Habitatselektion bei Libellen

HABITAT SELECTION IN DRAGONFLIES. — Habitat selec- tion is an important element in the reproductive behaviour of dragonflies. In order to optimize their fitness, individuals of both sexes must be able to choose sites for reproduction that are well suited for embryonic and larval development. In this paper selection of oviposition site and larval habitat is briefly reviewed with regard to problem definition, methods of study and presentation of results. The subject is considered from descriptive, causal and functional viewpoints. In order to determine what types of habitat dragonflies select, two quantitative methods are recommended: collecting exuviae and determining the number of adults or their duration of presence at a site. The water bodies selected by dragonflies may be described qualitatively and quantitatively with respect to their physical and biotic features, including size, depth, bottom structure, water regime, hydrochemical properties and vegetation. — In the study of the causal aspects, i.e. the proximate factors by which the habitats and oviposition sites are recognized, various experiments may be conducted, such as structural manipulations of natural habitats, choice experiments using natural substrates or dummies, and transplantations of larvae or adults to non-speciesspecific habitats. From the results available it may be concluded that sites are recognized mainly by visual cues. In the species so far studied positive reactions are evoked by structural features of the water surface (pattern of light reflection, vegetation). By physical examination of the site using the abdomen (in Somatochlora arctica) or the hind legs (in Perithemis mooma) the dragonflies may receive additional informations about the quality of the site. The use of thermal signals for choosing the precise localisation of the oviposition site was demonstrated in Leucorrhinia dubia. — Some of these proximate factors coincide with ultimate factors that select for habitat-seeking strategies of adult dragonflies. Investigation of the functional aspects (habitat binding) requires laboratory experiments with eggs and larvae. Hitherto only a few species have been studied with respect to factors favouring or limiting the survival rate of embryos and larvae. For Calopteryx the high oxygen requirement virtually limits members of the genus to running water. In some bog dwellers it could be shown that probably the thermal regime of the water — and not its hydrochemical composition — is crucial for habitat binding. — Besides vegetation and physical features, conspecific and heterospecific individuals, predators and possibly egg parasites also influence the selection behaviour of both sexes. Ovipositing females are frequently molested by conspecific males ready to copulate, and therefore try to avoid the latter in space and time. Conspecific and heterospecific competition between males may result in spacing out of individuals, some of whom are thereby relegated to suboptimal habitats. — On the basis of the results so far available a habitat selection model is suggested according to which the dragonflies approach their larval habitats stepwise using different sensory systems. — The research on habitat selection may be applied in bioindication and to nature conservation. Odonata are useful for the integral assessment of waters, especially as regards structure and landscape. It is suggested that the indication systems should comprise groups of species rather than single ones. When based on the regional stenotopic characteristics of many species, one system is valid only for one limited geographic area. — As all species of Odonata are completely protected in Central European countries such as Germany and Switzerland this order plays an essential role in nature conservation. The most important measures for the conservation of the Odonata fauna concern the larval habitats, i.e. their protection, management, regeneration and creation. It is stressed that scientific investigation of larval habitats and of habitat selection in adults is an indispensable basis for conservation.