The Rate of Reproduction - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 153

Figure 10.10A shows comparisons of growth factors after fi ve rounds
of reproduction for populations developing from different numbers of
founders and with different rates of reproduction (r), using the other
conditions described above. The results indicate that the highest growth
factors (not absolute census numbers) were achieved when fewer founders
were used, presumably because of decreased competition among offspring
for available safe sites (more grid points available for colonization).
However, this increase in relative population to founder growth rate with
fewer founders comes with the cost of increased losses of heterozygosity,
especially at lower values of r (Fig. 10.10B). A manager attempting to
achieve the highest growth rate by greatly subdividing the founders must
be willing to accept such losses.
Further, Fig. 10.11 shows that the number of unique alleles saved per
original founder is greater, at generation 5 under the given conditions, when
fewer founders are planted (graph A). In other words, more unique alleles
are preserved per unit effort expended in establishing founders. The number
of alleles saved per founder at generation 5 increases with increasing values
of r. Another way of considering this issue, however, is to examine what
percentage of the unique alleles brought by the founders still remain at a
given generation. Figure 10.11B depicts the percentage of founder unique
alleles remaining after fi ve rounds of mating for the populations with
increasing numbers of founders, and increasing r values, that have just
been under discussion. The lowest percentage is retained at low r values
and when the number of founders is low (trial population a). The greatest
percentages are retained when 400 founders are used (e.g., populations w,
x, y,...), and changing r does not greatly affect that percentage.
The above considerations involving spatial distribution and changing
values of r (for example, how growth factor, heterozygosity, unique alleles
retained per founder, or percentage of the original unique alleles preserved
across generations can change relative to the number of founders) lead to
such questions as: if one has only the resources and funding to establish
a population from 80 seeds for a restoration project, other than ensuring
that plants are within gamete exchange distance of at least some other
individuals, can the geometric pattern of introducing those 80 founders
affect the preservation of genetic diversity? Perhaps one is comparing the
rates of expansion of different populations. Or suppose two different natural
populations are established by 80 founders. Do differences in the geometry
of introduction infl uence the future population genetics trajectories of the
stands? For instance, since the growth factor is higher for smaller numbers
of founders, is population growth increased by dividing the 80 founders
into 8 smaller, more isolated groups of 10?
To examine these issues, consider a comparison of two trials (S versus
L) that are identical in the following characteristics: