Conclusions and Discussion - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 316

would still be at relatively high densities for a tree species, but not for many
annuals or short-lived perennials.
A major issue concerning the realism of NEWGARDEN modeling
is, does such density dependence occur in nature? Comita et al. (2010)
provide evidence that distribution pattern and abundance of most plant
species is often controlled by intraspecifi c density dependence. However,
the intensity of such dependence will vary from one species to another and
from one ecological situation to another, as well as depend on differences in
life history characteristics. For example, some species often disperse seeds
very locally and there is intense competition for space immediately around
a parent (e.g., Impatiens capensis with explosive dehiscence seed dispersal,
often of less than 1 m). In such cases where intraspecifi c competition may
be intense, introducing founders at densities lower than normally occur
for mature individuals of the species may increase rates of population
growth. But how about tropical lowland rain forest trees with average
densities of 1 to 2 reproductive individuals per hectare? Does this mean
rain forest tree founders should be placed further apart, say 5 ha? Perhaps
where competition dependent on intraspecifi c seed-seedling density, or
other effects, are not high, or where environmental conditions or history
have attenuated population density, such effects must be taken into account
when introducing founders. In these cases, it might be important to survey
to fi nd situations where the individuals of the species are at their most
dense, and use that density as the measure for modeling grid distances.
In making such assessments, one must also consider the effect of other life
history characteristics, such as seed versus pollen dispersal distance, on
genetic diversity retention.
Enormous effort has been expended in analyzing patterns of genetic
diversity across natural plant populations, and to provide evolutionary
explanations for those patterns. When comparing patterns of demography
and genetic diversity among natural populations, differences in initial
spationumeric deployment of founders should now be considered as one
factor that may contribute to interpopulational differences. Two “identical”
habitats may have identical overall founder density or seeding rates, but
changes in geometries of founding can lead to profound demographic and
genetic differences. These changes can sometimes be rather subtle and still
generate pronounced differences in population growth and diversity. Such
initial differences among populations may result in differing trajectories
for overall genetic diversity and local genetic diversity patterning, and
the consequences of such effects may persist once differentially affected
populations achieve more or less equal, large numbers of individuals.
Two stands may appear to be very similar demographically, but may
differ profoundly in genetic diversity because of spationumeric effects of
founder geometry alone. Differing trajectories of diversity maintenance