Geoscience Reference
In-Depth Information
The deductive approach uses known species' ecological requirements to
extrapolate suitable areas from the environmental variable layers available in
the
GIS
database. In fact, analysis of the species-environment relationship is
relegated to the synthesizing capabilities and wide experience of one or more
specialists who decide, to the best of their knowledge, which environmental
conditions are the most favorable for the existence of the species. Once the
preferences are identified, generally some sort of logical (Breininger et al.
1991; Jensen et al. 1992) or arithmetic map overlay operation (Donovan et al.
1987; Congalton et al. 1993) is used to merge the different
GIS
environmental
layers to yield the combined effect of all environmental variables.
When the species-environment relationships are not known a priori, the
inductive approach is used to derive the ecological requirements of the species
from locations in which the species occurs. A species' ecological signature can
be derived from the characterization of these locations. Then, with a process
that is very similar to the one used in deductive modeling but is generally more
objectively driven by the type of analysis used to derive the signature, it is used
to extrapolate the distribution model (Pereira and Itami 1991; Aspinall and
Matthews 1994).
In figure 11.2 we summarize the data flow of
GIS
-based species distribution
models for both the deductive and the inductive approaches. Whereas in the
deductive approach
GIS
data layers enter the analysis only to create the distri-
bution model, in the inductive approach they are used both to extrapolate the
species-environment relationship and the distribution model. Along with the
data flow, the steps that need validation are also evidenced in the figure. Vali-
dation is addressed in more detail later in this chapter, but it is interesting to
note here that validation procedures are needed at many different stages in the
flow diagram.
Both inductive and deductive models can be further classified according to
the kind of analysis performed to derive the species-environment relationship.
Essentially these can be subdivided into two main categories: the descriptive
and the analytical. Models pertaining to the first category use either the spe-
cialists' a priori knowledge (deductive-descriptive) or the simple overlay of
known location of the species with the associated environmental variable lay-
ers (inductive-descriptive) to define the species-environment relationship.
Descriptive models generally are based on very few environmental variable lay-
ers, most often just a single layer. They tend to describe presence and absence
in a deterministic way; each value or class of the environmental variable is asso-
ciated with presence or with absence (e.g., the species is known to live in
savanna with an annual mean temperature of 15-20°C, so savanna polygons
