Environmental Engineering Reference
In-Depth Information
Table 5.2
Types of population model.
Model type
Use
Lumped parameter
Default when no reason to use more complex models,
e.g. logistic growth model.
Age-structured
Long-lived species when biological processes are age-related
(e.g. juvenile/prime adult/senescent), e.g. many ungulates.
Sex-structured
When harvesting is sex-biased, e.g. for male trophies, and/or
when sex differences are biologically important, e.g. harem-
breeders.
Stage-structured
When size (weight or height) or stage (e.g. larva/adult) is
more important, biologically or for harvesters, than age, e.g.
trees, fish, insects.
Spatially
When dispersal is a key biological parameter, e.g. fragmented
structured
habitats, meta-populations and/or when management is
spatially structured, e.g. marine reserves.
Individual-based
Very small populations or when individuals are sessile or
territorial and spatial structure is important (e.g. duikers,
plants).
Many models nowadays are structured in several ways. For example, Skonhoft
et al
. (2002) produced a model of chamois hunting that included movement
between a protected area and a hunting zone (spatial structure) as well as age and
sex structure, because males are preferentially hunted for their horns. They also
included a socio-economic component, exploring which parameters had a major
effect on the hunting rate that would maximise overall profits.
Spatially structured models are becoming more widely used now, and it is
evident that space is a key component of the sustainability of harvesting, both from
the biological side and from the socio-economic side. For example, remote areas
can provide a refuge for over-exploited species (Ling and Milner-Gulland in press).
Dispersal rates between areas are key biological parameters determining the success
of spatial harvest controls such as no-take zones (Section 6.4.1)—too high, and the
animals disperse out of the protected area and are harvested, too low, and there are
few economic benefits to harvesters (Gerber
et al
. 2003).
5.4.1.1 Population viability analyses
Much early conservation modelling, particularly of small populations, and
particularly using packages such as Vortex and RAMAS, has been labelled as
Population Viability Analysis, or PVA. The distinguishing feature of a PVA, as
opposed to any other kind of population model for conservation, is that it aims to
estimate the
probability of population extinction
by using a stochastic model.
