Founder Number and Allelic Diversity - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 69

In reviewing recommendations on the number of founders to introduce
at a site, one gains the impression that, “demographic and genetic reasons
... support the conclusion that the founding population should be as large
as possible” (Guerrant 1996: 194), “that the default rule is bigger is better”
(Falk et al. 1996: 479), although practical considerations often place an upper
limit on this number. For example, the answer to “How many founders?”
is often constrained by the number of founders available (e.g., colonizers
taken from zoo, arboretum, or other limited breeding programs) or by
available habitat or other introduction resources. Obviously, one would
not want to reintroduce 10,000 turtles to 15 glade remnants each of which
has a carrying capacity of 30 individuals. Templeton (1996: 320) suggests
that when restoring animals to habitats with low carrying capacities (e.g.,
K < 100 individuals), “to minimize demographic stochasticity and genetic
drift, the release size should be close to the ultimate carrying capacity.”
Individuals would thus be introduced at approximately the average density
for the species. By exploring differing versions of such limited introductions,
estimation of the initial loss of unique alleles or heterozygosity relative to
a source population may be of value in planning introduction or future
management protocols.
Another way of thinking about this problem is the so-called Franklin/
Soul' 50/500 rule (e.g., see Meffe 1996: 53; or Finkeldey and Hattemer 2007:
123-127), which states that “a genetically effective population size of at
least 50 is necessary for short-term conservation of genetic diversity (i.e., a
few generations) and at least 500 is needed for long-term maintenance of
diversity.” Another interpretation of this rule is that a minimum effective
population size (Ne) of 50 would be needed to inhibit inbreeding depression,
and a size of 500 protects populations from diversity loss due to drift (Avise
1994: 370). Allendorf and Luikart (2007: 461) note, “Approximately 30 diploid
founders are required to have a 95% probability of sampling [at least one
copy of] an allele at frequency 0.05. However with 30 founders there is only
approximately a 45% probability of including an allele of frequency 0.01.”
On the basis of these conclusions, they go on to “recommend a minimum of
30 founders and preferably at least 50.” While it is widely understood that
the 50/500 rule is a convenient heuristic fi rst approximation, NEWGARDEN
allows us to explore the genetic diversity consequences of founding and
continuing populations in this size range when different source population
details and/or species life history characteristics apply.
A related issue concerns the founding of captive or arboretum
populations. For example, Frankham et al. (2004: 148) estimate that “if the
population is to encompass most of the genetic diversity in the wild and
minimize subsequent inbreeding, then a fully representative sample of at
least 20-30 unrelated contributing founders is required.” They go on to
estimate that 30 unrelated founders capture 98% of the source heterozygosity.