Abstract
The occurrence of seven southern Odonata species has been watched in Wallonia over the last two decades (from 1981 to 2000). They have clearly expanded in the meantime and this pattern is still highly significant when the data are corrected for the increase of sampling efforts. Moreover, reproduction evidences have been collected recently (from 1993 onwards) for all these species and several settled and have now resident populations in Wallonia. In a second step, all present regular and irregular resident species of Wallonia were looked for change in range size and observation rate per visit between two six years periods of a survey and monitoring scheme, from 1989 to 2000. Analysis was achieved on grid cells visited at the right time at both periods, a procedure designed to neutralize the spatio-temporal heterogeneity of sampling. The comparison of results in relation to the distribution types of species and their habitat preferences show a significant global trend toward an increase for southern species during the investigated time interval, contrasting with other groups of species. If there is a tendency to rise for species preferring eutrophic still waters, this proves to be clearly due to the southern species sub-group, the other dragonflies of this habitat type showing a stable or even decreasing trend. Three distinct hypotheses are examined and discussed as possible explanations of the expansion pattern of southern species: (1) global warming, (2) change in aquatic habitats, especially eutrophication, and (3) intrinsic population dynamics. The rise of temperatures appears to be the main factor explaining the observed expansions.