Unit 3: Populations
Chapter 5
Populations
Sea otters  Sea urchins  kelp
• Long ago, hunters hunted the sea otters
almost to extinction
• This made the sea urchin population grow
• Which in turn, made the kelp population do
what? __________________
Populations
• After sea otters were put on the endangered
species list, hunters were not allowed to hunt
them anymore
• With hunters out of the picture, sea otter
populations recovered
• Which greatly decreased the sea urchins
• Which greatly ___________ the kelp
Characteristics of Populations
• Three important characteristics of population
are:
1. Its geographic distribution
2. Its density
3. And its growth rate
Geographic Distribution
• Geographic Distribution, or range, is a term
that describes the area inhabited by a
population
Population Density
• Population Density is the number of
individuals per unit area
Growth Rate
• Growth rate is the number of births minus the
number of deaths in a given area, also taking
into affect immigration and emigration.
Factors that affect Growth Rate
• Three factors can affect population size
1. The number of births
2. The number of deaths
3. The number of individuals that enter (immigrate
into) or leave (emigrate out of) the population
Populations
• Generally, populations grow if more
individuals are born than die in any period of
time
Birthrate > Death Rate
Populations
Birthrate > Death Rate
**population growth**
Birthrate = Death Rate
**population remains the same**
Birthrate < Death Rate
**population shrinks**
Moving in and out
• Immigration – the movement of individuals
into an area
– can make a population increase
• Emigration – the movement of individuals out
of an area
– can make the population decrease
Exponential Growth
• If a population has abundant space and food,
and is protected from predators and disease,
then the organisms in that population will
multiply and the populating size will increase.
• Exponential growth – occurs when the
individuals in a population reproduce at a
constant rate (i.e. bacteria, humans)
Exponential Growth
• Under IDEAL CONDITIONS with UNLIMITED
RESOURCES, a population with grow
exponentially
• Will show a “J-Curve” graph
Logistic Growth
• Obviously, bacteria, elephants, and humans
cannot increase in population until we cover
the planet
• We reach a carrying capacity, or a maximum
quantity that the planet (or on smaller terms,
the ecosystem) can handle
Logistic Growth
• As resources become less available, the
growth of a population slows or stops
– Run out of space
– Run out of food
– Not enough clean water
• Logistic Growth occurs when a population’s
growth slows or stops following a period of
exponential growth.
What is CARRYING CAPACITY???
Carrying Capacity
• Carrying
Capacity- The
largest
number of
individuals of
a population
that a given
environment
can support
Checkpoint 
• What factors can change a population’s size?
• What is the difference between exponential
growth and logistic growth?
Do Fruit Flies and Rabbits Have Similar
Population Growths?
Fruit Fly Population
Growth
Days
Number of
Fruit Flies
5
10
10
50
15
100
20
200
25
300
30
310
35
320
40
320
• Graph the data
provided (make 2
graphs)
QUESTIONS
1. What type of
growth pattern is
the fruit flies?
2. Is it the same as
the rabbit’s
growth pattern?
Explain.
Rabbit Population
Growth
Generations
Number of
Rabbits
1
100
2
105
25
1,000
37
1,600
55
2,400
72
3,350
86
8,000
100
13,150
Fruit Flies vs. Rabbits
3. Does either graph indicate a carrying capacity?
4. If so, when does the population reach this
carrying capacity?
5. What is the maximum number of individuals
that can be supported at that times?
6. Predict: Animals such as foxes and cats often
prey on rabbits. Based on the growth curve of the
rabbit population, what might happen if a group of
predators move into the rabbits’ habitat during the
tenth generation and begin eating the rabbits?
PDN Review
1. List three characteristics that are used to
describe a population
2. What factors can change a population’s size?
3. What is the difference between exponential
growth and logistic growth?
4. What is meant by population density?
5. Define carrying capacity.
Limits to Growth
• Back to the sea otters,
– When their population declines, something has
changed with their birth rates or death rates
– Or between that rates of immigration and
emigration
• What caused the otter population to decrease
to greatly??
Factors that impact populations
• The hunters are considered a limiting factor
• A Limiting Factor is a factor that causes population
growth to decrease
– A limiting factor that is caused by or made worse by high
population (density) is known as a density-dependent
factor
– A limiting factor that is NOT caused by a high population
is known as a density-independent factor
Density-Dependent Factors
• A limiting factor that depends on population
size is called a density-dependent limiting
factor
• These factors become limited only when the
population density reaches a certain level
• Some examples: (YOU MUST KNOW THESE!!)
competition, predation, diseases, and
parasitism
Competition
• When populations become crowded,
organisms compete with one another for food,
water, space, sunlight, and other essentials
• Competition WITHIN the population
• Competition among members of the SAME
species is a density-dependent limiting
factor!!
Completion with others
• Competition can also occur between members of
different species
• This type of competition is a major force behind
evolutionary change
• When two species compete for the same
resources, both species are under pressure to
change in ways to win and thus decrease their
competition
Predation
• Populations in nature are often controlled by
predation
• A predator-prey relationship is one of the
best mechanisms for population control
• Ex: the sea otters controlled the sea urchin
population… what controlled the kelp?
Parasitism and Disease
• Parasites can also limit the growth of a
population
• Parasites can be microscopic bacteria to larger
organisms like tapeworms and leaches
• These organisms obtain nutrients from the
host similar to predation, but do not kill the
PREY… why not??
Density-Independent Factors
• Density-Independent Factors affect all
populations in similar ways BUT DO NOT
depend on the population (meaning they will
affect small groups and large)
• Unusual weather, natural disasters, seasonal
cycles, and certain human activities
– What kind of human activities do you think??
Human Activities
• Some human activities that affect populations
are:
– Damming of rivers
– Clear cutting forests
– Local pollutions
– Habitat destruction
• Environments are always changing, and most
populations can afapt to a certain amount of
change
• Populations often grow and shrink in response
to these changes
Checkpoint !!
1. List three density-dependent factors and
three density-independent factors that can
limit the growth of a population
2. What is the relationship between
competition and population size?
3. If an entire fox population disappears, what is
likely to happen to the hare population??
How about the grass in the rabbits’
ecosystem?
Human Population Growth
• How quickly is the world’s human population
growing?
• Like the populations of many other living
organisms, the size of the human population
tends to increase with time
Human Population
• About 500 years ago, the human population
began growing much more rapidly.
• WHY???
A Growing Population
• Agriculture and industry made life easier and
safer
• Food supplies became more reliable
• Essential goods were able to be shipped
around the globe
• Improved sanitation
• Improved health care
Demography
• The scientific study of human populations is
called demography
• Birthrates, death rates, and the age structure
of a population help predict why some
countries have high growth rates while other
countries grow more slowly
Age Structure
• Population growth depends on how many
people of different ages make up a given
population
• Demographers can predict future growth
using models called age-structure diagrams
Age-Structure Diagrams
• Equal numbers of people in each group –
predicts a slow but steady growth rate in the
near future
• Larger number of children than adults (fat
bottom) predicts a sharp population increase
• Fat top, or a larger number of adults than
children, predict a decrease in population
Future Population Growth
• To predict the overal human population
growth (the whole world) demographers need
to consider the following:
– Age structure of each country
– Prevalence of life threatening diseases
• AIDS, malaria, cholera
Human Growth Rate
• It is predicted that the human growth rate will
level off or even decrease by 2050
• A lower growth rate means that the human
population will be growing more slowly over
the next 50 years
• Because this growth rate is still above zero,
the human population will still rise
Review
1. What is the difference between immigration
and emigration?
2. How can a predator-prey relationship serve
as a population control?
3. Sketch the exponential growth curve of a
hypothetical population
• How is this different from a logistic?
4. How does age structure of a population
affect its growth rate?