Occupancy of artificial shelters by European tree frog (Hyla arborea) in Škocjanski zatok Nature Reserve: A non-invasive sampling method in ecologically sensitive habitats

Monitoring amphibians in non-breeding habitats is constrained by low detectability and potential disturbance to sensitive ecosystems. We tested the effectiveness of artificial shelters - PVC pipes - as a non-invasive method for monitoring the European tree frog (Hyla arborea) outside the breeding season in Škocjanski zatok Nature Reserve (Koper, Slovenia). Over three years (2022–2024), we installed 40- and 50-mm diameter shelter pipes at 24 sites and conducted regular inspections. A total of 338 encounters of the focal species were recorded, with frogs of all sizes using the shelters. Overall shelter occupancy declined across years, possibly reflecting a combination of factors including reduced recruitment, vegetation changes, demographic fluctuations, and weather conditions. A statistically significant trend was observed, with narrower (40-mm) shelter pipes having 1.6 to 1.7 times more captures than wider (50-mm) ones, despite no significant difference in the body size of frogs occupying each type. By using PVC artificial shelters, we greatly enhanced the detectability of H. arborea in the Škocjanski zatok Nature Reserve compared to classical monitoring methods such as acoustic surveys or dip-netting. Importantly, a non-invasive approach using artificial shelters minimized the risk of disturbance to breeding and migratory birds, which is essential in areas with high avifaunal conservation value. Our results support the broader use of artificial shelters for monitoring arboreal amphibians in ecologically sensitive areas and suggest future research should explore finer-scale shelter design to improve detection and ecological interpretation across life stages.