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Metapopulations

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The document defines key terms related to metapopulation dynamics. It describes metapopulations as populations composed of spatially discrete local populations between which migration is limited. Local populations exist in habitat patches surrounded by unsuitable matrix. Metapopulations are characterized by local extinction and recolonization of vacant patches over time. If extinctions exceed recolonizations, the entire metapopulation may go regionally extinct. The document outlines different types of spatially dynamic populations and models used to study metapopulation dynamics.

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Metapopulations Definitions
Outline Define Metapopulation Terms Define Types of Spatially Dynamic Populations Define Types of Models used to Study Metapopulation Dynamics
“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
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.
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.
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.
Harrison (1991) Types of Spatially Dynamic Populations Classic Levins Metapopulation Mainland-Island Metapopulation Patchy Population Non-equilibrium Populations
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.
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.
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
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.
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.
Modeling Metapopulations Spatially-Implicit Model Spatially-Explicit Model Spatially-Realistic Model
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.
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
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

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Metapopulations

  • 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