Conclusions and Discussion - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 336

higher rates of population growth. NEWGARDEN modeling can assist in
developing improved approaches to such interacting and often confl icting
issues.
In planning preservation and restoration projects, primary issues
always include the number of founders to be introduced, or the effects
of different population sizes. The organization most widely recognized
for the classifi cation of endangered species is the International Union
for Conservation of Nature and Natural Resources (IUCN: http://www.
iucnredlist.org/info/categories_criteria2001#introduction) . One of their
criteria for listing a species as either critically endangered or endangered
is that the census population size is estimated to number fewer than 50 or
250 mature individuals, respectively. In the many examples of developing
populations presented in this topic, several cases can be found in which
unique alleles are still being lost at signifi cant rates even when growing
populations exceed 250 individuals. As noted earlier, Ne is the ideal
population size, and if a census population (Nc) is behaving, in terms of
diversity maintenance, as though it were an ideal population, then Ne =
Nc. However, it is commonly the case for populations that the realized
Ne is much less than the Nc. For example, in animals, the average ratio
of Ne/Nc is approximately 0.1 (Frankham 1995). Recalling the Franklin/
Soule 50/500 rule, a minimum effective population size of 50 would be
needed to inhibit inbreeding depression, and a size of Ne= 500 protects
populations from diversity loss due to drift. This translates to an Nc of
5000 individuals to protect against loss due to drift. Since that number is
an average, numerous populations will require many more individuals
to prevent drift. NEWGARDEN analyses can assist in determining the
minimum census population sizes at which drift becomes negligible, or the
amount of drift occurring in a monitored population, through modeling
that population's history with an array of user-modifi able founder and life
history input conditions.
The many previous NEWGARDEN sample trials demonstrate that it
is imperative to gather as much accurate information as possible regarding
various life history attributes of a species to be restored or manipulated
before embarking on comparative NEWGARDEN trial analyses aimed
at improving conservation/restoration strategies. For example, in the
majority of analyses, placing founders at greater species-specifi c, average
reproductive density unit distances from one another promoted population
growth rate and genetic diversity retention. But there may be unfavorable
consequences of excessive distance between adjacent individuals (Allee
effects), whereby population growth will decline, at least initially, leading
to increased genetic diversity loss. Increasing the distance between founders
may also lead to higher F values (inbreeding). Further, these density
considerations are affected by numerous species-specifi c characteristics