Successful development of European tree frog (Hyla arborea Linnaeus, 1758) populations in the south-west and west of Luxembourg following reintroduction

In this article, we provide a comprehensive overview of our European tree frog reintroduction, from the sourcing and captive-rearing of donor animals, through reintroduction and population development, to the current status of the project. Before the start of the project, only one isolated population remained in the east of the country. Starting with two initial reintroductions (2012 to 2014), two tree frog populations have developed over the past decade and we have managed to save the species from extinction on the national level. While both populations have been expanding rapidly in recent years, they show noticeable differences regarding initial development and spread. Further reintroductions to reinforce these populations were carried out from 2019 to 2024. In total, more than 7,000 juveniles and tadpoles were released over the entire duration of the project, resulting in ca. 830 calling males at 61 sites in 2024. Our long-term annual monitoring provides detailed data on population development and spread, which are presented and discussed. In addition, comparisons with similar tree frog reintroduction projects in other countries are made and logistical and epidemiological aspects of the reintroduction project are highlighted. Finally, we give an outlook on practical implications for conservation management, upcoming challenges and future focal points of the project.