Geoscience Reference
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Figure 9.6 Effect of individual variation on population persistence. The three lines from top to bot-
tom have standard deviations of 0.1, 0.05, and 0.01 for the birth and death rates. Compare these
results with figure 9.3, where the standard deviation of individual variation is zero.
exactly a birth rate of 0.5 and a death rate of 0.5, let's select these values from
beta distributions with a mean of 0.5. The birth and death rates assigned to an
animal remain for its lifetime. As new animals are added to the population,
they likewise are assigned lifetime birth and death rates. How does persistence
of this new model compare with the results from the demographic model? The
answer is in figure 9.6.
The reason that increased individual heterogeneity increases population
persistence is that increased variation results in more chance that a few animals
have exceptionally high reproductive potential and high survival. Therefore,
these animals are unlikely to suffer mortality and be removed from the popu-
lation and can be relied on to contribute new births each year. As a result, the
population may remain small but will not go extinct as often. Individual het-
erogeneity has seldom, if ever, been included in a population viability analysis,
except as genetic variation. Yet as this simple example shows, individual het-
erogeneity not a function of genetic variation is a very important element in
maintaining viability.
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