Population Dynamics

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Population Dynamics

• Chapter 10

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Outline

• Dispersal
• In Response to Climate Change
• In Response to Changing Food Supply
• In Rivers and Streams
• Metapopulations
• Estimating Patterns of Survival
• Survivorship Curves
• Age Distribution
• Rates of Population Change
• Overlapping Generations

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Population Dynamics

• Population dynamics is the area of ecology
  concerned with the factors influencing the
  expansion, decline, or maintenance of populations.

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Dispersal

• Africanized Honeybees
• Honeybees (Apis melifera) evolved in Africa and
  Europe and have since differentiated into many
  locally adapted subspecies.

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Africanized Honeybees

• Africanized honeybees disperse much faster than
  European honeybees.
• Within 30 years they occupied most of South
  America, Mexico, and all of Central America.

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Dispersal

• Dispersal can increase or decrease local
  population densities.
• Ecologists must consider dispersal into
  (immigration) and out of (emigration) the local
  population.

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Collared Doves

• Collared Doves, Streptopelia decaocto, spread
  from Turkey into Europe after 1900.
• Dispersal began suddenly.
• Not influenced by humans.
• Took place in small jumps.
• 45 km/yr

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Dispersal

• Rates of expansion by animal populations varies
  from the rapid expansion of Africanized honeybees
  to the much slower rates of elk.

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Rapid Changes in Response to Climate Change

• Organisms began to spread northward about 16,000
  years ago following retreat of glaciers and
  warming climate.
• Evidence found in preserved pollen in lake
  sediments.
• Movement rate 100 - 400 m/yr.

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Dispersal in Response to Changing Food Supply

• Holling observed numerical responses to increased
  prey availability.
• Increased prey density led to increased density
  of predators.
• Individuals move into new areas in response to
  higher prey densities.

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Dispersal in Response to Changing Food Supply

• A 10-year study of voles and their predators
  showed that predator populations closely tracked
  the vole densities.
• If reproduction was responsible there would be a
  time lag.
• Dispersal in response to prey numbers.
• Supported by tracking study.

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Dispersal in Rivers and Streams

• Stream dwellers have mechanisms to allow them to
  maintain their stream position even in a strong
  current.
• Streamlined bodies
• Bottom-dwelling
• Adhesion to surfaces

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Dispersal in Rivers and Streams

• They still sometimes get washed downstream,
  especially in flash floods or spates.
• Downstream movement of stream organisms is called
  drift.
• Some organisms actively move downstream.

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Dispersal in Rivers and Streams

• Muller hypothesized populations maintained via
  dynamic interplay between downstream and upstream
  dispersal.
• Colonization cycle

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Dispersal in Rivers and Streams

• These snails from a tropical stream in Costa Rica
  provide an example.
• Larvae drift downstream to the Pacific. After
  metamorphosis, they migrate upstream in large
  aggregations (up to 500,000).

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Metapopulations

• A metapopulation is made up of a group of
  subpopulations living on patches of habitat
  connected by an exchange of individuals.

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Metapopulations

• A study of Parnassius smintheus included 20
  alpine meadows ranging in size from 0.8 20
  hectares.
• Fire suppression combine with global warming is
  causing the meadows to become smaller and more
  isolated.

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Metapopulations

• Lesser kestrels occur in breeding colonies
  scattered through the Ebro River valley in Spain.
• Kestrels in smaller subpopulations are more
  likely to emigrate.
• Kestrels were more likely to move from a small
  colony to a large one.

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Estimating Patterns of Survival

• A survivorship curve summarizes the pattern of
  survival in a population.

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Estimating Patterns of Survival

• Three main methods of estimation
• Cohort life table
• Identify individuals born at same time (cohort)
  and keep records from birth.
• Static life table
• Record age at death of individuals.
• Age distribution
• Calculate difference in proportion of individuals
  in each age class.
• Assumes differences from mortality.

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High Survival Among the Young

• Murie collected Dall Sheep skulls, Ovis dalli.
• Major Assumption Proportion of skulls in each
  age class represented typical proportion of
  individuals dying at that age.
• Reasonable given sample size of 608.

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High Survival Among the Young

• They constructed a survivorship curve for the
  sheep.
• Discovered bi-modal mortality.
• 9-13 yrs.

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High Survival Among the Young

• Similar curves can be seen for a plant, Phlox
  drummondii, and a rotifer, Floscularia conifera.

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Constant Rates of Survival

• The survivorship curves of many species are
  nearly straight lines.
• Individuals are equally likely to die at any
  point in life.
• Many birds show this pattern.
• The common mud turtle also shows this pattern
  after the first year.

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High Mortality Among the Young

• Some organisms produce huge numbers of offspring,
  few of which may survive.
• Marine fishes, like the mackerel show this
  pattern.
• Many plants show this pattern producing many
  seeds, with few survive.
• Cleome

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Survivorship Curves

• Type I Majority of mortality occurs among older
  individuals.
• Dall Sheep
• Type II Constant rate of survival throughout
  lifetime.
• American Robins
• Type III High mortality among young, followed by
  high survivorship.
• Cleome, many marine animals with planktonic
  larvae.

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Survivorship Curves
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Age Distribution

• Age distribution of a population reflects its
  history of survival, reproduction, and growth
  potential.

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Age Distribution

• Miller published data on age distribution of
  white oak (Quercus alba).
• Determined relationship between age and trunk
  diameter.
• Age distribution biased towards young trees.
• Sufficient reproduction for replacement
  indicating a stable population.

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Age Distribution

• Rio Grande Cottonwood populations (Populus
  deltoides wislizenii) are declining.
• Old trees not being replaced.
• Reproduction depends on seasonal floods.
• Prepare seed bed.
• Keep nursery areas moist.
• Because floods are absent, there are now fewer
  germination areas.

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Dynamic Population in a Variable Climate

• Grant and Grant studied Galapagos Finches.
• 1983 shows a regular distribution among age
  classes.
• Gap in age distribution.
• Drought in 1977 resulted in no recruitment.

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Dynamic Population in a Variable Climate

• The age distribution for 1987 is very different.
• Additional droughts in 1984 and 1985.
• Reproductive output driven by exceptional year in
  1983.
• Responsiveness of population age structure to
  environmental variation.

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Rates of Population Change

• A life table combined with a fecundity schedule
  can be used to estimate a variety of factors
  concerned with population change.
• Is it stable, growing, or declining?

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Rates of Population Change

• Birth Rate Number of young born per female.
• Fecundity Schedule Tabulation of birth rates for
  females of different ages.

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Estimating Rates for an Annual Plant

• P. drummondii
• Ro Net reproductive rate Average number of
  seeds produced by an individual in a population
  during its lifetime.
• Ro S lxmx
• X Age interval in days.
• lx pop. surviving to each age (x).
• mx Average number seeds produced by each
  individual in each age category.

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Estimating Rates for an Annual Plant
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Estimating Rates for an Annual Plant

• Weve already used the data in lx to produce the
  life curve for Phlox.
• The net reproductive rate is 2.4177.
• The population is growing (net reproductive rate
  1).

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Estimating Rates for an Annual Plant

• Because P. drummondii has non-overlapping
  generations, we can estimate growth rate.
• Geometric Rate of Increase (?)
• ?N t1 / Nt
• N t1 Size of population at future time.
• Nt Size of population at some earlier time.

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Estimating Rates for an Annual Plant

• For a time period of one year the geometric rate
  of increase in Phlox was the same as the net
  reproductive rate.
• This is because Phlox is an annual plant with
  pulsed reproduction.

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Estimating Rates when Generations Overlap

• Common Mud Turtle (K. subrubrum)
• About half turtles nest each year.
• Most nest once, some 2 or even 3 times.
• We want to keep track of female eggs.

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Estimating Rates when Generations Overlap

• The net reproductive rate for this population of
  mud turtles turns out to be 0.601.
• Population is declining (net reproductive rate 1.0).

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Estimating Rates when Generations Overlap

• Average generation time (time from egg to egg)
• T S xlxmx / Ro
• X Age in years
• Generation time for mud turtles 10.6 years