Population Demography in the Real World

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Population Demography in the Real World

• What is Population Demography?
• Mechanisms of Population Regulation
• Habitat-Specific Demography
• Population Viability Analysis
• Stochastic processes the small popn paradigm
• The Landscape Approach

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What is Population Demography?

• Intrinsic factors that contribute to a
  populations growth or decline (i.e., life
  history traits)
• Natality and mortality
• Rates of dispersal between populations
• Birth, Immigration, Death, Emigration
• Sex Ratio of breeding population
• Age Structure (proportion of pop. in each age
  group)

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Mechanisms of Population Regulation

• Why are some species rare? What factors regulate
  population size?
• Density dependent factors influence births and
  death rates depending on popn density
• Density independent factors influence birth and
  death rated independent of popn density

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Density - dependent factors

• Increased mortality or decreased natality due to
  shortage of resources (many examples exist)
• Increased mortality due to increased predation,
  parasitism, or disease
• Increased mortality or decreased natality due to
  increased intensity of intraspecific social
  interactions

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Density - independent factors

• Winter ranges of many birds in northern USA are
  defined by severe winter weather (Root 1988). A
  series of particularly cold years will greatly
  decrease population densities in a region ,
  regardless of the birds densities during the
  previous summer.
• Populations can be regulated by more than one
  factor

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Demography in the real world

• Understanding population fluctuations requires
  understanding the links between intrinsic (life
  history related) demographic processes (birth,
  death, immigration and emigration) and the
  environments in which populations exist
• .and the effects of stochastic processes
  especially as the popn get smaller.

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Habitat-Specific DemographySources and Sinks

• Population dynamics may depend on the relative
  quality of good and poor habitats
• Good habitats are called sources. R gt 1.
  Populations in source habitats produce an excess
  of individuals, who must disperse outside their
  natal patch to find a place to settle and breed
• Poor habitats (sinks) are areas Rlt 1where Without
  immigration from other areas, populations in sink
  habitats inevitably spiral to extinction (down
  the drain)

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Metapopulations

• Many species have both source and sink
  populations. Excess from sources disperse to sink
  habitat patches and maintain the population found
  in these poorer habitats. Metapopulation is a
  population that consists of several
  subpopulations linked together by immigration and
  emigration. In metapopulations, the demographic
  rates may or may not be the same in different
  patches of habitat.

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Metapopulations

• Theoretical models have shown that a small
  proportion of the total population may be located
  in the source habitat. I.e. as little as 10 of a
  metapopulation may be in the source habitat and
  still be responsible for maintaining 90 off the
  population found in the sink.
• very important from a conservation standpoint
  to identify critical habitats for endangered
  species

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Management conundrum

• Common practice critical habitats places
  where a species was most common.
• Source habitat is defined by demographic
  characteristics not density
• Determining sources and sinks requires a great
  deal of knowledge about the natural historyAdding
  habitat to a reserve can result in a smaller
  metapopulation if most of the additional land is
  sink habitat

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Metapopulation Concepts

• A collection of subpopulations of a species,
  each occupying a suitable patch of habitat in a
  landscape of otherwise unsuitable habitat.
• The fraction of suitable habitat patches
  occupied at time T represents a balance of the
  rate at which subpopulations go extinct in
  occupied patches and the rate of colonization of
  empty patches.

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Metapopulation Concepts cont.

• The subpopulation in each patch can fluctuate in
  size, and when a subpopulation is very small,
  local extinction can be prevented by occasional
  immigrants that arrive from neighboring patches.
  This is the rescue effect, which is a major
  factor in maintaining small populations (Brown
  and Kodric-Brown (1977)). It may also be
  responsible for maintaining high levels of
  species diversity because poor competitors will
  not be excluded from patches by locally well
  adapted species if the populations of the poorer
  competitors are maintained through immigration.

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PVA

• PVA The study of the ways in which habitat
  loss, environmental, demographic, and genetic
  stochasticity interact to determine extinction
  probabilities for individual species.

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Small popn paradigm

• The following are four categories of factors
  that influence the likelihood of population
  extinction.
• 1.   Demographic uncertainty (also called
  demographic stochasticity).
• 2.   Environmental uncertainty.
• 3.   Natural catastrophes.
• 4.   Genetic uncertainty (including the founder
  effect, genetic drift, and inbreeding).

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Small Population Paradigm

• Declining popn paradigm is concerned with why
  large popn become small (systematic processes)
• Small population paradigm is concerned with what
  happens to populations once they become small
  (stochastic processes)

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Population modeling in heterogenious landscapes

• Putting concepts to work

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Environmental Stochasticity

• Environmental stochasticity also called
  environmental variation affects vital rates in
  somewhat unpredictable ways (subject to weather)
• Extreme case is natural disaster

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Deterministic
Population Size
Stochastic
T
T1
Time
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Population Size
Risk line
Stochastic
T
T1
Time
Risk is potential (probability) of an adverse
event happening Risk Assessment is calculating
the likelihood of an adverse event occurring (in
this case a population falling below some level
over some time (40/1000) 0.04 extinction
probability
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200
Safe
Vulnerable
Endangered
Time (years)
100
Critical
0
0
0.2
0.4
0.6
0.8
1.0
Probability of Extinction
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Demographic StochasticityAdding reality
(complexity)

• Finite rate of increase (R) fecundity
  survival
• Fecundity rate is the number of offspring/female
  from time T to T1
• Survival rate is the probability of surviving
  from time T to time T1

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Adding more complexity stage or age specific
vital rates
ssd
ssm
sl
SSE
SSD
SSM
seeds
seedlings
small
large
Fs
Fl
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New Population Vector
Population Vector
Transition Matrix
x

029 0.4820 0.4814 0.4834 33
0.70329 020 014 034 20
029 0.71720 014 034 14
029 020 0.75114 0.746 34 36