Environmental Engineering Reference
In-Depth Information
When conducting sensitivity analyses of structured population models,
population growth is often projected over a long (two hundred to one thou-
sand year) time span. It is noteworthy that by doing this, the intention or
the goal is not to provide an accurate forecast of long-term future popula-
tion size.The intention, rather, is to provide a long-enough, stable time se-
ries so that one can obtain a reliable estimate of the degree of change in
population size that may arise as a consequence of changing a parameter
value by a fixed amount.The reason that one cannot assume the long-term
projection to be accurate is that under natural conditions, parameter values
do not remain fixed for indefinite time periods. As a consequence, struc-
tured modeling is normally used to conduct viability or risk analyses, rather
than forecast population size (Beissinger and Westphal 1998).
Viability of Loggerhead Sea Turtles
We are now equipped with the concepts and tools to revisit our original
question: Should management focus conservation effort at protecting sea
turtle eggs on the beach? Crouse et al. (1987) did such an analysis for log-
gerhead sea turtles.
Box 5.1 summarizes the average stage-specific survival values of logger-
head sea turtles based on cross-sectional studies conducted in the field. In
addition, nesting studies showed that novice breeders produce on average
127 eggs per breeding season and mature breeders average 80 eggs per
breeding season (Crouse et al. 1987).These values are not known with exact
certainty owing to wide variation about the average values. For example, egg
survival can vary between 3 and 90 percent. Reproductive females can pro-
duce between one and seven nests per year. Given this variability, we can ask
how sensitive the population is to a fixed percentage change in any one of
the survival and reproductive values (Caswell 2001). Such a sensitivity analy-
sis reveals where conservation should get its greatest “bang for its buck.”
In the case of loggerhead sea turtles, the population is most sensitive to
survival values rather than egg production. Moreover, the sensitivity is
higher for older stage classes, which spend their life in the ocean.This con-
clusion derives from computer simulations that examine the effects of
changing egg production and survival on absolute population growth rate.
Like the case of the Panda, this analysis is accomplished by using average
values to run a baseline computer simulation to estimate the long-term
population trend. Baseline runs are then followed by simulation runs that
systematically change one age-specific survival parameter value at a time.
