Input - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 32

Pollen Rate: Some Theoretical Considerations
The NEWGARDEN input fi le statements affecting pollen rates allow the
user to specify different degrees of likelihood of being a pollen contributor
depending on location and age class. Information on this topic is lacking for
most species, so the user will again need to make an educated guess. For
many organisms, however, the chances of being a microgamete contributor
are highest at maturity, being lower at younger or, with senescence, older
ages. In younger trees, for example, the height and volume of the crown
and resources available to create fl owers all act to lower pollen availability
relative to mature trees. As trees become overmature, their productivity may
drop, possibly lowering their relative pollination potential. Some perennial
individuals may become active as microgamete donors at earlier ages than
individuals become zygotic offspring producers. Some plants (e.g., Arisaema
spp.) and animals (e.g., fi sh) may have individuals that effectively change
sex throughout ontogeny. If separation in these phases is complete and
dependent on age, its effects can be modeled by assigning microgamete
versus zygotic offspring contribution capacities at distinctly different ages.
The random selection of the microgamete donor according to the relative
eligibility schedules, conditioned by age and/or distance, of all possible
donors refl ects the natural random variation in successful microgamete
dispersal that results in offspring production as it occurs in most species.
It is important to note that the Pollen Rate input specifi cation designates
the percentage of available randomly chosen males that will be included
in the actual pool from which a pollen donor is selected, and not the
probability that pollination will take place. For example, suppose that for
individuals of generation 20, pollen rate is designated as 1 (the maximum),
but for generation 7 it is designated as 0.1. This does not mean that, in a
particular bout of mating, the chances of any pollination taking place from
an individual from the generation 7 cohort is 10%; rather, it means that,
for that bout, all appropriate age 7 pollen sources are included in the pool
of potential sources, but for a particular reproductive event the chances
that any individual of age 7 is selected are 10% of the chances for a source
with age 20. If there are no individuals aged 20 and all possible sources
are aged 7, then one of these will be selected. This provision is meant to
model conditions where the process of pollination is highly effi cient: even
if there is only one eligible individual, pollination will be effected (but see
the effects of pollen dispersal distance below).
Mortality Rate
Rates of death (removal of an individual from further NEWGARDEN
processes) can be assigned. If an individual is an eligible reproducer