Changing
Populations
C21L2
Essential Question
How do populations change?
How Populations Change
A
population change can be
measured by the population’s
birthrate and death rate.
 A population’s birthrate is the number
of offspring produced over a given
period of time.
 The death rate is the number of
individuals that die over the same
period of time.
How Populations Change (cont.)
If
the birthrate is higher than the
death rate, the population increases.
If the death rate is higher than the
birthrate, the population decreases.
When a population is in ideal
conditions with unlimited resources, it
grows in a pattern called exponential
growth.
How Populations Change (cont.)
During
exponential growth, the larger
a population gets, the faster it grows.
E
coli bacteria are microscopic
organisms that undergo exponential
growth; the population doubles in size
every half hour.
How Populations Change (cont.)
Disease
and natural disasters such as
floods, fires, or volcanic eruptions can
decrease populations.
Predation—the hunting of organisms
for food—also reduces population
size.
How Populations Change (cont.)
If
populations continue to decrease
in numbers, they disappear.
An extinct species is a species that
has died out and no individuals are
left.
Extinctions can be caused by
predation, natural disasters, or
damage to the environment.
How Populations Change (cont.)
How Populations Change (cont.)
An
endangered species is a species
whose population is at risk of
extinction.
A threatened species
is a species at risk, but
not yet endangered.
How Populations Change (cont.)
How Populations Change (cont.)
Populations
also change when
organisms move from place to place.
Migration is the instinctive seasonal
movement of a population of
organisms from one place to another.
Ducks, geese, and monarch
butterflies are examples of organisms
that migrate annually.
How Populations Change (cont.)
How Populations Change (cont.)
Human Populations Change
 Human
population, like all other
populations, are affected by birthrate,
death rate, and movement.
 Unlike
other species, humans have
developed ways to increase the carrying
capacity of their environment.
 Scientists
estimate that there were about
300 million humans on Earth a thousand
years ago.
Human Populations Change
(cont.)
 Today
there are more than 6 billion
humans on earth.
 As human population grows, people need
to build more houses and roads and clear
more land for crops, which means there is
less living space, food, and other resources
for other species.
 Human energy use contributes to pollution
that affects other populations.
Human Populations Change
(cont.)
Human Populations Change
(cont.)
 Factors
that keep the human birthrate
higher than its death rate include food,
resources, sanitation, and medical care.
 Advances
in agriculture have made it
possible to produce food for billions of
people.
 Today
people have access to more
resources because of better transportation
methods.
Human Populations Change
(cont.)
Like other organisms, populations of
humans might
move when
more resources
become available
in a different place.