Impact of long-term floods on spatial dynamics of Myrmica scabrinodis, a host ant of a highly threatened Scarce large blue (Phengaris teleius)

Extensively used humid meadows, with high biodiversity are under threat in Europe due to anthropogenic pressure. Increasing frequency of prolonged floods is emerging as an additional threat to this fragile environment. In our study, we explored how prolonged floods impact the spatial distribution and its dynamic over time (caused by mortality and recolonization process) of the host ant species Myrmica scabrinodis , essential for the survivor of the threatened Scarce large blue ( Phengaris teleius ). The study was conducted on flood prone area Ljubljansko Barje at the southern edge of the P. teleius global range. Long-lasting floods in the study area, potentially affecting the past and recent distribution of the host ant M. scabrinodis , were recorded in 2010, 2013, and 2017. In 2020, we set up 160 ant traps to estimate host ant distribution in a system of meadows covering entire gradient of history of floods. Our results indicate that M. scabrinodis survive the flooding for up to three days and start to disappear if flooding persists longer. After the flood withdrawal, the ants gradually recolonize empty habitats from the surrounding upland refuge areas. Our spatial analyses predict that the average recolonization speed was about 29 meters per year, and that in one year of recolonization ants compensate the mortality effects of 1.8 days of flooding. These findings elucidate that flooding should be considered as an additional (in some areas a major) threat to the endangered Scarce large blue through its detrimental effects on the host ant species.