Environmental Engineering Reference
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Box 5.2 Modelling the effects of harvesting on green turtles.
Chaloupka (2002) gives a helpfully detailed account of the modelling process and
parameter values used to develop a stochastic stage-structured simulation model of
green turtle ( Chelonia mydas , Figure 5.4) population dynamics for the Great Barrier
Reef in Australia. The modelling process is fully laid out including the conceptual
model, a deterministic model, and a simulation in which parameter values are
picked from a distribution. Finally the effects of harvesting are included, showing
that even low levels of harvesting are likely to lead to population decline. There have
been several previous models of turtle population dynamics using elasticity ana-
lyses, which suggest that fertility is not an important determinant of population
growth (e.g. Crouse et al . 1987). These studies have cast doubt on nest protection
and head-starting of hatchlings as conservation tools and suggested instead that
conservation effort should focus on adult survival at sea. The problem with this is
that adult survival is difficult to influence, being mostly to do with interactions with
fisheries, while nest protection and head-starting are more feasible. However, using
this more realistic approach to evaluating the contribution of particular vital rates to
population growth, based on regression analyses over a plausible range of values,
Chaloupka showed that fertility is actually a key determinant of population growth.
Fig. 5.4 Green turtle nesting. © Nick Hill.
Then the value of an input parameter in a given year can be related to the output
parameter over the full range of all the parameter values, using a multiple regres-
sion. This approach is becoming increasingly widely used nowadays (e.g. Wisdom
et al 2000; Fieberg and Jenkins 2005; Katzner et al . 2006, Box 5.2).
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