Biology Reference
In-Depth Information
5000
5
4
5
4
4000
4
5
4
5
3000
4
5
4
5
4
5
3
2000
3
4
5
3
3
4
5
3
3
4
5
2
3
2
1000
4
3
5
2
3
4
2
5
3
2
3
4
2
5
3
2
4
5
3
2
2
3
4
5
2
3
2
4
5
2
3
4
2
3
5
2
3
4
5
2
0
3
4
5
2
2
2
3
4
5
2
3
4
5
2
2
0
7
14
21
GENERATION
Fig. 8.3
Population growth over 21 generations for populations initiated from 20 founders
placed at successively greater distances from the border of four different preserves. The symbol
2 denotes the population growth for the initial basic input fi le conditions described above
with the founders placed at the lower left corner of the preserve (this is the same growing
population depicted in
Fig. 8.1)
.
Populations grow more rapidly when the border recedes from
the founders (for symbol 3, the lower left border edges have receded 10 grid units from the
founders; recession for 4 = 40 grid units, while borders have receded 2,560 grid units for the
population denoted with a 5). Introducing the founders 40 grid units from the border may
be more economical than introducing the founders at the center of the preserve since no gain
in rate of population growth is gained in the latter case. After 21 generations, populations 4
and 5 are not statistically different (
t
test of means p value is > 0.05), although trials 2 and 3
do differ signifi cantly from each other, and also both from trials 4 and 5.
Figure 8.3 shows that population growth rate declines as the founders
are placed in closer proximity to the border. For these trials, fruit and pollen
dispersal have been held constant at 5 grid units and thus, when closer
to a border, fewer grid points for establishment are available, leading to
increased elimination of potential offspring since numerous offspring are
dispersed out of the preserve (as shown in
Fig. 8.4)
.
This effect lowers the
number of successful reproductive individuals in the next generation and
the consequent population growth rate over generations.
These results demonstrate that, while edge effects will affect population
growth rates under the given conditions, the effects appear to diminish
rapidly such that, in this case, a population developing from founders placed
approximately 40 grid units from the nearest border develops in a manner
almost indistinguishable from a population initiated with founders 2,560
grid units from a border (although some difference might emerge if the trials
were continued over more generations). Thus, introducing the population
at approximately 40 (or, say, 100 grid units to be safe!) from the border will
ensure that little or no population edge effects on population growth rate
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