Succession of the Odonate fauna at a small wetland in an overflow and seepage reservoir: results of a six-year study

An unintentionally created, small, permanent, groundwater-fed, and shallow wetland in Lower Austria was subject of an odonatological study carried out from 2016 to 2021. The water body (size 1.200 m2) was situated in an overflow and seepage reservoir created in 2014. As nobody – apart from the author – had access into the fenced area and as, therefore, there were no measures carried out, successional processes ran totally undisturbed. Main target of the study was to examine the odonate community´s response to changing habitat conditions, in particular as far as the rapid development of helophyte stands is concerned. A high number of observation days (236) within the investigation period of six years allowed detailed phenological analyses, for example of emergence, maturation, and flight periods. Investigations were primarily focused on counting tenerals, juveniles, and adults and on recording pairing wheels, tandems, and egg depositions. Exuviae, predominantly of Anisopterans, were sampled in the years 2016 and 2017. In the subsequent years, sampling of exuviae was widely waived in order to prevent damage to the reed stands associated with a “rejuvenation“ of parts of the wetland and the unintentional creation of new small pioneer habitats. In 2016, the site could be characterised as a mosaic consisting of open water areas, open, sparsely and densely vegetated riparian areas, floating submerged macrophytes, and emergent amphibious vegetation of different heights. As of the summer 2018, the wetland was completely covered by the high-growing Broadleaf Cattail Typha latifolia, Great Hairy Willowherb Epilobium hirsutum, and reed Phragmites australis. Within the six years, a total of 32 species were recorded with the highest number (27) in 2016. Species with highest abundances were Ischnura pumilio with about 350 individuals in 2016 (sum of both generations), and Sympetrum striolatum with about 150 individuals also in 2016. Orthetrum coerulescens, in turn, was the only species colonising the site when completely covered by helophytes. Based on the findings of the study, it can be concluded, that Pyrrhosoma nymphula was the only species classified as permanently autochthonous in the period under scrutiny – even if only as a small population. The dragonfly community responded in various ways on changing, for most species deteriorating habitat conditions: decrease of the number of individuals and of the number of breeding species; increase of the species´ number recorded only by single records; decrease of the duration of the species´ presence at the site and, thus, reduction of the site-specific flight period compared with the species-specific flight period. In the third and fourth year of its appearance, the bivoltinous I. pumilio produced only the spring/early summer generation; the same occurred in Ischnura elegans in its second and third year. In several species changes in the reproductive behaviour were observed. In this connection, key stimuli inducing and factors limiting pairing and egg deposition behaviour are discussed.