Interspecific competition in sympatric Sympetrum sanguineum (Müller) and S. flaveolum (L.) (Anisoptera: Libellulidae)

The influence of interspecific interactions between adults of sympatric S. sanguineum and S. flaveolum on the perching and flight behaviour of the males and the reproductive behaviour was studied at waters near Brunswick (FR Germany). Grid mappings show a horizontal habitat selection. S. flaveolum preferred perch and oviposition sites at earlier successional stages with lower height of the emergent vegetation. The overlapping indices and cumulations of the males’ perches were density-related. In areas with common oviposition sites there was a vertical separation of both species. The perches of the males of 5. sanguineum were higher vertical stalks whereas 5 flaveolum males remained perched lower on broad sloping stalks. The height of the perch site was influenced by the presence of heterospecific males. — Site-tenacious 5. flax’eolum males showed a longer total time of flight and more flights over longer distances. They were involved in interactions with males of the other species more frequently than 5. sanguineum was with conspecific males. Males with neighbouring heterospecifics were more site-fixed and their flight activity decreased. In 5. sanguineum 48% (versus 40% in S. flaveolum) of the ovipositing tandems were interfered with by heterospecific males mostly when changing the oviposition site. Possible ultimate factors selecting for the spacing out of the species (different horizontal habitat selection and interspecific interactions) are discussed.