Habitat Fragmentation
1. A reduction in total area
2. Creation of separate isolated patches
from a larger continuous distribution
3. Leads to overall reduction in
population size and reduced
migration among patches.
Population fragmentation
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Genetic diversity
inbreeding
Differentiation
Extinction
• These all depend on gene flow among
fragments
Population Fragmentation
• Number of fragments
• Distribution of population sizes in the
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fragments
Geographic distribution or spatial
pattern of populations
Dispersal ability of the species
Migration rates between fragments
Time since fragmentation
Habitat Fragmentation
Habitat Fragmentation
Population Fragmentation and H
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%.
Population Structure
• The genetic impacts of population
fragmentation can go from insignificant
to severe depending on the details of
fragmentation.
Population structure
1. Totally isolated population fragments with no
2.
3.
4.
5.
6.
7.
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
Fragmentation
• When a population is subdivided, individual
alleles and genotypes are distributed
among fragments. Fragments will be
genetically differentiated from the
beginning.
1. Fragmentation resulting in an initial genetic
2.
sub-division of a population
Cumulative diversification, through genetic
drift and inbreeding over time in each of the
population fragments
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.
Degree of Fragmentation
H declines with the number of fragments compared to a large pop.
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.
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
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.
Metapopulations
Genetically Viable populations
• So how many do we need????
• What is the minimum Ne to keep
genetics healthy?
• Minimum Viable Population Size
How Big??
One Horned Rhino
3 goals
1. Retaining reproductive fitness by
avoiding inbreeding depression
2. Retaining the ability to evolve in
response to changes in the
environment
3. Avoiding accumulation of new
deleterious mutations
Estimates of Ne
How large
• Generally the goal is 500 = Ne
• This means that there must be 5000
indivs.
• Maintain long term viability
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
GLT
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.
The fallacy of small surviving
populations
• A few small populations are surviving
but this is not the majority.