Input - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 44

purpose here. In general, for a great many organisms, dispersal is thought
to be leptokurtic, with the vast majority of the dispersules “landing” nearer
the parent and relatively few traveling further and even fewer reaching
very great distances. For example, most seeds are thought to be dispersed
only a few meters, or a few tens of meters at most (Howe and Smallwood
1982; Cain et al. 2000; Grace et al. 2004; and references). Cain et al. (2000)
considered seed dispersal to be a rare long-distance event if it exceeded 100
m (and see Nathan 2006). Likewise, microgamete dispersal is thought, for
many organisms, to be within the nearest tens of neighbors (e.g., for pollen,
see Dow and Ashley 1998 and references). However, for some organisms,
such generalizations are clearly not applicable (e.g., some tropical rainforest
species).
Further, for certain aspects of population biology, rare long-distance
dispersal may have disproportionate effects (e.g., immigrant genes reducing
population subdivision), although the importance of these effects is debated
(e.g., see Howe and Smallwood 1982). With the advent of more sophisticated
techniques for detecting long-distance dispersal (e.g., molecular genetics
analyses, seed isotope labeling and tracking), its importance is perhaps
coming to be seen as greater than previously thought (e.g., compare Cain
et al. 2000 with Levin and Kerster 1974). It is clear that we still do not
understand the “tail” of the dispersal curve for most organisms. For most
organisms then, both the dispersal of established offspring (e.g., not just
seed dispersal) and realized microgamete dispersal (dispersal resulting in
established offspring) can be extremely variable, being affected by organism
density, habitat closure, habitat patchiness, availability of dispersal vectors,
temperate versus tropical organisms, year-to-year environmental variation,
and numerous other factors. The realism of NEWGARDEN modeling will
ultimately depend on the values attributed to the species under study by the
user, and it will thus be most informative to examine trials with reasonable
potential input extremes.
When these extremes are assigned, the grid distances of offspring
versus microgamete dispersal will likely differ from one another. For
example, offspring establishment distance may be much greater than the
latter, as in some wind-pollinated plants with bird-dispersed fruit. Or, as
an example of the reverse, consider the Touch-me-not ( Impatiens ), in which
pollen microgametes can be dispersed tens of meters or more by bees or
hummingbirds while seeds are dispersed within 1 m or so of a parent by
elastic, coiling explosive dehiscence. When a species is reintroduced, other
species that normally would have affected these dispersal distances (e.g., a
predator or seed or pollen vector) may be missing or otherwise out of normal
balance. NEWGARDEN allows one to run different trials with differing
ratios of mega- and micro-dispersule distances. As will be seen, variation
of such ratios can have interesting effects on genetic diversity.