Input - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 42

between grid points should not be thought of as demarcating absolute
meters or feet or some other metric. Rather, two adjacent grid points are
intended to represent the average distance between individuals when
the population is at equilibrium. If every grid point were to be occupied,
the population would be at K, the carrying capacity, and in an ideal
NEWGARDEN population, all individuals would have an equal distance
between immediate neighbors. In a sense, the grid points can be thought
of as the maximum density and availability of “safe sites” for dispersal
and establishment.
If the average distance between individuals of a species in a given
equilibrium habitat is known (the distance may vary from habitat to habitat
owing to environmental differences), then that distance is the same as the
distance between NEWGARDEN grid points. Determining estimates of
this distance involves consideration of several factors and practitioners
interested in using NEWGARDEN should consult references dealing with
ecological density sampling for details on recommended methodologies
(e.g., Diggle 1983; Upton and Fingleton 1985; McGarvey et al. 2005). Suffi ce
it to say here that merely counting individuals in an area and dividing by
some measure of the total grid points in the sample area (e.g., measured
in meters or decimeters) may neglect patchiness in distribution due to
environmental and historical factors, and that some form of averaged
nearest-neighbor estimation may be more applicable. To determine safe
site density, reference to standing or historical populations of the species
to be introduced, or to similar species, can give a measure of the density at
which the species normally establishes.
Once an estimate of average individual density for the population
has been obtained, it can be compared to independently estimated
offspring and microgamete dispersal distance probability frames (as
described above) to determine the NEWGARDEN grid point frame
distance values to be entered for seeds and pollen. For example, suppose
that it is determined that the average distance between two mature sugar
maple trees is 10 m for a particular habitat. If the estimate for offspring
establishment is that, say, 95% of a tree's offspring establish within 60 m of
a tree (again, under NEWGARDEN conventions, offspring establishment
refers to offspring that establish to become reproductive, unless deleted
at random according to user-prescribed mortality schedules), then 95% of
the offspring will generally establish within 6 NEWGARDEN grid units
of the parent. Similarly, once actual distances for microgamete dispersal
have been estimated, they can be compared with average distance between
individuals to arrive at NEWGARDEN grid point estimates to be entered
for microgamete dispersal. Note that the grid point distance frame values
entered for offspring and microgamete dispersal are best thought of as limits
within which some percentage (e.g., 95%) of the dispersal has taken place.