Niche overlap, niche segregation and habitat selection in Somatochlora arctica (Zett) and S. alpestris (Sel.) in Switzerland (Anisoptera. Corduliidae)

In various regions of central Europe the sibling species S. arctica (AR) ans S. alpestris (AL) overlap with respect to their distribution. This raises the question how far the ecological niches overlap or segregate respectively. The problem was investigated in Switzerland by collecting ecological data on the larval habitats, by experiments on habitat selection and by exploiting the corresponding data from the literature available. AR is recorded at ca 90 localities. 83% of them are situated in the Alps, 6% in the Midlands and 11% in the Jura mountains. AL is found at more than 300 localities which are almost exclusively restricted to the Alps. The larval habitats of AR are situated between TL (thermic levels sensu K.- F. SCHREIBER (1977, Warmegliederung auf Grund von phdnologischen Geldndeaufnahmen in den Jahren 1969 bis 1973. Eidg. Justiz- und Polizeidepartement, der Delegierte filr Raumplanung) 4 and 10 (mainly TL 4-8) and those of AL between 1 and 8 (mainly TL 1-4). With respect to vertical distribution the two species overlap especially at TL 4 and 5 where they inhabit moorland biotopes. AR occurs at 40 localities alone, AL at 170, whereas the two species coexist at 36 places. Sometimes the larvae of AR and AL even develop successfully in the same miniature water body. At TL 4 and 5 emergence and flying season of AR begin two to four weeks earlier than in AL.