Conclusions and Discussion - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 314

Rates of population growth and genetic diversity retention in
comparative NEWGARDEN trials can be used to assess likely rates in
actual developing populations that are being monitored. NEWGARDEN
trials can be altered to more closely refl ect what has actually happened in
a developing population (e.g., growth rate or pattern of expansion). The
comparative NEWGARDEN trials can be used to establish “benchmarks”
of growth and diversity for the actual population in the coming years. Such
assessments can be used to design population manipulation plans that more
effectively meet conservation targets.
Although the 50/500 rule noted earlier (population size must be at
least 50 to prevent inbreeding and at least 500 to prevent random loss
of genetic diversity; e.g., Avise 1994: 370 or Meffe 1996: p. 53) refers to
effective population sizes, losses of genetic diversity in terms of decreasing
heterozygosity or loss of unique alleles, plus increasing rates of inbreeding,
continue for many of the NEWGARDEN trial populations examined earlier
even when census population numbers exceed 500 or more (in some of
our examples, > 5,000 census individuals). In some restoration projects, a
population of 500 potentially breeding individuals would be considered a
“large” population, but may still be capable of losing genetic diversity at
appreciable rates.
NEWGARDEN analyses such as those described in this topic can be used
to design “best practices” restoration and genetic diversity conservation
strategies. However, cost-benefi t analyses will have to take into account
which considerations are more important. For example, increasing distance
of establishment from a border may improve the growth and genetic
diversity retention of a population, but the costs of planting, monitoring,
maintaining, and manipulating populations may also increase with distance
of establishment from a border, or with subdivision of founders.
Comparative modeling of the establishment of a species over a range of
potential life history characteristics values is advisable to ensure that realistic,
seemingly slight changes in one of those values do not have pronounced
negative effects on population growth and diversity retention.
Comparative fi ndings under one set of conditions cannot necessarily be
generalized. Change in one input parameter, even if slight, may increase,
diminish, or reverse differences in population growth or genetic diversity
conservation revealed between comparative trials. Each species will have
to be analyzed separately, and for informative modeling it is crucial to
accumulate data that can be informative as to more accurate input conditions
(e.g., average species density, life history characteristics).
Even within species, various (sub-)populations will differ in population
growth rates and genetic diversity preservation because of local differences
in founder geometry, density levels dependent on resources, reproductive
values dependent on resources, wind effects on pollen dispersal distances,