Odonate assemblages of urban stormwater ponds: the conservation value depends on pond type

Urbanisation is among the most severe drivers of the recent biodiversity crisis. It has been shown that stormwater ponds have a high value for the conservation of dragon- and damselflies in urban areas. However, information on the relevance of different types of stormwater ponds is lacking so far. The aim of this study was to compare the Odonata assemblages of three types of urban stormwater ponds (n per type=10): (i) ponds only containing temporary water bodies (TEMP), (ii) ponds with temporary and small perennial water bodies (TEMP/PERE) and (iii) ponds with one large perennial water body (PERE). We observed distinct differences in environmental conditions and Odonata assemblages among the three types of stormwater ponds. In particular, vegetation structure and the partly interrelated microclimate differed considerably between TEMP on the one hand, and TEMP/PERE and PERE on the other hand. Odonate species richness and exuviae density of threatened species differed, too. Due to their early successional stages with low cover of riparian woodland and shallow water bodies, TEMP were characterised by a warmer microclimate than TEMP/PERE and PERE. Odonate species richness and exuviae density of threatened species were highest in TEMP and lowest in PERE. Moreover, indicator species were only identified for TEMP. Our study showed that stormwater ponds with a temporary hydroperiod play an important role for the conservation of odonates in urban areas. This is especially the case for specialised threatened species, such as Libellula depressa and Ischnura pumilio.