De oecologische groepen in de Standaardlijst van de Nederlandse flora 1975

The first standard-list for the Dutch flora for the year 1970, including indications for distribution frequency and socio-ecological group for all recognized native vascular plant species (VAN DER MAAREL, 1971) was followed by a second edition for 1975, which resulted from discussions amongst Dutch flora and vegetation experts (ARNOLDS & VAN DER MEIJDEN, 1976). It was planned to elucidate the revisions for the standard-list itself and the various indication figures in an accompanying booklet. This has now been postponed to the third edition (1980), but because of the general interest in the application of environmental indicators in flora statistics the new system of ecological groups should be outlined without further delay. The first system of 19 socio-ecological groups mainly reflected the major groups of higher phytosociological units (as described by WESTHOFF & DEN HELD, 1969). The present system consists of 37 groups and it is claimed to be more effective as to the indication of environmental qualities. It is based on four criteria: 1. The main natural landscape types with their semi-natural and agricultural replacement types are reflected. 2. The main environmental factor complexes: hydrology, pH, amount of organic material, and status of macronutrients can be indicated. 3. The size of the groups allows sufficient discrimination for floristic inventories of areas in the order of one sq. km (MENNEMA, 1973). 4. The groups are as homogeneous as possible with respect to the criteria 1 and 2. The term ecological group is considered the most appropriate one for such indicator groups. The main differences between old and new system are elucidated. (See table I, which presents a syntaxonomical characterization of the groups, as well as the coherence between the two systems). The assignment of species to one particular ecological group was acceptable to the experts involved in most of the cases. Difficulties were met with in the following cases: 1. Species which have become extinct or nearly so; 2. Species which indicate an intermediate position between two environmental types; 3. Species with a wide ecological amplitude; 4. Species with a narrow ecological amplitude, but indicating a gradient situation in which other factors are involved then those chosen as factors to be indicated (see above); 5. Species with different environmental preferences in various parts of the Netherlands. For case 3 some remainder groups were considered but again rejected because of their too small size. Some examples of the practical use of the ecological groups in landscape characterization and evaluation are mentioned and the principal distinction between the floristical and phytosociological approach is made clear.