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SURROGATE
IDENTIFICATION OF SPECIES
1.
Ecosystem
maps as surrogate identifiers of assemblages of species
In most of the
cases, the main structural classes of the UNESCO1
system, Closed Forests, Woodlands, Scrub, Dwarf Scrub, Terrestrial Herbaceous,
Deserts and other Scarcely Vegetated Areas, and Aquatic Plant Formation are
dominated by distinct species that together give shape to the vegetation
structure itself. Many trees, forbs, mammals and birds that have a preference
for forests are different from those that live in the semi-open spaces of wooded
savannahs or the much dryer open grasslands and semi-desserts. On the other
hand, fauna elements and fungi may be very instrumental in shaping the
vegetation structure. E.g. mammals may make clearings in the forest by grazing
and many trees need fungi for acquiring sufficient nutrients. At the highest
physiognomic level, the differentiation of assemblages of species of flora,
fauna and fungi is considerable, although even at this coarse level of
categorization, a number of species can be found in several or all classes
present in a study area. Many species of large mammals can be found to roam in
different vegetation formations, although their population densities may vary
considerably among them. E.g. the Puma, Pantera concolor, is spread from
North to South America while its habitat includes mountains ranges, forests and
plains. But also plant species may span plural UNESCO structures,
as Duivenvoorden et al. (2001) clearly demonstrate for Amazon lowland forests.
The differentiation of species on the basis of vegetation formations (and water
systems) does not apply to all species but only to a part of them, thus making
structural classes a selection mechanism of partially different species
assemblages. By subdividing
those structural formations, each resulting level of subdivision is likely to
have more species in common among the subdivisions, thus leading to a gradually
diminishing differentiation of distinction of species assemblages within a
classification hierarchy. Each
ecosystem thus identified has a partially distinct assemblage of species that
sets it apart from all other ecosystems with different physiognomic-ecological
characteristics. The greater the physiognomic-ecological differences between
ecosystems, the greater the differences in species assemblages. In the context
of protected areas informatics, ecosystems identified on the basis of
physiognomic-ecological modifiers serve well as proxy identification and
classification units for all taxa. Ecosystem
maps are geographically much less biased as they come from homogeneously sampled
data sets collected by satellites and the modifier identification is applied
across the entire dataset (image). Biodiversity representation analyses on the
basis of such datasets must be considered the least biased option with presently
available techniques. African
authors have claimed that physiognomic differentiation does not always lead to
different species compositions. Ecosystems with considerable variation in
dynamics over time – such as wooded savannahs in Africa and the coastal plains
of Belize– may show less floristic variation than one would expect on their
structural distinctiveness. Savannahs continuously go through different stages
of destruction and recovery, resulting in a differentiation of vegetation
structure varying from deciduous closed forest to almost treeless prairies. The
ecosystem goes through a cycle of burning, heavy grazing and recovery, which
creates mosaics in which usually patches of all phases are present at one place
or another. The populations of species belonging to the different phases
increase and decline proportionately to the sizes of their different stages in
the mosaic, but most of them being present throughout the savannah ecosystem.
Many fauna elements respond to this structural variation by taking benefit for
different functions in their daily routines (feeding, digesting, hiding, etc.)
as they often have preferences for specific vegetation structures, rather than
for species composition of the vegetation (Den Boer, pers. com., Oindo 2002).
Also mangrove systems show strong variation of dynamics in space, which leads to
considerable structural variation but very little species variation. Vegetation
structures with such considerable overlap of species due to mere temporal and/or
spatial development stages of the ecosystem are likely to be found in mosaics.
Combined field observations and expert judgement are sometimes necessary to
establish to which extent some ecosystems in a country or region must be
considered part of a common system and whether or not the development stages
must be mapped as separate classes or united into one. If unknown it is better
to distinguish them and later decide to treat them as a joint class. 1The
USNVC and LCCS also offer these classes, though organised in a slightly
different manner and using slightly different terminologies. An open water
category is lacking in both the UNESCO system and the USNVC; only the LCCS has
basic open water classes built in its design, but it requires further detail.
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