth
•
•
•
•
•
resources are unlimited
individuals grow and reproduce rapidly
few individuals die
many offspring are produced
both the population size and the rate of growth
increase rapidly
Lesson Overview
How Populations Grow
Phase 2: Growth Slows Down.
• rate of population growth begins to slow down
• limiting factors (space, food, competition) don’t
allow population to continue to grow
• population still grows, but the rate of growth
slows down
• population size increases more slowly.
Lesson Overview
How Populations Grow
Phase 3: Growth Stops
• rate of population growth drops to zero
• size of the population levels off
• under some conditions, the population will
remain at or near this size indefinitely
Lesson Overview
How Populations Grow
The Logistic Growth Curve
• S-shape that represents what is called logistic
growth
• occurs when a population’s growth slows and
then stops, following a period of exponential
growth
Lesson Overview
How Populations Grow
Carrying Capacity
When the birthrate and the death rate are the same, and
when immigration equals emigration, population growth
stops.
• dotted, horizontal line through the region of this graph
where population growth levels off
Lesson Overview
How Populations Grow
Carrying Capacity
• maximum number of individuals of a particular
species that a particular environment can support
• once a population reaches the carrying capacity
of its environment, a variety of factors (limiting
factors) act to stabilize it at that size
Lesson Overview
How Populations Grow
Factors That Affect Population Growth
Density-Dependent
Factors
Density-Independent
Factors
Competition
Natural Disasters
Predation
Environmental Changes
Parasitism/Diseases
Human Activities (Pollution,
Pesticides and Competition,
Removal of Predators,
Introduction of New
Species)
Food Supply
26
Lesson Overview
How Populations Grow
Density-Dependent Factors
Predator-Prey
Competitive
Exclusion
27
Lesson Overview
How Populations Grow
Predator-Prey Relationships
Lesson Overview
How Populations Grow
Food Webs
• A network of food
chains
• Graphic
representation of
relationships between
species
Lesson Overview
How Populations Grow
Self Check
What determines how large a population can grow?
Limiting factors – Living space, competition,
predation, diseases.
How does a density-dependent limiting factor affect
carrying capacity?
Space availability, food availability
How do density-independent limiting factors affect how
a population grows?
Natural disasters
30
Lesson Overview
How Populations Grow
HOT Question!
In the survivorship curve below, the probability of an individual
in this population surviving to 8 years old is about:
a. 100%
b. 80%
c. 60%
d. 50%
e. 20%
31
Lesson Overview
How Populations Grow
HOT Question!
2. As the population gets larger,
density-dependent factors ________ and density-independent
factors _____.
a. are unaffected; intensify
b. intensify; diminish
c. intensify; are unaffected
d. diminish; are unaffected
e. are unaffected; diminish
32
Lesson Overview
How Populations Grow
HOT Question!
3. What is the value of the carrying
capacity, for the population described
by the growth curve below?
a. 5,000
b. 4,000
c. 3,000
d. 2,000
e. 1,000
33
Lesson Overview
How Populations Grow
HOT Question!
4. Which of the following interactions are best
classified as predator-prey relationships?
I. A lion eats an antelope.
II. A virus enters a human cell.
III. A flea bites a dog.
IV. A dog gets tapeworms from the flea bite.
V. An antelope eats grass.
a. I , II and IV
b. II, III, and IV
c. I and V
d. I, IV, and V
e. II and III
34
Lesson Overview
How Populations Grow
HOT Question!
5. What is the maximum number of
individuals of a given species that a
particular environment can support for a
prolonged period of time?
A.
B.
C.
D.
population density
maximum growth rate
carrying capacity
biotic potential
35
Lesson Overview
How Populations Grow
HOT Question!
6. Which of the following is a density-dependent
factor that regulates population size?
A. flood
C. freezing
B. fire
D. disease
36