Dispersal - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 186

in centimetres or a few metres” (Fenner and Thompson 2005: 72; that book
reviews seed dispersal ecology). Even though that may usually be the case,
evidence also suggests that seeds of many such species are dispersed to
and establish at much greater distances at extremely low, but biologically
signifi cant, rates. Thus, there are many species with variously “fatter”
tails of the percentage of seeds dispersed to distant safe sites (e.g., Nathan
2006). Since NEWGARDEN modeling of the effects of different degrees of
long-distance dispersal cannot simply be done by extending one maximum
dispersal distance value, the input fi le allows the user to designate different
dispersal distance zones, each zone receiving a specifi ed percentage of the
dispersules.
For offspring, each dispersal zone refers to distance from the maternal
individual. As an example, suppose four zones are designated:
zone 1: dispersal within 5 grid units of the dispersing individual;
zone 2: dispersal within 6-12 units;
zone 3: dispersal within 13-21 units; and
zone 4: dispersal within 22-300 units.
In the NEWGARDEN input fi le, the user can not only specify the distance
limits for the zones, but also designate the percentage of times an offspring
will be dispersed to each of the zones. For example, if dispersal to zone 3
is given a probability of 12%, then in approximately 12 out of 100 matings,
a propagule will be dispersed to one randomly selected point within that
zone.
Microgamete Dispersal
The distances of microgamete dispersal (e.g., via pollen) are also generally
thought to follow leptokurtic distributions with variously fatter tails
depending on species and environmental conditions (e.g., Krauss et al.
2009). Whether genes are dispersed further via offspring or microgametes
varies greatly across species and circumstances. For microgamete dispersal,
the dispersal distance zones are specifi ed independently of the offspring
dispersal distance zones as follows.
As NEWGARDEN analyses proceed, when an individual is selected to
produce an offspring, a series of distance zones, as designated by the user,
are established around that individual (the target individual). Percentages
of dispersal from each distance zone are also input by the user that defi ne
the probability that, in a particular mating, a microgamete contributor will
be chosen at random from all eligible contributors in that zone. If a zone is
selected by NEWGARDEN, and no potential microgamete donor is available
in that zone, then the mating fails, although that failure is counted as one
of the scheduled matings.