Three important characteristics of a
1) Geographic distribution: A term used
to describe the area inhabited by
2) Population density: The number of
individuals per unit area.
3) Growth rate: How fast a population is
growing or shrinking.
• Populations can grow or shrink overtime.
• Factors that affect the growth rate are
1) Birth rate
2) Death rate
• If a population has ideal conditions it will
experience exponential growth.
1) Occurs when individuals reproduce
at a constant rate and forms a
2) Can not be sustained
• As resources become limited growth in a
population slows down.
• Logistic growth: Occurs when a
population’s growth rate slows or stops
after a period of exponential growth.
Forms a S-shaped curve.
• Carrying capacity: Largest number of
individuals in a population that a given
environment can support.
Limits to Population Growth
• Limiting Factor: A factor that causes
population growth to decrease or to stop.
1) Density Dependent Limiting Factor
2) Density Independent Limiting Factor
Density Dependent Limiting factor
• -Density-dependent limiting factors: A limiting factor that
depends on the size of a population.
1) Competition: When a population gets to big
individuals compete with each other for
limited resources. Can also occur between
2) Predation: As the size of predator and prey
populations increase and decrease they have
an effect on the other.
3) Parasitism and disease: Parasites and disease
are easier spread in very densely populated
Density Independent Limiting
• Density-independent limiting factors: Affects all
populations in similar ways regardless of
1) Unusual weather, natural disasters,
seasonal cycles and human activities
are all examples of density
independent limiting factors.
2) Populations tend to crash in response to
theses factors and then build back up
Human Population Growth
• Like the populations of many other living
organisms the size of the human
populations tends to increase with time.
1) In most developed countries the
growth rate is very low (<1%)
2) In developing countries the growth
rates tend to be higher (2-3%)
or 3 births/second.
• For most of human history the population has
grown very slowly to very harsh living conditions.
1) Because of this there was a very high
birthrate and death rate.
2) There was also a very high infant mortality
rate and short life span.
• About 500 years ago human populations started
to grow much faster due to
1) Agriculture and industry
2) Improved sanitary conditions
3) Medical technology
Human Population Growth
• The human population can not undergo
exponential growth forever due to
1) War, famine and disease can
all slow and stop human pop.
• Demography: The scientific study of human population
growth and the social and economic factors that
1) Birth rates, death rates, and age structures are
all factors that affect human population
Demographic Transition: A dramatic transition in the
birth and death rates of a country that tends
to slow population growth.
Age-growth structure: A diagram that depicts the
relative number of males and females and
their ages in a population..
1) These can be used to determine future
Figure 5-13 Age Distribution
U.S. Population Rwandan Population
Males Females Males Females