Transcript Chapter 16 Genetics and Management of Wild Populations

Chapter 16
Genetics and Management of Wild Populations
Although genetic issues are of critical importance, there
has been limited application of genetics in the practical
management of threatened taxa in natural populations.
Key Genetic Contributions to Conservation Biology:
Resolution of taxonomic uncertainties such that
managers are confident of the status of, and
relationships among, the taxa they strive to maintain.
Delineation of any distinct management units within
species, as biologically meaningful entities for
conservation.
Recognition that the effective sizes of populations,
that determine the genetic future of populations, are
frequently about an order of magnitude lower than the
census size.
Detection of declines in genetic diversity.
Development of theory to describe past, and predict
future, changes in genetic variation. All such categories
have a common, central theme -- genetic diversity is
dependent upon effective population size (Ne).
Recognition that the genetic diversity underlying the
quantitative variation in reproductive fitness is the
raw material for adaptive evolution through natural
selection. Loss of this class of genetic variation reduces
the capacity of populations to evolve in response to
environmental change.
Direct evidence for inbreeding depression in endangered
species in natural habitats.
At a practical level, potential inbreeding depression may
be inferred from its correlation with reduction in genetic
variation, assessed by markers.
Degree of fragmentation and rates of gene flow can be
inferred from the distribution of genetic markers
within and among population and calculation of various
population genetic statistics.
Approximate Order of Genetic Management Actions
for Wild Populations:
Resolve any taxonomic uncertainties and delineate
management units.
Increase population size
Diagnose genetic problems
Recover small, inbred populations with low genetic
diversity in naturally outbreeding populations.
Genetically manage fragmented populations.
Resolving Taxonomy and Management Units: The first
step in the genetic management of wild population of
threatened species is to ensure that the species’
taxonomy is correctly assigned and that any distinct
management units are defined -- Go back and review the
Dusky Seaside Sparrow.
Increasing Population Size: Once any taxonomic
uncertainties are resolved, the next step is to halt
decline and increase the size of populations.
This alleviates all the stochastic threats to species.
If populations have only recently declined from larger
size, say 50 to several hundred, then the genetic
impacts are usually minimal.
Despite the bottleneck, short-term reductions of this
magnitude allow little opportunity for variation to be lost
through genetic drift.
This step, increasing population size, is in the domain of
wildlife biologists and ecologists -- identification of the
cause of the decline.
While genetic information may help to alert conservation
biologists to the extent of endangerment, the
management action at this step involves little, or no,
genetic data.
However, recovery in numbers of highly inbred populations
can be substantially enhanced following introduction of
additional genetic variation.
Insecure wild populations can be augmented using
captive bred individuals.
The nene has been subjected to a long program of
augmentation from captivity as its wild population
does not appear to be self-sustaining.
Such programs may be counterproductive in the long-term
if the captive population adapts to reproduction in
captivity and its reproductive ability in the wild is
reduced.
Such is clearly a problem in fish where long-term
captive populations, used to stock wild habitats, have
lower reproductive fitness in the wild than residents.
Diagnosing Genetic Problems -- A necessary precursor to
genetic management of wild populations of threatened
species is to diagnose their status. We need to answer
three questions.
1. Has a threatened species/population lost genetic
diversity?
2. Is it suffering from inbreeding depression?
3. Is it genetically fragmented?
Answering these three questions have been the main
genetic contributions to the conservation of wild
populations thus far. However, using this information to
plan conservation management is still in its infancy.
We will now consider the genetic management actions
that should be taken to alleviate genetic problems.
Recovering Small Inbred Populations with Low Genetic
Diversity: An effective management strategy in the
recovery of small inbred populations with low genetic
diversity is to introduce individuals from other populations
to improve their reproductive fitness and restore genetic
diversity.
There is extensive experimental evidence that this
approach can be successful.
For example, it improved fitness in natural populations of
greater prairie chickens, Sweddish adders and a desert
topminnow fish.
In spite of the clear benefits of outcrossing to recover
small, inbred populations, there are very few cases
where it is being done.
Source 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 but
genetically differentiated from the population to which
they are being introduced.
When no unrelated individuals of the same taxon are
available, individuals from another sub-species can be
used to alleviate inbreeding depression.
This has been done for the Florida panther and the
Norfolk Island boobook owl.
If an endangered species exists as only a single
population, then the only possible source of additional
genetic material is from an unrelated, interfertile
species.
For example, American Chestnuts have been crossed to
Chinese Chestnuts to introduce genetic variation for
resistance to blight that severely depleted the American
Chestnut.
China is the source of the blight disease and the Chinese
chestnut possesses resistance.
The option of crossing a threatened species to a related
species requires very careful consideration. The potential
benefits need to be very large, as there may be serious
risk of outbreeding depression.
Management of Species with a Single Population Lacking
Genetic 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 has
been reduced is useful only as an indication of the
fragility of the species.
The lower the genetic diversity, the lower becomes the
evolutionary potential, and the higher becomes the
probability that the species has compromised ability to
cope with changes in its physical or biotic environment.
For fragile species, management regimes should be
instituted to:
Increase population size.
Establish populations in several locations to minimize
the risk of catastrophes.
For example, in the case of the black-footed ferret, where
there is low genetic diversity, the recovery plan calls for
re-establishment of 10 wild populations, in different
locations, to minimize the risks of disease and other
environmental catastrophes.
Maximize their reproductive rate by improving their
environment (e.g., removing predators and competitors).
Insulate them from environmental change. This should
include quarantining from introduced diseases, pests,
predators and competitors, and monitoring, so that
remedial action can be initiated as soon as new
environmental threats arise.
Genetic Management of Fragmented Populations:
Many threatened species have fragmented habitats and
the management options for these fragmented
populations is to maximize genetic diversity and
minimize inbreeding and extinction risks through:
Increasing the habitat area.
Increasing the suitability of available habitat
Artificially increase the migration rate via translocation
however, translocation of individuals among populations
may be costly, especially for large animals, and carries the
risks of injury, disease transmission and behavioral
disruption when individuals are released.
For example, introduced males lions regularly kill cubs.
Furthermore, sexually mature males of many species may
kill intruders.
The cost of translocations can be reduced by artificial
insemination for species where this technique has been
perfected.
Re-establish populations in suitable habitat where they
have gone extinct.
Create habitat corridors.
Corridors among habitat fragments can re-establish gene
flow among isolated populations.
Species vary in their requirements for a corridor to be
an effective migration path.
The most ambitious proposal of this kind is
“The Wildlands Project” which has the purpose of
providing corridors from north to south in North
America.
These corridors will link existing reserves and surround
both reserves and corridors with buffer zones that are
hospitable to wild animals and plants.
The time frame for achieving this goal is hundreds of
years, given the political, social, and financial
challenges.
Managing Gene Flow
This involves considerable complexity as many issues must
be 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 population
be genetically monitored.
Since there are so many variables to optimize, computer
projections will often be required to define and
redefine the required management.
The objective is to identify a regime that maintains
genetically viable populations with acceptable costs
that fits within other management constraints.
Re-establishing Extinct Populations
To maximize population sizes and minimize extinction risks,
populations that have become extinct should be
re-established from extant populations, if the habitat
can still support the species.
The important questions is “Which populations should be
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 genetic
differentiation among extant populations, as is common
in many plant species, the translocated individuals should
normally come from populations most likely to be best
adapted to the reintroduced habitat.
This is frequently the geographically closest population.
Care should be taken when island populations are being
considered as source populations for translocation as
they typically have low genetic variation and are
inbred.
Genetic diversity in populations available for restocking
should be compared and the most diverse populations
with the highest reproductive fitness, or a crossing among
populations, chosen.
Genetic Issues in Reserve Design:
There are many biological, ecological, political, and genetic
considerations to balance when considering the design of
nature reserves.
It has been suggested that the following three steps
need to be involved in the ecology and genetics of
reserve design:
1. Identify target, or keystone species, whose loss would
significantly decrease the value of biodiversity in the
reserve.
2. Determine the minimum population size needed to
guarantee a high probability of long-term survival for
the 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 genetically
viable population?
Remember from previous discussions that Ne is usually
0.1 that of N, thus to design a reserve to maintain
an effective population size of several hundred, you
would need several thousand individuals?
Is the species adapted to the habitat of the reserve?
Should there be one large reserve, or several smaller
reserves?
This relates to the “Single Large vs. Several Small
(SLOSS) reserves.
In general, a single large reserve is more desirable from
the genetic point of view, if there is a risk that populations
in small reserves will become extinct (a likely scenario for
many species).
However, protection against catastrophes dictates that
more than one reserve is preferable, or even obligatory.
The best compromise is to have more than one sizeable
reserve, but to ensure that there is natural or artificial
gene flow among them.
In practice, the choice of reserves has often been a
haphazard process, determined more by local politics,
alternative land uses and the need for reserves to
serve multipurposes, than biological principles.
Introgression and Hybridization
Introgression is the flow of alleles from one species, or
subspecies, to another.
Typically, hybridization occurs when humans introduce
exotic species into the range of rare species, or alter
habitat so that previously isolated species are now in
secondary contact.
Introgression is a threat to the genetic integrity of a
range of canid, duck, fish, plant, and other species.
Options for eliminating introgression include eliminating
the introduced species, or translocating “pure”
individuals into isolated regions or into captivity.
Impacts 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 hunting
moose and white-tailed deer would severely reduce genetic
diversity within short periods.
Hunting regimes reduced the effective population size by
64% to 79% in moose and 58% to 65% is deer, depending
upon the hunting regime and assumptions made.
Poaching has had a devastating effect on sex-ratio and
reproductive rate in Asian elephants.
Many wild species are selectively harvested by humans
who 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 pressure
that change the phenotype of the species, conflicting with
forces of natural selection and reducing the overall fitness
of populations.
For example, males with large antlers are favored prey
of hunters.
This is expected to select for smaller antlers, conflicting
with natural selection favoring large antlers in males.
As harvested species often occur in large, though
frequently declining numbers, an option is to preserve a
proportion of the population from harvest.
In this way, fully wild stock is maintained to introduce
into harvested areas so that the genetic impacts of
harvest are reduced.