Metapopulations

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Metapopulations

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

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