Dragonfly (Odonata) communities at three lotic sites with different hydrological characteristics

At the nature reserve “Bosco della Fontana” (Lombardy, Italy) the communities of larval and adult dragonflies were studied at three sites in small streams. The larval communities were investigated by collecting exuviae. Site 1, a stream section which falls dry in March–April due to human intervention, had a community of larval dragonflies where species typical of running water were almost absent. Here Ischnura elegans was the dominant species, accounting for more than 64% of the total number of exuviae. Additionally, other species typical for stagnant water were present. However, reproductively active adults of rheophilous species were dominant when flowing water was abundant at this site in summer. In contrast, at the two other sites, where flowing water is present throughout the entire year, rheophilous species were dominant in the larval community and in the adult communities. It is concluded that streams with a highly modified hydrology may represent “ecological traps” for specialized species and that dragonfly surveys solely based on the observation of adults can be misleading.