Dispersal - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 178

have dispersal distances that exceed average species founding densities, a
characteristic not needed for species that can facultatively self.
Equal Offspring Dispersal Distances and Spatial Subdivision of
172 Founders
In the previous examples exploring the effects of dispersal distance, the
172 founders were always placed in a square in the center of the preserve.
While, in many cases, central placement will minimize the risk of genetic
edge effects and loss of reproductive effort stemming from seeds dispersed
outside the preserve, it may also maximize the required human restoration
effort in terms of, for example, travel time, resource and equipment
movement, provisioning, and future monitoring time. Can more optimal,
less effort-intensive reintroduction designs be devised? Such designs might
include other establishment-promoting issues. For example, subdividing
the 172 founders into small groups that are introduced at some distance
from one another provides a type of “bet hedging” insurance: a fi re or
pathogen attack that might destroy all centrally and contiguously placed
individuals may not have such devastating effects when founders are placed
in widely separated subpopulations. Further, in the earlier chapter on edge
effects, dispersal distance was always held constant. What happens to the
intensity of edge effects as the founding population becomes subdivided
and dispersal distances increase?
The following comparisons of NEWGARDEN trial populations are
given as examples of how these matters can be explored. The many possible
permutations of life history features preclude an exhaustive study of these
topics, and only a few examples will be presented to demonstrate how
comparative analyses can be carried out. When more exacting life history
information is available for a given species targeted for introduction, these
issues can be explored more precisely.
As with the previous dispersal examples, in the following examples
demonstrating possible effects of subdivision, the NEWGARDEN
populations will all have the following identical input conditions:
Thirty loci, each with 100 different alleles of equal frequency = 0.01. A total
of 3,000 unique alleles are thus available across these loci.
Bisexual annuals.
Offspring reproduction rate = 1.4, with Poisson distribution across
reproducers.
Selfi ng rate is 0 with Random Mating value = false: these are completely
self-incompatible populations.