Chapter 16Chapter 16Genetics and Management of Wild PopulationsGenetics and Management of Wild Populations

Although genetic issues are of critical importance, therehas been limited application of genetics in the practicalmanagement of threatened taxa in natural populations.

Key Genetic Contributions to Conservation Biology:Key Genetic Contributions to Conservation Biology:

Resolution of taxonomic uncertainties such that managers are confident of the status of, andrelationships among, the taxa they strive to maintain.

Delineation of any distinct management units withinspecies, as biologically meaningful entities for conservation.

Recognition that the effective sizes of populations,that determine the genetic future of populations, arefrequently about an order of magnitude lower than thecensus size.

Detection of declines in genetic diversity.

Development of theory to describe past, and predictfuture, changes in genetic variation. All such categorieshave a common, central theme -- genetic diversity isdependent upon effective population size (Ne).

Recognition that the genetic diversity underlying thequantitative variation in reproductive fitness is the raw material for adaptive evolution through naturalselection. Loss of this class of genetic variation reducesthe capacity of populations to evolve in response toenvironmental change.

Direct evidence for inbreeding depression in endangeredspecies in natural habitats.

At a practical level, potential inbreeding depression maybe inferred from its correlation with reduction in geneticvariation, assessed by markers.

Degree of fragmentation and rates of gene flow can beinferred from the distribution of genetic markers within and among population and calculation of variouspopulation genetic statistics.

Approximate Order of Genetic Management ActionsApproximate Order of Genetic Management Actionsfor Wild Populations:for Wild Populations:

Resolve any taxonomic uncertainties and delineatemanagement units.

Increase population size

Diagnose genetic problems

Recover small, inbred populations with low geneticdiversity in naturally outbreeding populations.

Genetically manage fragmented populations.

Resolving Taxonomy and Management UnitsResolving Taxonomy and Management Units: The firststep in the genetic management of wild population ofthreatened species is to ensure that the species’taxonomy is correctly assigned and that any distinctmanagement units are defined -- Go back and review theDusky Seaside Sparrow.

Increasing Population SizeIncreasing Population Size: Once any taxonomic uncertainties are resolved, the next step is to haltdecline and increase the size of populations.

This alleviates all the stochastic threats to species.

If populations have only recently declined from largersize, say 50 to several hundred, then the genetic impacts are usually minimal.

Despite the bottleneck, short-term reductions of thismagnitude allow little opportunity for variation to be lostthrough genetic drift.

This step, increasing population size, is in the domain ofwildlife biologists and ecologists -- identification of thecause of the decline.

While genetic information may help to alert conservationbiologists to the extent of endangerment, themanagement action at this step involves little, or no, genetic data.

However, recovery in numbers of highly inbred populationscan be substantially enhanced following introduction of additional genetic variation.

Insecure wild populations can be augmented usingcaptive bred individuals.

The nene has been subjected to a long program ofaugmentation from captivity as its wild populationdoes not appear to be self-sustaining.

Such programs may be counterproductive in the long-termif the captive population adapts to reproduction in captivity and its reproductive ability in the wild isreduced.

Such is clearly a problem in fish where long-termcaptive populations, used to stock wild habitats, havelower reproductive fitness in the wild than residents.

Diagnosing Genetic ProblemsDiagnosing Genetic Problems -- A necessary precursor togenetic management of wild populations of threatenedspecies is to diagnose their status. We need to answerthree questions.

1. Has a threatened species/population lost geneticdiversity?

2. Is it suffering from inbreeding depression?3. Is it genetically fragmented?

Answering these three questions have been the maingenetic contributions to the conservation of wildpopulations thus far. However, using this information toplan conservation management is still in its infancy.

We will now consider the genetic management actionsthat should be taken to alleviate genetic problems.

Recovering Small Inbred Populations with Low GeneticRecovering Small Inbred Populations with Low GeneticDiversityDiversity: An effective management strategy in the recovery of small inbred populations with low geneticdiversity is to introduce individuals from other populationsto improve their reproductive fitness and restore geneticdiversity.

There is extensive experimental evidence that thisapproach can be successful.

For example, it improved fitness in natural populations ofgreater prairie chickens, Sweddish adders and a deserttopminnow fish.

In spite of the clear benefits of outcrossing to recoversmall, inbred populations, there are very few cases where it is being done.

Source of Unrelated Individuals for Genetic AugmentationSource of Unrelated Individuals for Genetic Augmentation:

The individuals chosen for introduction into inbred populations, for recovery of fitness and genetic diversity,may be either outbred (if available), or inbred butgenetically differentiated from the population to which they are being introduced.

When no unrelated individuals of the same taxon areavailable, individuals from another sub-species can beused to alleviate inbreeding depression.

This has been done for the Florida panther and theNorfolk Island boobook owl.

If an endangered species exists as only a singlepopulation, then the only possible source of additionalgenetic material is from an unrelated, interfertilespecies.

For example, American Chestnuts have been crossed toChinese Chestnuts to introduce genetic variation forresistance to blight that severely depleted the AmericanChestnut.

China is the source of the blight disease and the Chinesechestnut possesses resistance.

The option of crossing a threatened species to a relatedspecies requires very careful consideration. The potentialbenefits need to be very large, as there may be seriousrisk of outbreeding depression.

Management of Species with a Single Population LackingManagement of Species with a Single Population LackingGenetic DiversityGenetic Diversity:

From a genetic perspective, the worst situation is where an endangered species exists as a single, inbred population with no sub-species or related species with which to hybridize.

Information on the level to which genetic diversity hasbeen reduced is useful only as an indication of thefragility of the species.

The lower the genetic diversity, the lower becomes theevolutionary potential, and the higher becomes theprobability that the species has compromised ability tocope with changes in its physical or biotic environment.

For fragile species, management regimes should beFor fragile species, management regimes should beinstituted to:instituted to:

Increase population size.

Establish populations in several locations to minimizethe risk of catastrophes.

For example, in the case of the black-footed ferret, wherethere is low genetic diversity, the recovery plan calls forre-establishment of 10 wild populations, in different locations, to minimize the risks of disease and otherenvironmental catastrophes.

Maximize their reproductive rate by improving theirenvironment (e.g., removing predators and competitors).

Insulate them from environmental change. This shouldinclude quarantining from introduced diseases, pests,predators and competitors, and monitoring, so thatremedial action can be initiated as soon as newenvironmental threats arise.

Genetic Management of Fragmented Populations:Genetic Management of Fragmented Populations:

Many threatened species have fragmented habitats andthe management options for these fragmentedpopulations is to maximize genetic diversity andminimize inbreeding and extinction risks through:

Increasing the habitat area.

Increasing the suitability of available habitat

Artificially increase the migration rate via translocationhowever, translocation of individuals among populationsmay be costly, especially for large animals, and carries therisks of injury, disease transmission and behavioraldisruption when individuals are released.

For example, introduced males lions regularly kill cubs.

Furthermore, sexually mature males of many species maykill intruders.

The cost of translocations can be reduced by artificialinsemination for species where this technique has beenperfected.

Re-establish populations in suitable habitat where theyhave gone extinct.

Create habitat corridors.

Corridors among habitat fragments can re-establish geneflow among isolated populations.

Species vary in their requirements for a corridor to bean effective migration path.

The most ambitious proposal of this kind is “The Wildlands ProjectThe Wildlands Project” which has the purpose ofproviding corridors from north to south in NorthAmerica.

These corridors will link existing reserves and surroundboth reserves and corridors with buffer zones that arehospitable to wild animals and plants.

The time frame for achieving this goal is hundreds ofyears, given the political, social, and financialchallenges.

Managing Gene FlowManaging Gene Flow

This involves considerable complexity as many issues mustbe addressed including:

Which individuals to translocate?How many individuals should be translocated?How often should translocation occur?What are the source and recipient populations for

translocation?When should translocation begin and stop?

Answering these questions requires that the populationbe genetically monitored.

Since there are so many variables to optimize, computerprojections will often be required to define andredefine the required management.

The objective is to identify a regime that maintainsgenetically viable populations with acceptable coststhat fits within other management constraints.

Re-establishing Extinct PopulationsRe-establishing Extinct Populations

To maximize population sizes and minimize extinction risks,populations that have become extinct should bere-established from extant populations, if the habitatcan still support the species.

The important questions is “Which populations should beWhich populations should beused to re-establish extinct populations?used to re-establish extinct populations?”

To minimize inbreeding and maximize genetic diversity,the re-founding population should be sampled from most, or all, extant populations.

However, where there is evidence of adaptive geneticdifferentiation among extant populations, as is commonin many plant species, the translocated individuals shouldnormally come from populations most likely to be bestadapted to the reintroduced habitat.

This is frequently the geographically closest population.

Care should be taken when island populations are beingconsidered as source populations for translocation asthey typically have low genetic variation and are inbred.

Genetic diversity in populations available for restockingshould be compared and the most diverse populationswith the highest reproductive fitness, or a crossing amongpopulations, chosen.

Genetic Issues in Reserve DesignGenetic Issues in Reserve Design:

There are many biological, ecological, political, and geneticconsiderations to balance when considering the design ofnature reserves.

It has been suggested that the following three stepsneed to be involved in the ecology and genetics ofreserve design:

1. Identify target, or keystone species, whose loss wouldsignificantly decrease the value of biodiversity in thereserve.

2. Determine the minimum population size needed toguarantee a high probability of long-term survival forthe species.

3. Using known populations densities of these species, estimate the area required to sustain minimum numbers.

The genetic issues in reserve design are:

Is the reserve large enough to support a geneticallyIs the reserve large enough to support a geneticallyviable populationviable population?

Remember from previous discussions that Ne is usually0.1 that of N, thus to design a reserve to maintainan effective population size of several hundred, you would need several thousand individuals?

Is the species adapted to the habitat of the reserve?Is the species adapted to the habitat of the reserve?

Should there be one large reserve, or several smallerShould there be one large reserve, or several smallerreserves?reserves?

This relates to the “Single Large vs. Several Small(SLOSS) reserves.

In general, a single large reserve is more desirable fromthe genetic point of view, if there is a risk that populationsin small reserves will become extinct (a likely scenario formany species).

However, protection against catastrophes dictates thatmore than one reserve is preferable, or even obligatory.

The best compromise is to have more than one sizeablereserve, but to ensure that there is natural or artificialgene flow among them.

In practice, the choice of reserves has often been ahaphazard process, determined more by local politics,alternative land uses and the need for reserves to serve multipurposes, than biological principles.

Introgression and HybridizationIntrogression and Hybridization

Introgression is the flow of alleles from one species, orsubspecies, to another.

Typically, hybridization occurs when humans introduceexotic species into the range of rare species, or alterhabitat so that previously isolated species are now insecondary contact.

Introgression is a threat to the genetic integrity of arange of canid, duck, fish, plant, and other species.

Options for eliminating introgression include eliminatingthe introduced species, or translocating “pure”individuals into isolated regions or into captivity.

Impacts of HarvestingImpacts of Harvesting

Many species of wildlife and plants are harvested.

This may alter effective population size, genetic diversity, and generation length.

Usually, the effects are deleterious genetically.

For example, Ryman et al. (1981) showed that huntingmoose and white-tailed deer would severely reduce geneticdiversity within short periods.

Hunting regimes reduced the effective population size by64% to 79% in moose and 58% to 65% is deer, dependingupon the hunting regime and assumptions made.

Poaching has had a devastating effect on sex-ratio andreproductive rate in Asian elephants.

Many wild species are selectively harvested by humanswho favor particular phenotypes within populations.

These include elephants, rhinoceroses, deer, moose, fish,whales, crustaceans, forest trees, and many other plants.

Such selective harvesting may result in selection pressurethat change the phenotype of the species, conflicting withforces of natural selection and reducing the overall fitnessof populations.

For example, males with large antlers are favored preyof hunters.

This is expected to select for smaller antlers, conflictingwith natural selection favoring large antlers in males.

As harvested species often occur in large, thoughfrequently declining numbers, an option is to preserve aproportion of the population from harvest.

In this way, fully wild stock is maintained to introduceinto harvested areas so that the genetic impacts ofharvest are reduced.