Population structure and habitat connectivity of Phengaris teleius (Lepidoptera: Lycaenidae) in southern Slovenia

We studied the vulnerable Scarce Large Blue butterfly (Phengaris teleius) population in loosely connected wet grasslands in southern Slovenia. Our goal was to understand the distribution and demographic traits of this population within the Seasonal Lakes of Pivka Nature Park (KP PPJ). From 2021 to 2022, we mapped suitable habitat patches in KP PPJ with the larval host plant Sanguisorba officinalis and host ant Myrmica spp. Out of eight identified potential patches, five were inhabited for at least one year. Our dispersal model, using an inverse-power function, indicated that all but one suitable patch could be connected by migrations of at least 1% of either sex, suggesting a metapopulation structure. The clustered habitat distribution (nearest neighbour index = 0.14) highlights the role of each patch as a “stepping stone” within the system. We also assessed demographic parameters in a subset of three patches: Klenik, Kalec, and Zagorje. In 2021, mark-release-recapture estimated the populations in two inhabited patches (Klenik, Kalec) at 197 males (95% CI: 159–265) and 315 females (95% CI: 205–525), showing a female-biased sex ratio (males : females = 1 : 1.6). We recorded a relatively long average life span (4.4 and 8.4 days for males and females, respectively) and a weak protandry, however about three-quarters of the population was present simultaneously, mainly in the largest patch (Klenik: > 85%). Recorded demographic traits (female-biased sex ratio, long lifespan, simultaneous peaks of both sexes, and high dispersal) support the persistence of this metapopulation. However, conservation efforts and a denser network of suitable patches are crucial to maintain its viability at this range’s southern edge.