Environmental Engineering Reference
In-Depth Information
Figure 5.2.
Hypothetical “spread sheet” for depicting aging and reproduction within
six stage classes of loggerhead sea turtles over the course of three years.The value
n
with
age specific subscript represents the number of individuals in the population belonging
to a specific age class. The total population size can be calculated by summing all age-
specific numbers in a column.
an age class. (Incidentally, these are the kinds of statistics that insurance ac-
tuaries use to calculate life or health insurance premiums.) This informa-
tion can then be used to follow the fate of different-aged individuals over
time (figure 5.2).
There is one problem with this modeling approach.That is, males in a
population cannot produce offspring. So the model will distort dynamics
if we include males in the current formalism.This issue is usually resolved
by assuming that the sex ratio in a population is 50:50 (i.e., the number of
males equals the number of females) and that females produce female off-
spring only. Under theses circumstances, we simply forecast the number of
females in a population from one time period until the next and then dou-
ble that number to forecast the total population size. The assumption of
equal numbers of male and females is reasonable for most populations
(Caswell 2001).When it isn't, we can build more complex models that ac-
