Die Bedeutung der Vegetation für die Habitatwahl con Ceriagrion tenellum (Villers) in Südwest-Deutschland (Zygoptera: Coenagrionidae)

THE SIGNIFICANCE OF VEGETATION FOR HABITAT SELECTION BY CERIAGRION TENELLUM(V1LLERS) IN SOUTHWESTERN GERMANY (ZYGOPTERA : COENAGRION1DAE). — In Southwestern Germany Ceriagrion tenellum occurs nearly exclusively in permanently wet calcareous, spring-fed mires and marshes, as well as in springfed lakes with Cladium mariscus, and here it is considered to be a very stenotopic species. — In dragonflies, as generally in animals, the mechanisms and course of habitat selection are still largely unknown. In this paper the hypothesis is proposed that habitat choice of C. tenellum proceeds by means of a sequence of consecutive selection steps, with selectivity being differentially high in the individual steps. — In a first step, completed in the warm period after the last Ice Age, larger regions and particular landscapes within the present range of the species were colonized. Mountainous regions, regions with continental climate, and areas far from river valleys were not colonized. — After emerging, the imagoes of C. tenellum leave their home water in order to use adjacent areas as a maturation habitat. It was shown by comparing the observed abundances that areas are more densely colonized, the nearer they are situated to the breeding water and the more their vegetation is vertically structured in distinct layers. Differences in microclimate and quantity and quality of the food supply are considered as possible causes for the differentially high abundances in the respective study areas. — C. tenellum behaves very selectively in choosing its breeding habitat, i.e. a particular mire or a particular lake. The Cladietum marisci in the reed zone of oligotrophic lakes is used by C. tenellum as an oviposition site (breeding habitat) although it is very unsuitable for colonization due to its specific structure. Therefore the strong coincidence between Cladium waters and occurrences of C. tenellum is apparently an indirect relationship, with the Cladium stands serving for the dragonfly species as a “proximate factor” for necessary abiotic factors of the breeding water. Furthermore, it is shown that C. tenellum accepts only those Cladium Stands at the border of large lakes that are situated in contact with particular plant communities. — However, the choice of the sub-habitat (e.g. particular small pools in a spring mire), as well of the oviposition site and substrate within a breeding habitat just colonized, is made far less selectively. Preferences for particular characteristics of the waters or for particular plant species were determined, but in these selection steps C. tenellum also proceeds by using the strategy of “trial and error”. — In Southwestern Germany C. tenellum apparently has a very narrow ecoscheme that for the most part may consist of the following elements: vegetation at the breeding water, the characteristics of the water and the mosaic of adjacent plant communities.