Abstract. Devillers, P., Devillers-Terschuren, J. & Vander Linden, C.- Orchids as bio-indicators of the state of the environment: first reflections on the experiment conducted in the Walloon Region [Natural. belges 82  (Orchid. 14) (2001): 19-37]. The Section Orchidˇes dÕEurope des Naturalistes Belges has, since 1989, conducted an annual evaluation of the biological value of a sample of sites, estimated on the basis of a measure of their richness and of the pondered richness in orchids. This effort is part of a programme to monitor the state of the Walloon environment, conceived and refined by the Institut Royal des Sciences Naturelles de Belgique, the Universitˇ de Li¸ge and the Universitˇ de Louvain, in collaboration with several non-governmental organisations, and based upon a protocol of plotting the richness of sites in relation to several taxonomic groups. Orchids bring a particular contribution to this type of experiment, that of a group adapted to the exploitation in low density of relatively hostile oligotrophic habitats, those that themselves generally harbour the highest biological diversity, or at least the most important contributions to the regional biological diversity. With ten years of results, it is possible to analyse the sociological constraints and the biological reliability of this approach (Figs 1-4, Table 1). It is also possible to attempt a first interpretation of the results. They are summarized by figures 5 and 6. Figure 5 illustrates the variation of the average normalized value of the sites over the course of the 11 years of the project. This value varies within a relatively narrow span, in general around 10% on either side of the value 100 characteristic of an average site. It shows nevertheless a net erosion over the course of time, of the order of 8% for the 11 years of the project. Figure 6 expresses the average of the inter-annual variations of the values of all the sites censused during two consecutive years. This parameter also evolves over the course of time in the direction of a loss of value, but it is, especially at the beginning of the project, greater than 1. Taken together, the two figures suggest a progressive loss of sites of great biological value, detected in spite of increased expertise of observers. In the last part of the paper, table 2 and figures 7-15 illustrate the orchidological data gathered in the course of the project. Site species diversity decreases (Fig. 7) while the total number of individuals of all species taken together increases (Fig. 8). A majority of species displays stability or near stability in average number of individuals per record (Figs 9-10). A few species increase significantly (Fig. 11), some of them displaying a recovery after a period of slump (Fig. 12). Five species, Epipactis leptochila, Epipactis palustris, Dactylorhiza incarnata (Fig. 13), Coeloglossum viride (Fig. 14) and Goodyera repens (Fig. 15) show very significant decreases.

Key-words: Orchidaceae, Belgium, Walloon Region, conservation biology, environmental monitoring, bio-indicators, population trends.