NATURE WORLDWIDE: BIODIVERSITY

WORLD INSTITUTE FOR CONSERVATION & ENVIRONMENT, WICE

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.

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