Decline in the number and size of populations of two Lymnaeidae living in central France over the last decade

Field investigations in 2013–2014 on acid soils and in 2016–2017 on sedimentary soils showed that populations of two lymnaeids had decreased in number in central France since 1998. As several heat waves occurred in this region in 2018, 2019 and 2020, it was useful to check whether this decline had further increased in recent years. Surveys in 56 farms in the north-west and west of the Haute Vienne department (acid soils) and 37 farms in the south of Indre (sedimentary soils) were therefore carried out in 2020 and 2021 and the results obtained were compared with those recorded by our team in the same farms in 2013–2014 and 2016–2017. On acid soils, the overall number of populations decreased by 34.7% for Galba truncatula (out of 813 populations in 2013–2014) and 22% for Omphiscola glabra (out of 550), while it decreased by 25% for G. truncatula (out of 361 in 2016–2017) and 15.2% for O. glabra (out of 205) on sedimentary soils. Similarly, the overall density of overwintering snails significantly decreased by 80.7% for G. truncatula and 70.2% for O. glabra on acid soils, while it significantly decreased by 64.1% and 38.3%, respectively, on sedimentarym soils. In both cases, these decreases were more marked on acid soils than on sedimentary soils. In contrast, the habitats of G. truncatula and most of those colonized by O. glabra showed no significant variation in their area between the two periods of study. The decline in the number of these lymnaeid populations, observed since 1998, is still continuing today in central France and may be due in part to heatwave episodes s that occurred in 2018, 2019 and 2020 in this region.