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Metapopulations

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  • 1. Metapopulations Definitions
  • 2. Outline
    • Define Metapopulation Terms
    • Define Types of Spatially Dynamic Populations
    • Define Types of Models used to Study Metapopulation Dynamics
  • 3. “A Population of Populations”
    • Unlike a continuous population, a meta- population has spatially discrete local populations (or subpopulations), in which migration between populations is significantly restricted.
    Metapopulation example Continuous population example
  • 4. Habitat patches
    • In metapopulations, local populations are found in “patches” of suitable habitat.
    • These islands of suitable habitat are surrounded by intervening, unsuitable habitat called the matrix.
    • Mortality risk is generally higher in the matrix, limiting movement between local populations.
  • 5. Extinction and Recolonization
    • A metapopulation also must have a nontrivial probability of extinction for one or more of its local populations. Due to their small size, local populations are much more influenced by stochastic events (infertility, drought, disease, etc.)
    • As extinctions occur in local populations, animals from other local populations periodically recolonize the now vacant patches that were formerly occupied in a process called turnover.
  • 6. Regional Extinction
    • If extinction rates are higher than recolonization rates within a metapopulation, the extinction of all local populations (the metapopulation) may occur.
    • The measure of time until all local populations in a given metapopulation become extinct is called the persistence time of the metapopulation.
  • 7. Harrison (1991) Types of Spatially Dynamic Populations
    • Classic Levins Metapopulation
    • Mainland-Island Metapopulation
    • Patchy Population
    • Non-equilibrium Populations
  • 8. The Classic Levins Metapopulation (1969)
    • “ A nexus of patches, each patch winking into life as a population colonizes it, and winking out again as extinction occurs.” (Wilson 1980)
    • Much higher levels of interaction between individuals within a patch than between patches
    • All patches relatively small
    • All patches have a nontrivial probability of local extinction.
    Fig. 1a. Harrison and Taylor 1997.
  • 9. The Mainland-Island Metapopulation
    • Several small “island” patches are within dispersal distance of a much larger “mainland” patch.
    • Though smaller patches have a high probability of local extinction, there is a highly improbable chance that the mainland population will ever become extinct.
    • A steady migration of organisms out of the mainland to the islands, called propagule rain, is independent of the number of patches vacant or filled.
    • Helps explain source-sink dynamics observed in some metapopulations
    Fig. 1b. Harrison and Taylor 1997.
  • 10. Sources and Sinks
    • Source patches- At low density and without immigration, pop. growth rate is positive.
    • Sink patches- At low density and without immigration, pop. growth rate is negative.
    • Without emigrants leaving source patches, sink patches would decrease to extinction.
    • “ Rescue effect” allows for the persistence of local populations with negative growth rate.
    Tittler et al. 2006 Thomas et al. 1996
  • 11. Patchy Population
    • Local populations exist in habitat patches, but dispersal between patches is high.
    • Population structure is clumped, but interbreeding between patches is frequent
    • The metapopulation concept is not very useful under this scenario, and most researchers do not consider this a metapopulation.
    Fig. 1c. Harrison and Taylor 1997.
  • 12. Non-Equilibrium Population
    • Local populations are patchy, but local extinctions greatly exceed recolonization
    • Vacant patches are rarely or never recolonized
    • Not considered a functional metapopulation
    • Frequently found in anthropogenic fragmented landscapes (e.g. formerly forested agricultural fields)
    Fig. 1d. Harrison and Taylor 1997.
  • 13. Modeling Metapopulations
    • Spatially-Implicit Model
    • Spatially-Explicit Model
    • Spatially-Realistic Model
  • 14. Spatially-Implicit Model
    • Type of model used in Levins (1969)
    • Simple assumptions, including all local populations are equally connected and have independent local dynamics
    • Instead of focusing on distance between patches and population density of each patch, the model keeps track of the proportion of patches occupied at any one time.
  • 15. Spatially-Explicit Model
    • More complex than spatially-implicit models
    • Can model density-dependent migration by organizing patches as cells on a grid
    • Assumes that local populations are only interacting with nearest patch(es).
    • Also only considers presence/absence of a species in each patch
  • 16. Spatially-Realistic Model
    • First used in 1994 by Hanski as the incidence function (IF) model
    • Uses GIS to assign attributes, georeferenced coordinates, stochasticity parameters, and a patch’s geometry to a metapopulation.
    • Can make quantitative predictions about metapopulation dynamics (unlike other two models)
    Invasive plant IF model map from Montana State University