Monitoring Endangered Species Populations: Gene Dispersal Can Have Pronounced Effects
on the Relationship between Census Size and Genetic Diversity
1933
been proposed that the Ne 50/500 numbers are too small,
and that populations with Ne sizes approaching at least
1000 to 5000 are needed in the long term to prevent
“mutational meltdown” stemming from the accumulation
of deleterious mutations in smaller populations (e.g., [8-
10]).
However, many factors contribute to the realized Ne
values and rates of drift for populations through time, in-
cluding variable mating systems, demographics, and gene
dispersal distances. Thus, estimating the Ne, and more im-
portantly for endangered plant conservation considera-
tions, the rates of loss of genetic diversity in natural po-
pulations, are difficult. Census size may be deceptive as
to the genetic diversity retained by a population, but, short
of extensive genetic surveys, how can a manager model
what has occurred in the population to gain an estimate
of current diversity retained?
Adding to the complexity of these considerations, for
many endangered species, obtaining and successfully es-
tablishing 500 or 5000, or even 50, genetically distinct
viable propagules can be a challenge, and restoration me-
thodologies that preserve genetic diversity in such small
populations are needed. But conducting sufficiently rep-
licated field experiments to determine general guidelines
on how best to manipulate newly founded populations to
secure stand maintenance and genetic variation going
forward is cost-prohibitive and not feasible, especially
given the array of life-history characteristics distributed
across the wide range of endangered plant species.
To statistically explore the above issues, we have pro-
duced the computer program NEWGARDEN [11,12],
which generates virtual plant populations that develop
through generations as conditioned by input parameters
concerning available habitat size and shape, the size and
geometric placement of the founding population, and the
life history characteristics of the study species. Future
generations develop via matings and offspring produc-
tion processes emerging from the initial input parameters.
Throughout a user-specified number of generations (bouts
of mating), the program provides statistically analyzable
output for the total population (including all age classes)
concerning population growth and genetic diversity (see
below). In the comparative trials presented here, we use
NEWGARDEN to explore, for an annual plant (lacking a
seed bank), how varying founder number and density,
combined with varying gene dispersal distances, can af-
fect population growth and genetic diversity of popula-
tions. Our results demonstrate that plant populations ini-
tiated with the same number of founders can attain iden-
tical subsequent population sizes that retain genetic di-
versity to widely varying degrees due to differences in
gene dispersal. Replicated plant preservation projects
may appear to have similar population sizes, but such
stands may have very different levels of genetic varia-
tion.
2. Materials and Methods
All trials were analyzed using NEWGARDEN version
2.2 (4apr2012). One set of user-defined input parameters
is called a “trial”. For each trial, the user specifies the
number of “runs”, that is, replicate re-runs of those par-
ticular trial input conditions. With output statistics being
produced and stored for each individual run, the ultimate
summary output (which is what we report below) is pro-
vided in the form of mean values across runs with stan-
dard deviation for each output value. In all of the trials
analyzed here, such summary output values (see below)
represent the mean of 100 replicate runs.
Except as noted below, the following input parameter
conditions were held constant across all trials (parame-
ters are discussed in order that they appear in input files).
Each individual was analyzed at 20 loci, each locus hav-
ing 100 alleles at equal frequencies in the source popula-
tion (frequency = 0.01). Each individual was cosexual.
The expected offspring production per generation per adult
was 1.017, distributed among eligible individuals accord-
ing to the Poisson distribution. All individuals could pro-
duce pollen. The species is an annual. The isolated pre-
serve is 2001 by 2001 grid points in size (carrying capac-
ity, or K = 44,004,001 grid points). Individuals can exist
only on grid points, which are meant to represent the ave-
rage minimum distance between individuals when the lo-
cal population is at K. If the distance between points here
is 1 m, then the preserve is approximately 2 km on a side.
Populations developed over 34 bouts of mating (genera-
tions) unless noted. The source population from which
alleles were drawn for the founders was at Hardy-Wein-
berg equilibrium.
Other conditions varied among trials. Although foun-
ders were always scattered randomly throughout the pre-
serve, comparative trials were initiated with either 1000
(high density) or 300 (low density) founders. For each
density condition, 14 trials were run which differed either
in the maximum distances from which pollen in a given
mating could arrive, or to which offspring were dispersed.
These differences in gene dispersal will be described
where results of comparative trials are discussed below.
To explain an example, in one trial, maximum offspring
dispersal distance might be five grid points with a pollen
dispersal maximum of 250 grid points. This means that
in a given randomly selected mating, offspring can be
dispersed to any point that is within five grid points in
the x or y direction (each axis distance randomly selected
separately), and pollen will come from a randomly se-
lected pollen source that is within 250 x and y grid points
of the selected pistillate individual. Again, x and y dis-
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