DEMOGRAPHY
READINGS:
FREEMAN, 2005
CHAPTER 52
Pages 1192-1196
What is a Population?
• A group of individuals living in a
particular area.
• Individuals that interact while seeking
resources and in producing offspring.
• Members of a group that are subject to
the same local conditions of the
environment.
• Members of a single species.
Patterns of Dispersion
Individuals in a population may be
distributed according to 3 basic patterns
of dispersion:
* Random
* Uniform
* Clumped
Random
• What?
Scattered; no
regularity and
affinity
• Why?
Environment uniform;
individuals solitary
Uniform
• What?
About equal distance
apart; regular with no
affinity
• Why?
Resource competition;
antagonism
Clumped
• What?
Grouped in some
places, absent in
others; irregular with
affinity
• Why?
Resources patchy;
individuals aggregate
The Frog Problem
Dr. T., an ecologist, wanted to find out how
many frogs live in a small pond. On the first
trip to the pond, 55 frogs were caught,
banded, and released. The second trip to the
pond, 72 frogs were caught, of those 72
frogs, 12 were banded. Assuming the banded
frogs had thoroughly mixed with the
unbanded frogs, how many frogs live in the
pond?
CAPTURE/RECAPTURE OF
FROGS
How many frogs in the pond?
If X= number of frogs in pond, the
proportion of marked frogs on the 1st day
must equal the proportion of marked frogs
on the 2nd day.
X = 72 or X = 55 x 72 = 330 frogs
55 12 12
ABUNDANCE
• Most population studies begin with a statement of
abundance.
• The number of individuals in a population may be
obtained by:
1. census- Counting all individuals.
2. sampling- Counting a known fraction to
arrive at an estimate of total number.
• Many ecological studies require use of sampling,
such as capture/recapture or the plot method.
PLOT METHOD OF
SAMPLING
1. Subdivide an area into
equal sized plots.
2. Randomly sample a
known proportion of
the area.
3. Calculate the average
number of individuals
per plot.
4. Multiply this average
times the number of
plots in the area.
A Maple Tree Problem
A biology student wanted to know how
many maple trees there were in his
hometown. He knew the town covered
an area of 520 blocks. To estimate how
many trees, he counted all trees in a
random sample of 10 blocks and found
that there were 45 maple trees. How
many trees are there in his home town?
PLOT METHOD FOR MAPLE
TREES
How many maple trees in the town?
Let X= number of maple trees in town. If a
known proportion of equal sized plots are
sampled at random, then the number of trees in
the area is equal to the average number of
trees per plot times the number of plots.
X = 45 or X = 520 x 45 = 2340 trees
520 10 10
ACCURACY OF ESTIMATES
• Both the capture/recapture and the plot
methods are most accurate when
distribution of individuals is either
uniform or random. Clumped
distributions of individuals are highly
subject to error.
• Larger sample sizes provide the more
accurate estimates.
Understanding Check
A biologist was concerned about reports that
fewer small mouthed bass were being caught
in Joe’s Pond. He conducted a capture-
recapture study to estimate the size of this
fish population. He caught and marked 200
bass and returned them to the pond. The
following day, 240 bass were caught of which
60 were marked. When the fish population of
any species falls below 500, the pond should
be closed to fishing. Should the pond be
closed?
POPULATION ECOLOGY
• The study of how and why the number of
individuals change over time.
• Changes in abundance are made through
comparison of direct counts or estimates in
numbers of individuals.
• Changes in density or numbers of individuals
per unit area or volume are often used where
population sizes are very large or difficult to
sample.
MORTALITY, NATALITY AND
MOVEMENT
• New individuals are added to a
population by NATALITY (BIRTHS) or
IMMIGRATION (IN MOVEMENT).
• Existing individuals are removed from a
population by MORTALITY (DEATHS)
or EMIGRATION (OUT MOVEMENT).
POPULATION DYNAMICS
• If natality (births) and immigration (in
movement) exceed mortality (deaths)
and emigration (out movement), then
populations increase.
• If mortality (deaths) and emigration (out
movement) exceed natality (births) and
immigration (in movement), then
populations decrease.
POPULATION DYNAMICS
• If natality (births) and immigration (in
movement) equals mortality (deaths)
and emigration (out movement), then
populations are stationary; there is no
increase or decrease in number.
• Stationary populations are rare, but
minor fluctuations around a mean or
average population size is common.
A POPULATION DYNAMICS
PROBLEM
A prairie is inhabited by a ground squirrel
population for which:
natality is 25 per year
mortality is 20 per year
immigration is 5 per year
emigration is 10 per year
This population is:
a. increasing.
b. decreasing.
c. stationary.
DEMOGRAPHY
• A study of deaths, births and
movements and predictions of how
these factors determine the size and
structure of populations through time.
• Involves construction of life tables,
survivorship curves, fecundity tables
and calculation of reproductive output.
POPULATION
PROJECTIONS
Can be made using:
1. Life table, maturnity tables and
reproductive outputs.
2. Age structures.
3. Mathematical models that
incorporate birth rates, death
rates and doubling times.
BASIC DEMOGRAPHIC
TOOLS
• Life tables are constructed from age
specific deaths. lx is the proportion of
individuals surviving to a given age.
• Age specific fecundity. mx is the number
of female births to females of a given
age.
• Net reproductive output, Ro , is sum lx mx
over all age classes.
RO AND POPULATION
DYNAMICS
The following rules can be used to
determine if a population is stationary,
increasing or decreasing. The rules are:
• If Ro = 1, then population is stationary.
• If Ro > 1, then population is growing.
• If Ro < 1, then population is declining.
LIFE TABLE
• A device for showing mortality changes
associated with an age interval (X).
• The number of deaths at a given age (DX) is
recorded.
• The number of survivors at the beginning of
an age interval (SX) is determined.
• The proportion of “newborns” that survive to a
given age interval (lX) is calculated.
A SIMPLE LIFE TABLE
X (AGE) DX SX
O-1 500 1,000
1-2 200
2-3 100
3-4 100
4-5 100
5
A SIMPLE LIFE TABLE
X (AGE) DX SX
O-1 500 1,000
1-2 200 500
2-3 100
3-4 100
4-5 100
5
A SIMPLE LIFE TABLE
X (AGE) DX SX
O-1 500 1,000
1-2 200 500
2-3 100 300
3-4 100
4-5 100
5
A SIMPLE LIFE TABLE
X (AGE) DX SX
O-1 500 1,000
1-2 200 500
2-3 100 300
3-4 100 200
4-5 100 100
5 0
AGE SPECIFIC
SURVIVORSHIP (lx)
• The proportion of live births that survive
to the beginning of any age interval is
defined as age specific survivorship (lX).
• The proportion of the original
population alive at age X0 is always
100% or 1.00. Thus, l0 = 1.00
• lX for any subsequent age interval is
Sx/S0.
A SIMPLE LIFE TABLE
X DX SX lX
O-1 500 1,000 1.00
1-2 200 500 500/1000
2-3 100 300
3-4 100 200
4-5 100 100
5 0
A SIMPLE LIFE TABLE
X DX SX lX
O-1 500 1,000 1.00
1-2 200 500 0.50
2-3 100 300 300/1000
3-4 100 200
4-5 100 100
5 0
A SIMPLE LIFE TABLE
X DX SX lX
O-1 500 1,000 1.00
1-2 200 500 0.50
2-3 100 300 0.30
3-4 100 200 0.20
4-5 100 100 0.10
5 0 0
SURVIVORSHIP CURVE
• The proportion of individuals living to
various ages is the survivorship of a
population.
• A survivorship curve is constructed by
plotting age specific survivorship (lx) and
age (X).
• Survivorship curves indicate those ages
at which mortality is high.
TYPE I
SURVIVORSHIP
• Some juvenile
mortality
• Secure middle age
• High mortality at old
age
TYPE II
SURVIVORSHIP
• Some to substantial
juvenile mortality
• Constant mortality
thereafter
TYPE III
SURVIVORSHIP
• Heavy juvenile
mortality
• Relative security
thereafter
A GRAY SQUIRREL
POPULATION IN A WOOD
LOT
• This squirrel
population living in
an Ohio woodlot has
a type II survivorship
curve.
• Typical of a
population with
accidental death.
A PALM TREE POPULATION
WITH A TYPE III
SURVIVORSHIP
A POPULATION OF
DRUMMOND’S PHLOX
DEMOGRAPHY
READINGS:
FREEMAN, 2005
CHAPTER 52
Pages 1192-1196