(e.g. juvenile survival, adult survival, age at first reproduction, etc.) are affected
by density. These relationships would then be combined in a structured model
(see 13.6.4) to provide an alternative to equations (13.1) and (13.13) that
captured more of the dynamics specific to the animal of interest.
13.6.3Other driving forces
In the simple models discussed above, the only factors that affect population
growth are density and harvest. But, in reality, many other factors can affect popu-
lation growth, sometimes to such a degree that the density-dependent relation-
ships are not discernable. For instance, reproduction, and hence population
growth, of Mallard ducks (Anas platyrhynchos) in North America are strongly
influenced by the availability of water on the prairie breeding grounds, so much
so that annual harvest management decisions for this species are conditional on
observed number of prairie ponds (Johnson et al. 1997). Increased understand-
ing of the range of forces that affect the population dynamics can enhance the
management of exploited populations.
13.6.4Model structure
The models presented in equations (13.1) and (13.13) treat the population as
a homogeneous group of individuals, all with the same life-history traits and
vulnerability to harvest. The heterogeneity of real populations, due, for instance,
to differences by sex, age, size, and reproductive status, can affect the growth
rate and density-dependence of a population, and hence the impacts of harvest
(Box 13.2). Further, some exploitation strategies target specific classes of
320 |Exploitation
Box 13.2Case study: assessing the population dynamics of the MaleoMaleo (M. maleo) are burrow-nesting megapodes endemic to Sulawesi, Indo-
nesia, that incubate their eggs in communal nesting sites on beaches and in soils
heated by volcanic activity (Argeloo 1994). The eggs of the Maleo, like those of
most other megapodes, have probably been harvested for millennia ( Jones et al.
1995). In recent decades, traditional indigenous restrictions, which had served to
make egg harvesting sustainable, have broken down, leading to overexploitation
(Argeloo and Dekker 1996). Using a population dynamics approach to assess the
potential growth rate would require knowledge of several life-history parameters:
number of eggs laid per female per breeding season, hatching success rate, survival
rate of hatchlings until sexual maturity, and adult survival rate. From those para-
meters, a structured population model could be built, and a growth rate could be