Population dynamics across multiple sites

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Population dynamics across multiple sites
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Multiple populations

• How many populations are needed to ensure a high
  probability of survival for a species?
• To what extent should multiple populations be
  clumped together in space versus spread apart?
• Can small populations or those occupying sites
  with low habitat quality substantially add to the
  regional viability of a species?

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Terminology

• Site a discrete piece of habitat that has some
  potential of maintaining a population of the
  species of interest.

Separated
Juxtaposed
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Terminology

• Population the group of individuals living on a
  site
• While the individuals across all sites will be
  called the Total or multi-site population

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Terminology

• Metapopulation sets of discrete, largely (but
  not entirely) independent populations whose
  dynamics are driven by local extinction and
  recolonization via movement from other
  populations (Levin 1969)

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Data needs

• Modeling the operation of a set of populations
  requires all the information to do a good job of
  a single-population PVA for each site, plus data
  on movement rates between populations, plus
  estimates of how temporal fluctuations in
  population processes are correlated between
  population

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Characteristics of plant PVAs (n90)
Characteristic Alternative studies
Classification method Stage or size 80
Classification method Other 68
Length of the study 1-5 years 70
Length of the study gt5 years 30
Number of populations 1 84
Number of populations gt1 16
Advanced modeling features Environmental Stoch. 24
Advanced modeling features Density dependence 12
Advanced modeling features Spatially explicit 5
Advanced modeling features Demographic Stoch. 3
Advanced modeling features Genetics 3
Modified from Menges 2000, TREE 15 51-56
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Requirement 1

• Site specific Population dynamics.
• Information about the quality of the population
• But how likely is that for every population of an
  endangered species, many years of census data,
  let alone estimates of all vital rates, will be
  available?

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Common approaches

• Assume that population growth rates or vital
  rates are identical at all sites, but carrying
  capacities differ among sites.
• Assume that most demographic rates are identical
  across sites, but to allow a handful of rates,
  about which more information is available to
  differ.

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Cowbird parasitism
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Requirement 2

• The importance of correlations.
• Through safety in numbers multiple populations
  can strikingly decrease the risk of total
  extinction of a species.
• However, this benefit critically relies on a lack
  of correlation in the dynamics, and hence risks
  of extinction, of the different populations

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Hypericum cumulicola
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The California clapper rail
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The California clapper rail
Population Numbers µ s2 P(ex)
Mowry 70 0.043 0.051 0.06
Faber 29 -0.002 0.041 0.79
Laumeister 33 0 0.051 0.72
0.060.790.720.034
Harding et al. 2001
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Pearson correlation coefficients
Population Mowry Faber
Mowry 1
Faber 0.995 1
Laumiester 0.896 0.938
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Among sites Pearson correlation coefficients of
H. cumulicola vegetative-small adult transition
(TSFgt15 years)
Population 62 93 59
1 -0.681 0.476 0.599
62 -0.150 -0.527
93 -0.393
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Correlations in population growth rates
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Correlations in population growth rates
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Joint-rank correlations for Delphinium
uliginosum patch level data
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The Lake Wales Ridge
World distribution of Hypericum cumulicola
Archbold
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Patch level Archbold Biological Station
110 Rosemary scrub patches
Patch 45
H. cumulicola occupancy 58
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Hypericum cumulicola occupancy related to patch
size and patch isolation (plt0.001)
Quintana-Ascencio Menges. (1996)
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Fire intensity and location in burn unit 58 E,
1967, 1968, 1976 and 1980
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Requirement 3

• The importance of movement.
• If movement rates are quite high, then multiple
  sites do not truly harbor multiple populations,
  but instead a single one that utilizes a
  dispersed set of habitat patches.
• If movement occurs at low rates, it may
  nevertheless play an important role in supporting
  multi-site viability by allowing rescue of
  populations

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How to quantify movement?

• Capture-recapture analysis

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Capture-recapture methods

• use resighting data to estimate the actual
  numbers of individuals in each class including
  those not directly seen
• To accomplish this, you must not only count and
  relocate the marked animals, but also estimate
  the number of unmarked animals in each site

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Fitting functions
Fitted
Dispersal
Observed
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A classification of multi-site scenarios
Correlations in population Movement rates Movement rates Movement rates
Vital rates Essentially none Low to medium High
Significantly negative Separate pops, multiple strongly beneficial Highly effective Metapop Multiple sites, very different habitat
Non different from zero Separate pops multiple strongly beneficial Somewhat effective Matapop Multiple sites Somewhat different habitat
Significantly Positive Separate pops Multiple not very effective Ineffective Metapop One population