Geoscience Reference
In-Depth Information
more than one environmental factor on a single map. The habitat type,
defined as a mappable unit of land “homogeneous” with respect to vegetation
and environmental factors, circumvented this problem and was the basis of the
land system (land concept) maps developed in the 1980s (Walker et al. 1986;
Zonneveld 1989). However, it is based on the assumption that environmental
factors show an interdependent change throughout the landscape and that the
environmental factors are constant within the “homogeneous” area. Thus to a
certain extent the land unit meaning of the term
habitat
arose as a way to over-
come operational difficulties in species distribution mapping. Nevertheless,
given that the variation of one environmental factor affecting the distribution
of a species often tends to be independent of the other environmental factors,
homogeneity is seldom the case, so there is seldom a true relationship between
species and habitat types.
The advent of
GIS
has made it possible to store the variation of environ-
mental factors independently and subsequently integrate these independent
environmental surfaces into a map displaying the suitability of land as a habi-
tat for a specific species.
The first examples of such
GIS
-based habitat mapping were published in the
second half of the 1980s (e.g., Hodgson et al. 1988). Since then there has been
a steady increase of the number of
GIS
-based habitat models (figure 11.1). The
increase illustrates a move away from the general habitat-type mapping appli-
cable for multiple species toward more realistic species-specific habitat maps.
At the same time, the habitat type loses its usefulness because of the
decreasing need to classify land in homogeneous categories. In other words,
species-specific habitat mapping is increasingly incorporating independent
environmental databases processed using information on the preferences of
the species concerned. In view of the anticipated move toward species-specific
habitat models, we prefer to use the original species-related concept of habitat
instead of a land-related concept; to avoid confusion, in this chapter we will
use the terms
species-environment relationships
and
ecological requirements
in-
stead of the terms
species habitat
and
habitat requirements.
General Structure of GIS-Based Models
j
The rationale behind the
GIS
approach to species distribution modeling is
straightforward: the database contains a large number of data sets (layers), each
of which describes the distribution of a given measurable and mappable envi-
ronmental variable. The ecological requirements of the species are defined
