International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
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Graph 1. Publications on application of remote sensing and GIS
techniques in mapping invasive species
Mapping the type and extent of bio-invasions, the impact of
invasions or potential risks of invasions requires accurate
assessment and modelling species distributions. So far no
synoptic literature review has been published in the field of
mapping invasive species. To sketch the possibilities,
limitations and challenges of remote sensing techniques in
mitigation of invasive species, this paper provides an overview
of the application of remote sensing and GIS technologies in
mapping biological invasions. We addressed the following
questions.
1. What mapping techniques have been used to map and
predict the potential distribution of invasive species?
2. What sensors and what image processing and
classification techniques have been used to map the
actual distribution of invasive species?
3. For what species groups (canopy versus non canopy
members, plant versus animal species) has successful
mapping been reported? Is there any evidence that the
reported successful applications tend to be biased
towards any particular species groups?
4. To what extent has sensitivity to scale and the
reliability of the mapping product been addressed?
S. Which available mapping techniques so far not
applied to invaders could be used to improve the
mapping of invasive species?
We searched for articles on biological invasions using several
electronic databases (AgECONCD, GEOBASE and SOILCD),
covering international agricultural, economical and rural
development literature, Journals, monographs, conferences,
books and annual reports. We also searched other sources such
as scientific abstracts, worldwide web, CD ROMs and libraries
within the Netherlands. Several experts were contacted who
provided additional references.
2. MAPPING ACTUAL AND POTENTIAL
DISTRIBUTION
2.1 From global to local scale
At national or continental level, maps of invasive species
distribution are mainly interpolations from recorded
observations compiled and stored in herbaria, zoological
collections and research institutes. Maps are often generated by
manually drawing polygons (boundaries) around areas where
the species is known to occur or alternatively using some
automated interpolation — procecures. For example the
distribution map of Chromolaena odorata, one of the world's
worst invaders is displayed in Figure 1. The map displays the
distribution as a continuous surface. This suggests that the
species occurs throughout the area represented by the map
polygons. In reality, species are not homogeneously distributed
across their distribution range. Instead they prevail in certain
environments while they are absent from others. Maps showing
discontinuous patches would more realistically represent such a
distribution pattern. However, at small scale (typically « | to a
million) we prefer to use interpolations, while realizing that
they are generalizations, displaying the broad geographic range
within which the species is known to occur.
Figure 1. The world distribution of Chromolaena odorata: area
under infestation (Black) and native range (Grey). Source: map
drown by the author based on global invasive species database
(ISSG, 2004)
The need to display the discontinuity and patchiness in
distribution patterns emerges while moving towards larger
scales. Here, it would be impractical to derive maps through
interpolation, because it would require sampling every patch, a
costly operation particularly when larger areas are to be
mapped.
2.2 Remote sensing techniques
Remote sensing gives a synoptic view of the surface of the
earth. Aerial photography is the oldest remote sensing technique
(Sabins, 1987; Lillesand & Kiefer, 1994). There is a wide
choice of films and spectral sensitivity (visible part of the
spectrum versus those that include the infrared). Aerial
photography has been used to assess vegetation and plant
species attributes such as canopy architecture, vegetative
density, leaf pubescence and phenological stage (Everitt et al,
2001a). Digital camera photography and videography are
recently introduced as cheap, easily available and flexible
alternatives to standard photography, particularly when the data
are to be transferred onto a computer system. There are systems
available that cover the near infrared (NIR) and infrared (IR) as
well. Multispectral scanners register reflectance in a number of
spectral bands throughout the visible, near- to far-infrared
portions of the electromagnetic spectrum. Broad-band scanners
have few spectral bands of one hundred or more nm wide.
Hyperspectral scanners have more (tens up to several hundreds)
but narrower (from tens to a few nm wide) spectral bands.
Broad-band scanners have been successfully applied to
discriminate between broad land cover types such as forest
versus bare soil and built up area. The higher spectral resolution
of hyperspectral scanners allows discrimination of more subtle
differences such as those between individual species (Graph 2).
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