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)
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
0.060.790.720.034
Harding et al. 2001
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Pearson correlation coefficients
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Among sites Pearson correlation coefficients of
H. cumulicola vegetative-small adult transition
(TSFgt15 years)
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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