Section 8Genetically Viable Populations

Habitat loss = loss of living space for threatenedand endangered species.

mammals 56%birds 53%reptiles 62%amphibians 64%fish 56%gymnosperms 32%angiosperms 9%

Currently, about 2,000 endangered vertebratespecies require captive breeding.

Space exists for only about 800 species.

Important questionImportant question: “How large must populationsbe to be genetically viable?”

We must consider what is the minimum sizerequired to maintain a population that suffers noreduction in reproductive fitness or evolutionarypotential over thousands of years!

For a particular population or species:

Is the population size large enough to avoidloss of reproductive fitness?

Does the species have enough genetic diversityto evolve in response to environmental changes?

We do not know precisely how large populations must be to avoid meaningful inbreeding depressionin the long-term.

However, the required size clearly is much largerthan an effective size of 50.

Disturbingly, about 1/2 of all captive populationsof threatened mammals have N of less than 50!

Genetic Goals in the Management of WildGenetic Goals in the Management of WildPopulationsPopulations

Only a few examples of management plans forendangered species in wild where genetic objectivesare defined.

Genetic objectives for theGolden Lion Tamarin is toretain 98% of the geneticdiversity for 100 years.

Genetic Goals in the Management of WildGenetic Goals in the Management of WildPopulationsPopulations

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 WildGenetic Goals in the Management of WildPopulationsPopulations

Currently:

630 individuals in the wild360 individuals in the wild from reintroductions500 individuals in captivity.

Ne/N ratio must exceed 0.31 to attain the genetic goals based on all animals, or 0.5 for wildanimals.

Since this is unlikely, the genetic goals are NOTbeing achieved.

Lack of available habitat to allow expansion of thepopulation is the primary obstacle to reachingthese goals.

Recovery Targets for Population Sizes Used toRecovery Targets for Population Sizes Used tode-list Species.de-list Species.

Recovery programs often identify a targetpopulation size, the size at which the species wouldbe removed from the endangered list.

While most target sizes are in the thousands, theyare generally less than genetic arguments wouldrequire based on a Ne/N ratio of about 0.1.

Peregrin Falcon = 900 to delistPeregrin Falcon = 900 to delist

California Condor only 450 to delistCalifornia Condor only 450 to delist

Number of Ozark big-eared batsNumber of Ozark big-eared batsto delist = ????to delist = ????

Genetic goals in management of captive populationsGenetic goals in management of captive populationsA CompromiseA Compromise

Much fewer captive resources available thanrequired to maintain all species deserving captivebreeding, especially considering the recommendedNe=500 per species.

Zoos house about 540,000 mammals, birds,reptiles, and amphibians.

About half the species are suitable for propogatingendangered animals.

About 2,000 vertebrate species require captivebreeding to save them from extinction.

To maintain each of these at Ne = 500 (assumingNe/N = 0.3 in captivity) would require3,300,000 animal spaces, about 12 times the spacecurrently available.

At an average census size of 500, only 540 speciescan be accommodated.

Currently only 15% of mammal spaces in zoos housethreatened species.

The current compromise is to manage endangeredspecies in captivity to conserve 90% of the wildpopulations genetic diversity for 100 years.

The 100 year time frame was chosen because itis estimated that wild habitat may become available following the predicted human populationdecline 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 inverselyproportional to the generation length for thespecies in question.

Ne required to maintain 90% or originalheterozygosity for 100 years is:

475 for a species with 1 generation/year

18 for Carribean Falmingos with 1 generation every26 years.

1,759 for white-footed mice with a generation length of 14 weeks.

Maintaining 90% of genetic diversity for 100years is a reasonable goal.

However, many species are being maintained withlesser goals (and smaller sizes) due to shortageof resources.

The cost of this compromise is increased ininbreeding & reduced reproductive fitness:

F = 1 - (Ht/Ho)

Accepted 10% loss of heterozygosity correspondsto a 10% increase in F with concomitant inbreedingdepression.

After 100 years, individuals will be related to eachother to a degree somewhere between that offirst cousins (F = 0.0625F = 0.0625) and half-siblings(F = 0.125F = 0.125).

This will reduce juvenile survival on average byabout 15% and total fitness by about 25%, in captivity.

These values ignore the bottleneck associated withfounding populations and assumes that populationnumbers can immediately rise to the desiredlevel.

Captive populations typically have few foundersand grow slowly to their final size. These factorslead to greater loss of genetic diversity early incaptive breeding programs than predicted.

Consequently, effective population sizes requiredto retain 90% of genetic diversity for 100 yearsare typically greater than predicted!!!!!!