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preserve by automobile is possible) is preferable to marching 5,120 grid
units to establish and monitor founders at the center of this preserve. The
former strategy also causes less disturbance and spreads the risk. However,
if the species is highly dispersed in the preserve, than the long march would
be preferred.
The results presented in this section suggest that the factors that select
for increased dispersal do not just involve selection for inbreeding avoidance
and increased mixis, but also include selection for more rapid population
growth with increasing dispersal distance. Given two species derived in
allopatry very recently from the same ancestral species, and thus with very
similar characteristics, some of the results above suggest that, if the two new
species come into sympatry, the species with the greater average distance
dispersal may have a competitive advantage since its populations should
be able to grow more rapidly (as long as the increase in average dispersal
distance is not so great that it brings about Allee effects). Often, the evolution
of increased dispersal of either seeds or microgametes involves increased
resource investment in reproduction (e.g., increased allocation to the food
attractant for the dispersule vector, increased investment in structures
promoting wind dispersal to greater distances, increased allocation to
display features that attract vectors). In such cases the advantages of
increased dispersal can evolve simultaneously with the increasing costs
of provisioning such changes. If some species do increase fitness by
increasing the average dispersal distance for dispersules, thereby increasing
population growth rates, then the above cost/benefi t evolutionary trade-off
can be added to the list, provided by Willson and Traveset (2001), of the
selective forces acting to increase dispersal distance: escape from natural
enemies, avoidance of sibling interactions (they do not mention avoidance
of intraspecifi c competition, which drives the NEWGARDEN patterns seen
above), and increased probability of fi nding a safe site.
Amount of Dispersal Can Differ with Distance
Offspring Dispersal
In all of the previous examples, there was only one class of dispersal
distance. However, as outlined earlier, when there is only one class of
maximum dispersal distance, average dispersal distance is not just half
the dispersal distance. As dispersal distance increases, the total number
of spaces available for dispersal increases by a square, so the average
dispersal distance is somewhat greater than the maximal distance * 0.5.
Note: some associated explicit equations are given in an earlier section.
However, for a great many species, perhaps the large majority, dispersal is
highly leptokurtic, with most dispersules traveling “distances measured
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