Input - Genetic Diversity in Establishing Plant Populations: Founder Number and Geometry - page 37

<Offspring_Distribution method=“bracket”/>
Using the “bracket” option, the general number of offspring produced per
individual is found from the input Reproduction_Rate versus age input
specifi cations, but instead of rounding, the program chooses between the
whole numbers that lie on either side of a user-specifi ed fractional number
of offspring in such a way that the average number of offspring produced
by plants of a given age is that specifi ed in the parameter fi le. If the user
specifi es 1.67, then the program will choose for an individual 1 or 2 in
such a way that across all individuals, the average offspring produced per
individual equals the closest achievable approximation of 1.67.
If the offspring distribution method = “round” option is entered,
for eligible females, NEWGARDEN rounds the age-specifi c offspring
reproduction rate to the nearest whole number. All reproducing adults of
a given age class have an equal probability of reproducing at the rounded
rate. For example, if the program interpolates a fractional reproduction
rate at a specifi c age, the nearest whole number of offspring will be used
as the rate.
Note that the accuracy of the application of the poisson, bracket, and
round options is subject in each run of a trial to the constraints of the number
of offspring to be produced, and to the nature of the grid system and the way
it is occupied by pre-existing individuals. Likewise, with all three options,
the chosen offspring distribution method selection-of-mates processes will
be conditioned by other input commands that affect offspring production
(e.g., pollen and offspring dispersal distances, mortality).
Offspring Distribution Method: Some Theoretical Considerations
Obviously, the nature of variation in offspring production across individuals
of plant populations is completely unknown for most species, and the
effect of year-to-year environmental variation on variation in reproductive
output per reproducer is unpredictable. While the “round” option holds
reproduction equal for all females of a given age class, perhaps more
realism can be achieved by employing the “bracket” or especially the
“poisson” options, where females of one age class vary in reproductive
output randomly to fi t the option constraints. This will also have the effect
that, in perennial iteroparous species, the same female will vary in relative
reproductive output from generation to generation. Such variation perhaps
conveys a more realistic aspect to the transmission and maintenance of
genetic variation across generations. Note that using the “bracket” and
“poisson” option will also indirectly cause pollen donors to contribute at
more variable rates, also perhaps bringing more realism to trial results.