Metapop Case Study


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Metapop Case Study

  1. 1. Extinction and isolation gradients in metapopulations: the case of the pool frog ( Rana lessonae) - Per Sjögren 1991 Chris Asquith
  2. 2. Outline <ul><li>Background Information, System of Interest, and Study Questions </li></ul><ul><li>Experimental Methods </li></ul><ul><li>Results and Findings </li></ul><ul><li>Metapopulation and Conservation Implications </li></ul>
  3. 3. Concept Review <ul><li>Not all local population extinction in metapopulations is due to catastrophic or deterministic causes. </li></ul><ul><li>Demographic fluctuations or inbreeding depression can also contribute to local extinctions. </li></ul><ul><li>Though anecdotally isolated populations as well as highly fluctuating populations are more likely to go extinct, whether the driving factors are demographic or genetic has not been thoroughly investigated. </li></ul>
  4. 4. Study Questions <ul><li>What are the relative importance of demographic and genetic factors in local extinctions? </li></ul><ul><li>How can we apply this information in conservation management? </li></ul>
  5. 5. Pool frogs ( Pelophylax lessonae ) All reptile and amphibian species are protected by the Swedish Environmental Protection Agency. It is illegal to kill, injure, collect, or move any of these animals.
  6. 6. <ul><li>Note 1 </li></ul><ul><li>Note 2 </li></ul><ul><li>Note 3 </li></ul>
  7. 7. Methods <ul><li>Survey data from Forselius (1962) and Sjögren (1983, 1987-89) for 60 pools in central eastern Sweden </li></ul><ul><li>Presence/Absence was determined by calling males </li></ul><ul><li>Viable population determined by presence of egg masses. </li></ul><ul><li>Subset of populations examined for genetic viability (proportion of eggs fertilized v. eggs laid). </li></ul><ul><li>Population size, survivorship and immigration estimates determined by mark-recapture methods using hand nets, drift fences, and toe clippings. </li></ul>
  8. 8. Methods cont.
  9. 9. Environmental Variables <ul><li>Area of Water Surface </li></ul><ul><li>Alkalinity </li></ul><ul><li>Distance to closest pond w/ local extinction </li></ul><ul><li>Distance to closest occupied pond </li></ul><ul><li>Distance to Baltic Sea </li></ul><ul><li>Presence/Absence of Pike ( Esox lucius ) </li></ul><ul><li>Mean water temp. in May </li></ul><ul><li>Calcium ion concentration </li></ul>
  10. 10. Temp. by Neighbor Distance Plot <ul><li>Using stepwise regression, surface water area, distance to nearest neighbor, and distance to nearest extinct pond were only significant variables in the model. </li></ul>
  11. 11. Pike and Pool Frog Incidence Plot
  12. 12. Proportional fertilized eggs by nearest neighbor distance plot <ul><li>Using stepwise logistic regression, nearest neighbor distance did not significantly explain the data. </li></ul><ul><li>Alkalinity (an environmental variable) was significant and explained 77% of the data variation. </li></ul>
  13. 13. Synthesizing Results <ul><li>Data suggest environmental factors (alkalinity) explain infertility, not isolation. </li></ul><ul><li>Inbreeding does no likely play an important role in non-deterministic extinction. </li></ul><ul><li>Reduced frog populations in Pike-inhabited waters will likely lead to demographically driven extinction. </li></ul><ul><li>Since only 1-2% of eggs survive to adulthood in pike-free populations, immigrants are very important. </li></ul>
  14. 14. Discussion <ul><li>Erlich and Murphy found similar results in Checkerspot Butterfly (1983,1987) </li></ul><ul><li>Both species show high degrees of variation in population size that is environmentally induced. </li></ul><ul><li>Inbreeding depression may not be expected if both species have undergone several bottlenecks in recent past. </li></ul><ul><li>Genetic history is likely important in determining importance of inbreeding depression. </li></ul>
  15. 15. Conservation Implications <ul><li>Regardless of whether genetic or demographic risks are greater for local populations, patch isolation increases risk of local extinction. </li></ul><ul><li>Rescue Effect importance for the persistence of many metapopulations. </li></ul><ul><li>Habitat fragmentation carries the double-edged sword of deterministic and isolation-dependent causes of population reduction. </li></ul><ul><li>Reserves should be designed with connectivity and vacant patches in mind. </li></ul>
  16. 16. Quote of the week <ul><li>“ In addition to the increased risk of extinction following from a reduction in population size, increased isolation of the remaining populations beyond a critical degree is likely to increase the risk of local and region extinction further, and especially in taxa susceptible to environmental stochasticity. As many frog populations belong to this category, this may similarly explain the sudden disappearance of individual populations from seemingly pristine , but isolated reserves highlighted in a recent conservation conference on the global decline of amphibians.” </li></ul>