Transcript Understanding & Measuring Population Notes

Understanding Populations
The Human Population
•
From 1900 to 2003, the
population tripled in size to
reach 6.3 billion people
•
Today, the human
population count is over 7
billion and we are projected
to reach 9 billion within the
next 50 years
•
While our population has
increased, others have
dramatically decreased
Population of…
China: 1.3 billion
India: 1.2 billion
Human population growth
What factors have
contributed to this
exponential growth
pattern?
adding 75 million/year
20137 billion
Significant advances
in medicine through
science and technology
Industrial Revolution
Bubonic plague "Black Death"
1650500 million
Properties of Populations
For example, all of the bass
in a pond are considered a
population, because they are
isolated in the pond and
don’t interact with bass
living in other ponds
• Populations are a
group of organisms
that belong to the
same species and live
in a particular place at
the same time
• They can be widely
distributed , or
confined to a small
area
Properties of Populations
•
Boundaries can be
imposed by environmental
features, or they can be
arbitrarily chosen to
simplify a study of the
population
•
Population studies focus
on a population as a whole
– how many individuals
are born, how many die,
and so on
Properties of Populations
• Three main characteristics of populations:
• 1) Population Size
• 2) Population Density
• 3) Dispersion
Population Size
• The number of
individuals that the
population contains
How many blades of
grass are in this football
field?
• Size is a fundamental
population property
and can be difficult to
measure directly, so
sometimes we must
estimate
Population Size
• How many oak trees
are in this forest that
is 10 km2?
•
•
Extrapolate
Count how many are
in 1 km2 and
multiply by 10.
Measuring population size
 How do we measure how many
individuals in a population?
number of individuals in an area
 mark & recapture methods

Difficult to count a moving target
AP Biology
sampling populations
Population Density
• A measure of how
crowded a population
is
• Expressed as the
number of individuals
per unit of area or
volume
Dispersion
• From Latin dis- meaning out and spargeremeaning to scatter
• The spatial distribution of individuals
within the population
Types of Dispersion
• Clumped –
individuals clustered
together
• Uniform – separated
by a fairly consistent
distance
• Random – each
individual’s location
is independent of the
locations of other’s
Clumped Distributions
• Occur when resources
such as food or living
space are limited
• Occur because of
species social
behavior (flocks)
Uniform Distributions
• Result when
individuals within the
same habitat stay as
far away from each
other as possible
• When a bird locates
its nest so it’s not
close to other birds
nests.
Random Dispersion
• Few populations are
truly randomly
dispersed
• Usually they show
degrees of clumping
or uniformity
• Also depends on the
scale at which the
population is being
observed
Population Dynamics
• All populations are
dynamic – they
change over time
• Look at birth rate,
death rate, and life
expectancy
Population Dynamics
• Birth Rate: the number
of births occurring in a
period of time
• Death/Mortality Rate:
the number of deaths in
a period of time
• Life Expectancy: How
long an individual is
expected to live
US Population Dynamics
Birth Rate/yr
4 million
Death Rate/yr
2.6 million
Life Expectancy
M = 74 yrs
F = 80 yrs
Comparing Population Properties
and Population Dynamics
Population Properties
Population Dynamics
Comparing Population Properties
and Population Dynamics
Population Properties
Population Dynamics
There are aspects of the population
that can be measured
These describe how the population
changes over time
Describe the population as a whole
group
Describe how the properties of the
population change
Properties include size, density, and
dispersion
Include the birth rate, death rate, life
expectancy, and age structure
Age Structure
• The distribution of
individuals among
different ages in a
population
• Often presented as
graphs
• Many important
population processes
vary with age
Age structure
 Relative number of individuals of each age
What do the data imply about population growth in
these countries?
A
AP Biology
B
C
Patterns of Mortality
• Mortality data tends to
match one of three
curves on a graph
• Known as survivorship
curves
• These show the
probability that
members of a
population will survive
to a certain age
Patterns of Mortality
• Type I – Death late in
life (humans)
• Type II – Probability
of dying doesn’t
change (birds)
• Type III – More
likely to die young
(salmon)
TRUE OR FALSE: A population
consists of individuals of the same
species living together in one place
at the same time.
TRUE
TRUE OR FALSE: Dispersion is the
term for how populations are
distributed within a ecosystem.
FALSE – IT REFERS TO THE
DISTRIBUTION OF THE INDIVIDUALS
OF A PARTICULAR POPULATION
WITHIN A PARTICULAR AREA
TRUE OR FALSE: The birth rate in
a population equals the death rate.
FALSE – BOTH CAN FLUCTUATE
Measuring Populations
A single pair of elephants can
increase to a population of 19 million
individuals within 750 years! Why
haven’t they increased their numbers?
Population Growth Rate
• The amount by which a
population’s size changes in
a given time
• Whether a population
grows, shrinks, or remains
the same size depends on:
•
•
•
•
Birth
Death
Immigration
Emigration
Population Growth Rate
• Immigration – movement of individuals
into a population
• Emigration – movement of individuals out
of the population
• Immigration & birth add to a population
• Emigration & death subtract from a
population
• Assume immigration = emigration
Population Size
• Demographers divide large populations into groups
of 1,000 and to present data per capita, meaning per
individual
• Birth rates, death rates, and growth rates for large
populations are usually expressed per capita
Population Size
• Example:
• If there are 52 births and 14 deaths per 1000
individuals per year:
• Birth Rate = 52/1000 = 0.052 births per capita per yr
• Death Rate = 14/1000 = 0.014 deaths per capita per yr
• Growth rate can be calculated by:
• Birth rate – Death Rate = Growth Rate
Population Size
• Calculating per capita growth:
0.052 births per capita – 0.014 deaths per capita
= 0.038 growth per capita
• A positive growth rate means population is
growing; negative means it’s shrinking
Population Size
• To find the number
of new individuals
that will be added to
the population in a
year, just multiply
the per capita growth
rate by the number of
individuals in the
population
Ex] Population = 50,000
Growth = 0.038 per capita
0.038 x 50,000 = 1900
The Exponential Model
• At a steady positive growth rate, the
population will add a larger number of
individuals with each generation
• A pattern of increase in number due to a
steady growth rate is exponential growth
The Exponential Model
• A graph of the
population size over
time for exponential
growth makes a Jshaped curve
• Population size grows
slow when small, but
increases as
individuals join the
population
The Exponential Model
• Leads us to predict that
population size will
increase indefinitely
and by a greater
number with each time
period
• Do you think this trend
will continue? What
will the graph look like
in the future?
Applying the Exponential Model
•
This model matches observed
patterns of growth of real
populations, but only under a
certain number of conditions
and for a limited period of time
•
Example] Bacteria can grow
exponentially if provided with
an abundance of food and space
and if waste is removed
Applying the Exponential Model
•
•
•
This doesn’t apply to most
populations because
resources aren’t unlimited
and harmful waste
accumulates
Any factor, such as space,
that restrains the growth of a
population is called a
limiting factor
All populations are limited
by their environment
Applying the Exponential Model
• As a population grows,
competition intensifies
for resources
• Thus, each individual’s
ability to fight off
disease, grow, and
reproduce decreases
• This results in a
decreasing birth rate and
increasing death rate
The Logistic Model
•
Builds on the exponential
model but accounts for the
influence of limiting
factors
•
Carrying capacity (K) is
the number of individuals
the environment can
support over a long period
of time
The Logistic Model
•
•
•
•
The graph of this model
looks like a stretched-out
letter S
When population is small,
birth rates are high and
death rates are low, so looks
like exponential growth
As size approaches K, the
growth rate slows
At K, the birth rate = death
rate and growth stops
The Logistic Model
• Contains some
assumptions
•
•
K is constant and
doesn’t fluctuate
with environmental
changes
Reality is, it does.
Ex] It is greater
when prey is
abundant and lower
when it is scarce.
The logistic and exponential
models are not universal
representations of real
populations – but – they are an
important tool that scientists use
to explain population growth and
regulation.
True or False: Carrying capacity is the number of
individuals the environment can support for an
extended period of time.
True!!!
True or False: Population growth can be predicted
using only birth- and death-rate statistics.
False – Immigration and
emigration rates are important
factors to consider!!!
How many new individuals will there be next year in a
population of 85,000 people if there are 98 births and 75
deaths per thousand people?
0.098 births per capita – 0.075 deaths per capita = 0.023 growth per capita
0.023 x 85,000 = 1955 new individuals