Warm Up
“What kind of relationships do
organisms have?”
 “What factors might change a
population size?”

Question: What does it mean when
a population increases or decreases
in size with respect to birth rates
and death rates?
Ch. 5 Objectives – Sections 1 and 2
 List the characteristics used to describe a
population.
 Identify factors that affect population size.
 Differentiate between exponential and
logistic growth.
 Identify factors that limit population
growth.
 Differentiate between density-dependent
and density-independent limiting factors.
How Populations Grow
What is a population?
 A group of organisms of the same
species living in the same area
Characteristics of a Population
 4 ways to describe a population:
1. Geographic distribution: where the pop. lives
2. Population density: # of individuals / unit
area
o
Ex. 150 frogs live in a pond that is 3 square
kilometers. What is the population density?
3. Population growth rate
4. Age structure
Population Growth
 Is a measured a rate of how fast (or
slow) a population is growing. It is the
size of a population over time.
 A negative value for population growth
rate means the population is
decreasing.
Population Growth
3 factors that affect pop. size:
# births
# deaths
# individuals that enter or leave
Growth = (Births + immigrants) –
(Death + emmigrants)
Population Growth
Immigration: individuals move
into an area
Emigration: individuals move out
of an area
Exponential Growth
 “J-curve”
 Occurs when indiv. in
population reproduce at
constant rate
 Needs ideal conditions and
unlimited resources
 From graphs – what would
happen if populations grew
uncontrollably??
Logistic growth
 “S-curve”
 Population’s growth
slows/stops following
exponential growth
 Occurs when resources
are less available
 Population reaches
carrying capacity: largest
# of indiv. an environment
can support
Limits to Growth
Limiting Factors
 Causes pop. growth to decrease
Density-Dependent Factors
 Limiting factor that depends on
population size
 Usually strong when pop. is large and
dense
Density-Dependent Factors
 Include:
 Competition
 Predation
Predator-prey relationship: regulates
population, best-known mechanism of
pop. control
 Parasitism
 Disease

Predator-prey relationship
Density-Independent Factors
 Affect all pops. the same, regardless of
pop. size
 Usually results in crash in pop. size,
followed by building back up
Density-Independent Factors
 Include:
 Unusual weather
 Natural disasters
 Seasonal cycles
 Certain human activities
Exit Slip

Give at least 2 factors that can be
classified as density dependent and
2 density independent factors.
Explain how density dependent
factors affect the carrying capacity
of an environment.
Human Population Growth
Objectives
Describe how the size of human
population has changed over
time.
Explain why population growth
rates differ from country to
country.
Human Population Growth
 The human population is still showing
exponential growth
http://www.census.gov/main/www/popclock.html
 Population as of 10/26/09
 U.S. 307,790,831 - World 6,793,050,389
 Population as of 9/28/10 at 11:04am
 U.S. 310,359,253 - World 6,871,700,813
Patterns of Population Growth
Demography: scientific study of
human populations
Birth/death rates and age structure
of pop. help predict why countries
grow at different rates
Demographic transition
 A dramatic change in birth/death rates
 Change from high birth rate and high
death rate to low birth rate and low
death rate
 So far, countries that have completed
this transition include U.S., Japan,
much of Europe
Age Structure
 Shows how many people of different
ages make up population
 Constructing an age pyramid can give
information about the growth rate.
Age-structure pyramids for the human population of Kenya (growing at 2.1% per year),
the United States (growing at 0.6% per year), and Italy (zero growth) for 1995
What’s going to happen
to human population
growth????
Estimates for
Earth’s carrying
capacity range
from 2 billion to
40 billion.