Spearhead blues: How threats to the damselfly Coenagrion hastulatum changed over time

Abstract: Given the rapid response of insects to environmental changes, their most prominent threats may change quickly as well. For effective insect species conservation it is therefore necessary to discriminate between former and current drivers of decline and to focus conservation efforts on the latter. We investigated how various environmental pressures, including former drivers of decline, have affected populations of the regionally endangered damselfly Coenagrion hastulatum and how their relevance has evolved over time. In our analysis, we compared water quality, vegetation properties and population trends across three distinct time spans: 1921–2000, 2001–2015 and 2016–2021. We included all known reproduction sites in the Netherlands, both current and historical, and also considered adjacent sites that were never inhabited. Ponds suitable for the species were characterised by mesotrophic conditions and high coverage of emergent and floating vegetation. Never inhabited ponds differed from inhabited ponds in being either more acidic or more eutrophic. Ponds where C. hastulatum disappeared between 2001 and 2015 had less of the mentioned vegetation structures and higher concentrations of various minerals. Ponds where the species disappeared after 2015 experienced severe droughts during 2018–2020. Most primary threats to C. hastulatum have shifted over time. In the past, changes in human use of ponds and increased sulphur and nitrogen deposition posed prominent threats. Presently, severe droughts, alongside nitrogen deposition have become the dominant concerns. Consequently, restoration of groundwater systems and rewetting measures are now first conservation priorities. Implications for insect conservation: Our study highlights how threats to insect populations can rapidly evolve. Consequently, conservation strategies need regular evaluation and adjustment.