section 12 genetics management for reintroduction reintroduction is the process of releasing...

Section 12Section 12Genetics Management for ReintroductionGenetics Management for Reintroduction

Reintroduction is the process of releasing captive-bornindividuals back into the wild to re-establish or supplementexisting wild populations.

While reintroduction programs currently exist for manyspecies, conditions are not suitable for reintroduction forthe majority of species as native habitat is not availableor threats to wild populations still exist.

Nevertheless, many captive breeding programs aim toretain sufficient levels of genetic diversity anddemographic variability over the long-term to eventuallyreintroduce animals back into the wild -- if and whenif and whenthe situation presents itselfthe situation presents itself.

There are three genetic scenarios for captive populations.

First, the threatening process in the wild may becontrolled with relative ease, for example by exterminationof exclusion of introduced predators or competitors froman island.

In this case, the endangered species may be reintroducedafter only a few generations of population expansion incaptivity.

The only genetic issues are representative sampling during the foundation and avoidance of inbreeding.

Second, loss or degredation of habitat may be so severethat reintroduction is not a realistic proposition.

This may be the case for some species of large, naturallywide-ranging mammals.

Here, the genetic management may, ultimately be“domestication”, deliberate selection of passiveindividuals, capable of tolerating close proximity to humans and other animals, and easily maintained on cheap,non-specialist diets.

The majority of captive endangered species lie betweenthese extremes and constitute the third scenario and will be the focus of the following several lectures.

These species require many generations of captive breeding, but release remains a viable option.

Genetic Changes in Captivity that Affect Reintroduction Genetic Changes in Captivity that Affect Reintroduction SuccessSuccess

Captive populations typically deteriorate in ways that reduce reintroduction success such as:

Loss of genetic diversityInbreeding depressionAccumulation of new deleterious mutationsGenetic adaptation to captivity.

We previously discussed the first three of these components and therefore will focus on GeneticAdaptation to Captivity.

While genetic adaptation to captivity has beenrecognized since the time of Darwin (domestication), ithas, until recently, been considered only a minor problemin captive breeding.

However, there is now compelling evidence that it can bea major threat to the success of reintroductions asall populations adapt to their local environmental conditions.

When we compare populations across a range of environments, we usually observe that they have highestfitness in their own environment and have lower fitness inother environments.

This is due to genotype X environment interactions.

Thus, for populations maintained in captivity for manygenerations, adaptation to this novel environment mayseverely reduce their performance upon return tonatural environments.

When wild populations are brought into captivity, theforces of natural selection change.

Populations are naturally or inadvertently selected fortheir ability to reproduce in the captive environment.

Selection for tameness is favored by keepers andflighty animals such as antelope, gazelle, wallabies,and kangaroos may kill themselves by running intofences.

Predators are controlled, as are most diseases & pests.

Carnivores are no longer selected for their ability tocapture prey.

Further, there is usually no competition with other speciesin captivity, and limited competition for mates withinspecies.

Natural selection on all of these characters will be relaxed,or, if there are trade-offs with other aspects of reproductive fitness, they may actually be selectedagainst.

To minimize the deleterious impacts of adaptation to captivity on reintroduction success, we need to considerwhat factors determine the annual rate of adaptationto captivity and include:

Number of generations in captivity (years/length = y/Lyears/length = y/L)Selection differential in captivity (SS)Additive genetic variation for reproductive fitness (hh22)Effective population size of the captive population (NeNe)Proportion of the population derived from migrants (mm)Generation length in years (LL).

Rates of genetic adaptation (GA) can be predicted from:

GA~(ShGA~(Sh22/L)/L)[1 - 1/(2Ne)][1 - 1/(2Ne)]y/Ly/L(1-m(1-mii))

Where mi is the proportion of the genetic material fromimmigrants in the ith generation.

Selection in captivity is dependent upon the mortalityrate and upon the variance in family size.

If the captive environment is very different from thewild, selection in captivity will be strong and the populationwill evolve rapidly to adapt.

As selection is more effective in large than smallpopulations, genetic adaptations to captivity is greaterin larger than smaller populations.

Immigrants introduced from the wild, will slow the rate ofgenetic adaptation.

However, for many endangered species, wild individuals areeither not available or too valuable to use in augmenting acaptive population.

Species with shorter generation lengths show fastergenetic adaptation per year than ones with longergenerations.

Adaptation to Captivity can be Reduced By:Adaptation to Captivity can be Reduced By:

Minimizing the number of generations in captivityMinimizing selection in captivityMinimizing the heritability of reproductive fitness in

captivityMinimizing the size of the captive populationMaximizing the proportion of wild immigrants and the

recency of introducing immigrantsMaximizing generation length

Population Fragmentation as a Means for MinimizingPopulation Fragmentation as a Means for MinimizingGenetic Adaptation to CaptivityGenetic Adaptation to Captivity.

The competing requirements to maintain genetic diversityand avoid severe inbreeding depression, but also to avoidgenetic adaptation to captivity, indicate that neitherlarge nor small populations are ideal for breeding incaptivity, when reintroduction to the wild is envisioned.

Small populations suffer loss of genetic variation andinbreeding depression, but minimize genetic adaptation tocaptivity.

Large populations retain genetic variation and have onlyslow accumulation of inbreeding but suffer most fromgenetic adaptation to captivity.

A compromise could be achieved by maintaining largeoverall population, but fragmenting it into partiallyisolated sub-populations.

The sub-populations are maintained as separate populationsuntil inbreeding builds to a level where it is of concern(F ~ 0.1 -- 0.2F ~ 0.1 -- 0.2).

Immigrants are exchanged among sub-populations at thispoint.

The sub-populations are then maintained as isolatedpopulations until inbreeding again builds up.

This structure is expected to maintain more geneticdiversity than a single population of the same total sizeand to exhibit less deleterious changes to captivity.

If all sub-populations are combined to produce individualsfor reintroduction, the pooled population has a lower levelof inbreeding and more genetic diversity than a singlelarge population.

A critical requirement in the use of this design is thatnone of the sub-populations becomes extinct and therefore,this strategy is NOTNOT recommended for wild populations.

Captive populations are already fragmented.

Individual zoos & wildlife parks have limited capacity andendangered species are dispersed over several institutionsto minimize the risk from catastrophes.

Currently, individuals are moved among institutions tocreate, effectively, a single large population.

The fragmented structure will reduce costs and the riskof injury, and reduce disease transmission.

Captive Management of ReintroductionsCaptive Management of Reintroductions

Choosing sites for reintroductionChoosing sites for reintroduction -- sites for reintroduction should maximize the chances of successful, re-establishment in the wild.

The environment should match, as closely as possible,the environment to which the population was adapted,prior to captive breeding.

Reintroductions should therefore be carried out withinthe previous range of the species and ideally into prime,rather than marginal habitat.

This minimizes the adaptive evolution required in thereintroduction site.

Choosing individuals for reintroductionChoosing individuals for reintroduction -- Individualsused for reintroduction should maximize the chance ofre-establishing a self-sustaining population.

Thus, healthy individuals with high reproductive potential,low inbreeding coefficients and high genetic diversity areideal.

When an individual is transferred to the wild, its geneticdiversity is added to the reintroduced population, butremoved from the captive population and both of theseeffects need to be assessed when evaluating individualsfor reintroductions.

Since survival in the natural habitat is expected to bemuch lower than in captivity, it is undesirable to depletethe captive population of genetically valuable individualsto benefit the wild population.

This is particularly important at the beginning of areintroduction program as mortality is frequently high.

Conversely, the reintroduced population is highly relatedto its source captive population and an otherwise idealreintroduction candidate may be closely related to individuals previously released and its introduction may actually reduce genetic diversity of reintroduced population.

How many reintroduced populations should be established?How many reintroduced populations should be established?

Where several suitable reintroduction sites and ampleexcess captive bred individuals are available, a number ofreintroduced populations should be established to maximize the numbers of reintroduced individuals.

This minimizes the loss of genetic diversity and willminimize inbreeding if individuals are translocated amongdifferent sites.

Additionally, several populations reduce the risk of extinction due to natural hazards, disease, & stochastisity.