Transcript Populations

Populations
Populations

A population is all the individuals of a species
that live in the same area.
Characteristics of Populations

Four characteristics:




Geographic distribution
Density
Growth rate
Age structure
Geographic Distribution

The area inhabited by
a population.

The range can vary in
size from a few cubic
centimeters to millions
of square kilometers.
Population Density

The number of
individuals per unit
area.

The number can vary
depending on species
and ecosystem.

Ex. Saguaro cactus in
the desert plant
community has a low
density. Sage brush
has a high density.
Population Dispersion


Population dispersion is the way in
which individuals of a population
are spread in an area or volume.
Clumped dispersion


Uniform dispersion


Individuals may live close together
in groups in order to facilitate
mating, gain protection, or access
food resources.
Territoriality and intraspecies
competition for limited resources
lead to individuals living at specific
distances from one another.
Random dispersion

Individuals are spread randomly
within an area or a volume.
Survivorship Curves

A survivorship curve is a
generalized diagram
showing the number of
surviving members over time
from a measured set of
births.

By measuring the number of
offspring born in a year and
following those offspring
through until death, survivorship
curves give information about
the life history of a species.

There are three basic patterns of
animal survivorship curves.
Survivorship Curve

Type I



This represents a life history
that is common among large
mammals.
It shows a low infant mortality
and a population that will
generally survive until old age.
A behavior shared by
organisms showing Type I
survivorship curves is parental
care for the young.
Survivorship Curve

Type II


Organisms such as
birds, small mammals,
and some reptiles show
a survivorship rate that
is roughly equal at all
ages of an organism’s
life.
At all times, these
species have an equal
chance of living and
dying, whether as a
result of predation or
from disease.
Survivorship Curve

Type III



These organisms have a very
high birth rate and also a
very high infant mortality rate.
These species are generally
invertebrates, fish,
amphibians, and plants.
Many of their offspring will
die from predation, but
inevitably, a few will survive
to adulthood and pass on
their genes to the next
generation.
Population Growth


Natural populations
stay the same size
from year to year.
Three factors affect
population size:



Number of births
Number of deaths
Number of individuals
that enter or leave the
population.
Population Growth

A population can grow when its birthrate is
greater than its death rate.
Population Growth

If the birthrate equals the death rate, the
population stays the same size.
Population Growth

If the death rate is greater than the birthrate,
the population shrinks.
Population Growth

Immigration



The movement of
individuals into an area.
Causes population growth.
Emigration


The movement of
individuals out of an area.
Causes population decline.
Exponential Growth

If a population has abundant space and food, and is
protected from predators and disease, then
organisms in that population will multiply and the
population size will increase.
Exponential Growth

Exponential Growth



Occurs when the
individuals in a population
reproduce at a constant
rate.
It will grow slowly at first,
then faster and faster.
Under ideal conditions
with unlimited resources,
a population will grow
exponentially.
Exponential Growth


The current population of
African elephants is 470,000
to 690,000 individuals (down
from 1.3 million)
 An elephant can produce
offspring only every 2 to 4
years (they’re pregnant for
22 months!)
 The baby’s take 10 years to
mature and cannot
reproduce until ~13 years of
age.
If all the offspring of a single
pair of elephants survived and
reproduced for 750 years
there would be nearly 20
million elephants!
Logistic Growth


As resources become
less available, the
growth of a population
slows or stops.
Logistic growth

Occurs when a
population’s growth stops
or slows down after a
period of exponential
growth.
Logistic Growth

Will occur when:




Death increases
Emigration increases
Birthrate decreases
Immigration decreases
Logistic Growth

Carrying Capacity (K)


The environment can only support so many individuals.
The number that represents the largest amount of
individuals an environment can support at any one time is
the carrying capacity.
Limits to Growth
Limiting Factors

Limiting Factor

Any factor that
causes population
growth to decrease





Competition
Predation
Parasitism and disease
Drought and other
climate extremes
Human disturbances
Density-Dependent Factors

Density-Dependent Factor

A limiting factor that depends on
population size.




Factors become limiting only
when a population has reached a
certain size.
Factors operate most strongly
when a population is large and
dense.
They do not affect small,
scattered populations as greatly.
Competition, predation,
parasitism, and disease are all
density-dependent factors.
Density-Dependent Factors

Competition



When populations become
crowded, organisms
compete with each other
for food, water, space,
sunlight, and other
essentials.
Can occur between
members of the same
species
Can occur between
members of different
species

Is a major source of
evolutionary change
Density-Dependent Factors
Density-Dependent Factors

Predation


Predator-prey relationships are
the best known mechanisms of
population control.
Ex. Wolves and Moose on Lake
Superior


When the moose population
increases, the wolf population
increases shortly afterward.
As the wolf population
increases, the moose
population decreases (due to
predation), followed shortly by a
decrease in wolf population.
Density-Dependent Factors

Parasitism


Parasitic organisms can range
in size from microscopic to 30
centimeters or more in length
(tapeworm)
Parasites are similar to
predators

They take nourishment at the
expense of their hosts,
weakening them and causing
disease or death as a result.
Density-Dependent Factors

Disease

The closer packed a
population is, the
faster disease will
spread.

The Black Plague killed
about 75 million people
in 1340.
Density-Independent Factors

Density-Independent
Factors

Affect all populations in
similar ways,
regardless of the
population size.

Unusual weather,
natural disasters,
seasonal cycles, certain
human activities
(damming rivers, clearcutting forests).
Density-Independent Factors

Regardless if the
population has
5,000,000 individuals
or 5,000, a
hurricane, fire, or
drought can kill them
all off.