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Section 8
Genetically Viable Populations
Habitat loss = loss of living space for threatened
and endangered species.
mammals
birds
reptiles
amphibians
fish
gymnosperms
angiosperms
56%
53%
62%
64%
56%
32%
9%
Currently, about 2,000 endangered vertebrate
species require captive breeding.
Space exists for only about 800 species.
Important question: “How large must populations
be to be genetically viable?”
We must consider what is the minimum size
required to maintain a population that suffers no
reduction in reproductive fitness or evolutionary
potential over thousands of years!
For a particular population or species:
Is the population size large enough to avoid
loss of reproductive fitness?
Does the species have enough genetic diversity
to evolve in response to environmental changes?
We do not know precisely how large populations
must be to avoid meaningful inbreeding depression
in the long-term.
However, the required size clearly is much larger
than an effective size of 50.
Disturbingly, about 1/2 of all captive populations
of threatened mammals have N of less than 50!
Genetic Goals in the Management of Wild
Populations
Only a few examples of management plans for
endangered species in wild where genetic objectives
are defined.
Genetic objectives for the
Golden Lion Tamarin is to
retain 98% of the genetic
diversity for 100 years.
Genetic Goals in the Management of Wild
Populations
To obtain the required population size:
Ht/H0 = e-t/2Ne
Substituting Ht/H0 = 0.98 and t = 100/6 = 16.7:
0.98 = e-16.7/2Ne = e-8.3333/Ne
Ne = -8.33/ln 0.98 = 412
Genetic Goals in the Management of Wild
Populations
Currently:
630 individuals in the wild
360 individuals in the wild from reintroductions
500 individuals in captivity.
Ne/N ratio must exceed 0.31 to attain the
genetic goals based on all animals, or 0.5 for wild
animals.
Since this is unlikely, the genetic goals are NOT
being achieved.
Lack of available habitat to allow expansion of the
population is the primary obstacle to reaching
these goals.
Recovery Targets for Population Sizes Used to
de-list Species.
Recovery programs often identify a target
population size, the size at which the species would
be removed from the endangered list.
While most target sizes are in the thousands, they
are generally less than genetic arguments would
require based on a Ne/N ratio of about 0.1.
Peregrin Falcon = 900 to delist
California Condor only 450 to delist
Number of Ozark big-eared bats
to delist = ????
Genetic goals in management of captive populations
A Compromise
Much fewer captive resources available than
required to maintain all species deserving captive
breeding, especially considering the recommended
Ne=500 per species.
Zoos house about 540,000 mammals, birds,
reptiles, and amphibians.
About half the species are suitable for propogating
endangered animals.
About 2,000 vertebrate species require captive
breeding to save them from extinction.
To maintain each of these at Ne = 500 (assuming
Ne/N = 0.3 in captivity) would require
3,300,000 animal spaces, about 12 times the space
currently available.
At an average census size of 500, only 540 species
can be accommodated.
Currently only 15% of mammal spaces in zoos house
threatened species.
The current compromise is to manage endangered
species in captivity to conserve 90% of the wild
populations genetic diversity for 100 years.
The 100 year time frame was chosen because it
is estimated that wild habitat may become
available following the predicted human population
decline in 100 - 200 years.
This requires different population sizes for
species with different generation lengths.
Ht/H0 ≈ e-t/2Ne
taking natural logarithm, substituting 0.9 for Ht/H0,
100/L for t (where L is generation length in years),
and rearranging we obtain:
Ne = 475/L
Consequently, the required size is inversely
proportional to the generation length for the
species in question.
Ne required to maintain 90% or original
heterozygosity for 100 years is:
475 for a species with 1 generation/year
18 for Carribean Falmingos with 1 generation every
26 years.
1,759 for white-footed mice with a generation
length of 14 weeks.
Maintaining 90% of genetic diversity for 100
years is a reasonable goal.
However, many species are being maintained with
lesser goals (and smaller sizes) due to shortage
of resources.
The cost of this compromise is increased in
inbreeding & reduced reproductive fitness:
F = 1 - (Ht/Ho)
Accepted 10% loss of heterozygosity corresponds
to a 10% increase in F with concomitant inbreeding
depression.
After 100 years, individuals will be related to each
other to a degree somewhere between that of
first cousins (F = 0.0625) and half-siblings
(F = 0.125).
This will reduce juvenile survival on average by
about 15% and total fitness by about 25%,
in captivity.
These values ignore the bottleneck associated with
founding populations and assumes that population
numbers can immediately rise to the desired
level.
Captive populations typically have few founders
and grow slowly to their final size. These factors
lead to greater loss of genetic diversity early in
captive breeding programs than predicted.
Consequently, effective population sizes required
to retain 90% of genetic diversity for 100 years
are typically greater than predicted!!!!!!