Levels and Spatial Patterns of Effective Population Sizes in the Southern Damselfly (Coenagrion mercuriale): On the Need to Carefully Interpret Single-Point and Temporal Estimations to Set Conservation Guidelines.

The effective population size (N _e) is a key parameter in conservation and evolutionary biology, reflecting the strength of genetic drift and inbreeding. Although demographic estimations of N _e are logistically and time-consuming, genetic methods have become more widely used due to increasing data availability. Nonetheless, accurately estimating N _e remains challenging, with few studies comparing N _e estimates across molecular markers types and estimators such as single-sample methods based on linkage disequilibrium or sibship analyses versus methods based on temporal variance in allele frequencies. This study aims at bridging this gap by analysing single-sample and temporally spaced populations in the southern damselfly (Coenagrion mercuriale), a bioindicator Odonata species of conservation concern found in southwestern Europe's freshwater stream networks. A total of 77 local populations were sampled from a semi-urbanised area located in eastern France near Strasbourg city, yielding 2842 individuals that were genotyped with microsatellites and 958 of which were also genotyped for 2092 SNPs. Spatial genetic structure was stable over time, suggesting porosity between alternate-year cohorts. When accounting for spatial genetic structure, single-sample and temporal estimations of N _e were consistent for each set of molecular markers. Biologically meaningful results were obtained when the effect of migration was minimising by considering metapopulation N _e estimates based on the level of genetic differentiation and population boundaries. In terms of applied conservation and management, most depicted metapopulations displayed large N _e, indicating no immediate need for conservation measures to mitigate anthropogenic pressures, provided that a continuous suitable freshwater network is maintained. However, urbanisation negatively impacted N _e levels in populations close to Strasbourg city. Because N _e is used to inform conservation decisions, caution is crucial in interpreting N _e estimates, especially in continuously distributed populations undergoing migration. Altogether, our study highlights the challenge of obtaining robust N _e estimates and the necessity of careful interpretation to set relevant conservation guidelines.