DEMOGRAPHY

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DEMOGRAPHY

• READINGSFREEMAN, 2005
• CHAPTER 52Pages 1192-1196

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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.

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Patterns of Dispersion

• Individuals in a population may be distributed
  according to 3 basic patterns of dispersion
• Random
• Uniform
• Clumped

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Random

• What?
• Scattered no regularity and affinity
• Why?
• Environment uniform individuals solitary

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Uniform

• What?
• About equal distance apart regular with no
  affinity
• Why?
• Resource competition antagonism

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Clumped

• What?
• Grouped in some places, absent in others
  irregular with affinity
• Why?
• Resources patchy individuals aggregate

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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?

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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

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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.

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PLOT METHOD OF SAMPLING

• Subdivide an area into equal sized plots.
• Randomly sample a known proportion of the area.
• Calculate the average number of individuals per
  plot.
• Multiply this average times the number of plots
  in the area.

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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?

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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

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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.

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Understanding Check

• A biologist was concerned about reports that
  fewer small mouthed bass were being caught in
  Joes 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?

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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.

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A SIMPLE LIFE TABLE
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A SIMPLE LIFE TABLE
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A SIMPLE LIFE TABLE
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A SIMPLE LIFE TABLE
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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.

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A SIMPLE LIFE TABLE
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A SIMPLE LIFE TABLE
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A SIMPLE LIFE TABLE
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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.

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TYPE I SURVIVORSHIP

• Some juvenile mortality
• Secure middle age
• High mortality at old age

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TYPE II SURVIVORSHIP

• Some to substantial juvenile mortality
• Constant mortality thereafter

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TYPE III SURVIVORSHIP

• Heavy juvenile mortality
• Relative security thereafter

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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.

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A PALM TREE POPULATION WITH A TYPE III
SURVIVORSHIP
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A POPULATION OF DRUMMONDS PHLOX
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DEMOGRAPHY

• READINGSFREEMAN, 2005
• CHAPTER 52Pages 1192-1196