Biology Reference
In-Depth Information
12
CHAPTER
Dispersal
Offspring Dispersal Distance and Pollen Dispersal Distance
Both offspring dispersal distance and pollen dispersal distance can
separately affect the dynamics of populations. To demonstrate this, we
will consider one simple example of offspring dispersal distance before
exploring how interactions between offspring and pollen dispersal distances
can affect population growth and genetic characteristics.
In this example, two sets of comparative trials were run. All conditions
for these trials were identical except as follows. The set of trials denoted
“x” had a reproductive rate set to 4 (r = 4), while those denoted “o” had
r = 2. In this example, we are interested only in population growth rates,
so no detailed explanation of the input conditions will be given except that
the plants were annuals, there was no selfi ng, each plant was bisexual,
trials began with 20 founders placed in the center of a very large preserve
(> 26,214,000 species average density grid points available for colonization),
and trial populations were allowed to grow for four generations. Trials
given the same symbol differed only in the distance range specifi cations
to which offspring could be dispersed from the parent plant (e.g., up to 5,
or up to 13, grid units from a parent).
Results from these trials are shown in Fig. 12.1. In both sets of trials
(r = 2 versus r = 4), the population growth after four generations increases as
offspring dispersal distance increases until at a certain “critical” distance a
plateau is reached. Gains in population growth when offspring are dispersed
beyond that critical distance become negligible. Since individuals in all of
these trials are reproducing at the same rate, the patterns seen indicate that
when dispersal distance is low, population growth is hindered by density-
dependent effects (lack of grid points for establishment).
These curves show that the geometric arrangement of founders can
interact with dispersal distance to affect population growth. In the examples
here, all other considerations aside, if population growth rate is strongly
selected for, individuals and/or populations with mechanisms that better
promote a dispersal range up to or slightly beyond the critical distance
should be selected for. Evolutionary costs invested beyond that critical
distance do not produce large increases in population growth rate and may
result in less effi cient pollination (greater distances among individuals),
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