Habitat Fragmentation

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

6. Habitat Fragmentation

7. Population Fragmentation and H

8. 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%.

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

10. 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

13. 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

14. 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.

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

16. 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.

17. 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

18. 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.

19. Metapopulations

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

21. How Big??

22. One Horned Rhino

23. 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

24. Estimates of Ne

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

26. 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

28. GLT

29. 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.

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