Stem Cells, Cancer, and Human Health

Growth of Populations
How Many Organisms Live in a
Particular Environment, and Why?
• Population ecology is the study of the number
of organisms in a particular place
• Population ecology is essential for solving realworld problems, such as protecting
endangered species or controlling pest species
What Is a Population?
• A population is a group of
interacting individuals of a
single species located within
a particular area
• Population size: total number
of individuals in the
• Population density: the
number of individuals per
unit of area.
Ex: 200 humans/square mile
There are three populations in
this picture
Changes in Population Size
• Whether a population increases or decreases
in size depends on the birth and death rates
and immigration and emigration rates
• Birth and death rates and immigration and
emigration rates are all affected by
environmental factors
Calculating Changes in Population Size
ΔP = [N+I] – [M+E]
I = immigration
E = emigration
Density= # or organisms/ unit of area
Exponential Growth:
J-Shaped Growth
• Endless exponential
growth of a population is
limited by the availability
of resources and is not
seen under natural
• Doubling time: The time it
takes a population to
double in size
The No. 1 rabbit-fence in Western Australia (1926)
Rabbits around a waterhole in the myxomatosis
trial site on Wardang Island in 1938
"The introduction of a few rabbits could do
little harm and might provide a touch of
home, in addition to a spot of hunting.” Thomas Austin
Erosion of a gully in South Australia caused by
Population Growth Has its Limits
• Growth is limited
by essential
resources and
• Populations cannot
increase in size
Limiting Factors
• A Limiting Factor is any living or non-living
factor that restricts the existence, numbers,
reproduction, or distribution of organisms.
• Factors that limit one population will affect
other populations (Remember all living things
in a committee are part of a whole).
• Water, space, food, temperature, soil, etc.
Population Growth Has its Limits
• Space and nutrients are
important limiting factors
in the population growth of
an organism
• Habitat is the type of
environment in which an
organism lives and is an
environmental factor that
limits population growth
Abiotic Factors
Abiotic Factors:
Physical and
Characteristics of
the environment
Ex: Solar Energy,
Oxygen, CO2,
Humidity, pH,
Biotic Factors
Biotic Factors: living
aspects of the
environment. Plants,
animals, etc.
Logistic Population Growth:
S-Shaped Growth Pattern
• Logistic growth:
– What we see in nature most of
the time
– Represented by an S-shaped
– Consideration that growth rates
change as resources become
• Carrying capacity:
– The maximum population size
that can be sustained in an
– At the carrying capacity, the
population growth rate is zero
Density-Dependent Factors
Birth rates and
death rates are
because they
change as the
density of the
population changes
Factors include
competition, and
Biotic factors
Density-Independent Factors
• Density-Independent
Factors affect all
populations regardless
of their density
• Abiotic factors
• Temperature, storms,
flood, drought, or
other natural habitat
Population Cycles
• In a population cycle, the
population sizes of two or
more species change
together because at least
one of the two is
influenced by the other
Predator Prey
encourage the health
of an ecosystem:
• Usually, in prey populations, the weak,
young, old, or injured members are caught
• Keeps population size within the limits of
available resources
• Improves the genetic make up of the
surviving members of that population
Patterns of Population Growth:
Irregular Fluctuations
• Populations of
the same
species may
patterns of
depending on
the environment
Patterns of Population Growth:
Irregular Fluctuations
• In1800 estimated
100,000 breeding
pairs present
• By the early 1960s,
population counts
indicated that only
417 breeding pairs
of eagles remained
in the lower 48
Human Population
• Which will we experience?
The jury is still out.
• How are we different that the
rest of the animal Kingdom