Populations
Populations
• Populations: groups of individuals that belong to the
same species and live in the same area
Characteristics Of A Population
• There are three important characteristics of a
population:
1. Geographic Distribution, 2. Density, and 3. Growth
- Geographic distribution, or range, is the area in
which a population lives
- Population density is the number of individuals per
unit area. An example is the number of people per
square kilometer.
- Growth rate is how quickly a population increases or
decreases in size.
Geographic Distribution Of
Termites
Population Density
Population Density
• Dispersal patterns
– Random
– Clumped
– Uniform
Random
Clumped
Uniform
Patterns
• Reproductive pattern = life-history pattern
– Two main types of patterns:
• Rapid life-history patterns
• Slow life-history patterns
Patterns
• Rapid life-history
patterns
– Changing or
unpredictable
environment
– Small species
– Mature rapidly
– Reproduce early
– Short life span
Patterns
• Slow life-history
pattern
–
–
–
–
–
–
Large species
Stable environments
Reproduce slowly
Matures slowly
Long life span
Stay at or near carrying
capacity
Factors That Affect
Population Size
• Three factors can affect population size: 1. the # of
births, 2. the # of deaths, and 3. the # of individuals that
enter and leave the population.
• Populations increase through births and immigration.
Populations get smaller through deaths and emigration.
– Immigration is the movement of individuals into an
area.
– Emigration is the movement of individuals out of an
area
How Populations Grow
• Exponential growth occurs when members of a
population reproduce at a constant rate. (This growth
pattern is shown by a J-shaped curve.)
– As the population grows, the number of reproducing
members keeps rising. Thus the population grows
faster and faster.
– Under ideal conditions with unlimited resources, a
population will grow exponentially.
– In nature, exponential growth does not go on for
long.
• Resources are used up in time.
• This causes population growth to slow or stop.
• Predators and disease also slow exponential
growth.
Exponential Growth Graph
How Populations Grow
• As resources become less available, the growth of a
population slows or stops. This is called logistic
growth. (Logistic growth is shown by an S-shaped
curve.)
– Logistic growth occurs when a population's growth
slows or stops following a period of exponential
growth. Example, if food resources start to run out or
space becomes limited, a population that was
growing exponentially may start to exhibit logistic
growth.
– The population size when growth stops is called
carrying capacity.
Carrying capacity is the number of individuals of a
specific species that a given environment can support.
Limits To Growth
• A limiting factor is anything that slows population
growth. There are two kinds of limiting factors.
– Density-dependent limiting factors
– Density-independent limiting factors
Limiting Factors
• Density-dependent limiting factors rely on population
size. They include competition, predation, parasitism,
and disease.
– Competition occurs when organisms are using the
same ecological resources at the same time.
– Predation occurs when one organism captures and
feeds on another organism. The organism that gets
eaten is the prey. The organism that eats the prey is
the predator.
• Predator-prey relationships can affect the size of
both the predator population and the prey
population.
Limiting Factors
– Parasitism is a symbiotic relationship in which one
organism benefits at the expense of another species
– Disease is an abnormal condition affecting the body
of an organism.
• Diseases can come from bacteria, viruses, and
genetics
– Examples: cancer, heart disease, AIDS
Limits To Growth
• Limiting factors continued
– Density-independent limiting factors do not rely on
population size.
• They include natural disasters and human
activities such as damming rivers.
• When such factors occur, many species show a
rapid drop in population size.
Human Population
Human Population Growth
• The size of the human population tends to increase with
time.
• For most of human existence, the population grew
slowly.
• About 500 years ago the population began growing
faster.
– Agriculture and later industry made food more
available.
– Improved sanitation and medicine reduced death
rates.
– However, birthrates stayed high in most places. This
led to exponential growth. Today, the human
population continues to grow exponentially.
Human Population Growth
Industrial
Revolution
begins
Agriculture
begins
Plowing
and
irrigation
Bubonic
plague
Human Population Growth
Human Population Explosions
The Population Explosion
– Around 1650, improvements in hygiene, diet, and
economic conditions further accelerated population
growth.
– After World War II, the human population grew at the
fastest rate in history, largely because of better
sanitation and medical care in poorer countries.
Human Population Growth Due To
Medicine Practices And Sanitation
Advances in Human Technology =
Growth
Human Population Growth Vs Species
Extinction
Study Of Population Growth
• Demography is the scientific study of human
populations.
• Demographers try to predict how human populations
will change over time.
• 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.
Study Of Human Population Growth
• Over the past century, population growth in the U.S.,
Japan, and much of Europe slowed greatly.
• Demographers call this shift a demographic transition.
– The transition began as death rates fell, causing a
brief rise in population growth. Then birthrates fell,
slowing population growth.
– Most people live in countries that still have high
population growth.
Human Population Growth
Human Population Growth
Study Of Human Population Growth
• Demographers use age-structure diagrams to help
predict population growth. An age-structure diagram is
a bar graph.
• An age-structure diagram shows how many people of
each gender are in each age group in the population.
• To predict how the world population will change,
demographers need to think about the age structure of
each country as well as the number of people with fatal
diseases, including AIDS.
Age Distribution
U.S. Population
Males
Females
Rwandan Population
Males
Females
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