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Habitat Fragmentation

Habitat Fragmentation. A reduction in total area Creation of separate isolated patches from a larger continuous distribution Leads to overall reduction in population size and reduced migration among patches. Population fragmentation. Genetic diversity inbreeding Differentiation Extinction

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Habitat Fragmentation

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  1. Habitat Fragmentation • A reduction in total area • Creation of separate isolated patches from a larger continuous distribution • Leads to overall reduction in population size and reduced migration among patches.

  2. Population fragmentation • Genetic diversity • inbreeding • Differentiation • Extinction • These all depend on gene flow among fragments

  3. Population Fragmentation • Number of fragments • Distribution of population sizes in the fragments • Geographic distribution or spatial pattern of populations • Dispersal ability of the species • Migration rates between fragments • Time since fragmentation

  4. Habitat Fragmentation

  5. Habitat Fragmentation

  6. Population Fragmentation and H

  7. Natural Disasters • Hurricane Hugo in 89 wiped out 87% of the active nests in Francis Marion NF and killed 63% of the population. Through intensive management it has recovered by 33%.

  8. Population Structure • The genetic impacts of population fragmentation can go from insignificant to severe depending on the details of fragmentation.

  9. Population structure • Totally isolated population fragments with no gene flow • Effectively single large – fragments where gene flow is sufficient to result in effectively a single large pop. • Island models where migration is equal among equally sized islands • Linear stepping stone models where only neighboring populations exchange migrants • Two-dimensional stepping-stone models where only surrounding populations exchange migrants • Mainland-Island • Metapopulations

  10. Fragmentation • When a population is subdivided, individual alleles and genotypes are distributed among fragments. Fragments will be genetically differentiated from the beginning. • Fragmentation resulting in an initial genetic sub-division of a population • Cumulative diversification, through genetic drift and inbreeding over time in each of the population fragments

  11. Degree of Fragmentation • A population of size N separated into f totally isolated equal sized fragments, the size of each fragment is N/f. • Each frag will become inbred and lose genetic diversity at a rate dependant on N/f. • A single pop. Of the same total size will become inbred and lose GD at a slower rate dependant on its size N.

  12. Degree of Fragmentation H declines with the number of fragments compared to a large pop.

  13. Fragments • Single pop N=500 ind./Gen. • Over 50 generations this pop. loses 5% • While 2 populations of 250 lose 10% each and 5 populations of 100 lose 22% each of their initial H.

  14. Divergence of Allele frequencies • Will vary due to random genetic drift • Each fragment will fix of lose different alleles • Drift among fragmented populations reduces overall heterozygosity across all fragments to below the expected HW

  15. Impacts of pop structure on repro fitness • In island and stepping-stone models migration and population size are key • No migration then Ne of each frag. • In Source-sink structures then the effective pop. size will depend on Ne of mainland, thus inbreeding and loss of fitness will be much higher in this type than a SL. • Metapopulations typically have Ne that are smaller than the number of breeding adults due to extinction and recolonization.

  16. Metapopulations

  17. Genetically Viable populations • So how many do we need???? • What is the minimum Ne to keep genetics healthy? • Minimum Viable Population Size

  18. How Big??

  19. One Horned Rhino

  20. 3 goals • Retaining reproductive fitness by avoiding inbreeding depression • Retaining the ability to evolve in response to changes in the environment • Avoiding accumulation of new deleterious mutations

  21. Estimates of Ne

  22. How large • Generally the goal is 500 = Ne • This means that there must be 5000 indivs. • Maintain long term viability

  23. How large are endangered species pop’s • Generally population size is smaller then the recommended 500. • Not doomed for immediate extinction but they will suffer depletion of genetic resources

  24. GLT

  25. Genetic goals in management • The GLT goal is to retain 98% of the genetic diversity for 100 years. • 630 wild + 360 reintro. • The Ne/N ratio must be .31 for total or .5 for wild animals to meet the goal.

  26. The fallacy of small surviving populations • A few small populations are surviving but this is not the majority.

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