Conclusions and Discussion - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 315

or local barriers, for example. The population growth and genetic diversity
consequences of such differences can be modeled with NEWGARDEN.
In restoration projects, or for naturally establishing populations
identical except for the geometry of founder placement, geometry-
induced differences in rates of expansion of contrasting populations can
be pronounced, and can have important implications for the genetic and
demographic future of such populations.
We have shown that two populations, identical in all things except for
the geometric placement of founders, can lose low-frequency unique alleles
at differing rates. Of the two expanding populations, the one that loses such
alleles at a higher rate will lose copies of more frequent alleles at higher
rates, and as a consequence, will have loci exhibiting greater variance in
allele frequencies across replicate trials. The greater loss of low-frequency
alleles, and the greater loss of copies of alleles with higher frequencies, will
lead to greater variance in the probabilities of the occurrence of particular
multilocus genotypes (see Effects on Evolution, below).
NEWGARDEN can be used to assist in learning about several factors that
can infl uence the population genetics of developing plant populations.
Additional Issues
Many of the examples presented above demonstrate that even slight changes
in the spatial distribution of founders, often changes that do not include
differences in overall density but only of geometric shape of the founder
assemblage, can have profound effects on population growth and retention
of genetic diversity. Increasing the distance between founder individuals
to greater than the average equilibrium (carrying capacity) spacing of
individuals for a species can provide more inhabitable safe sites for the
establishment of offspring in the region of the founders, thereby increasing
population growth rate and the preservation of low-frequency unique
alleles. These population trends can persist through several successive
generations.
Differences in life history characteristics can infl uence the effects of
the spationumerics of founding on the genetic diversity of developing
populations. To give just one example of the importance of this issue, Allen et
al. (2001) recommend tree restoration plantings of seed or seedlings 0.9-1.8
m apart in rows separated by 2.7 m (Allen et al. 2001: 46-47), well above
the natural densities for most hardwoods. But they also note that losses of
75% to 90% or more of the planted individuals before reproductive maturity
are not uncommon. In a square planting of 800 individuals separated by
1.8 m with 25% reproductive survivorship, 200 individuals would remain
with an average distance among individuals of approximately 6 m, which