Occupancy changes over the summer of the rare fen raft spider <i>Dolomedes plantarius</i> in a drained fen strongly depend on vegetation structure

Abstract Fens are among the most diverse ecosystems in Europe. They are also highly threatened, mostly by drainage and climate change. Understanding how drought can affect biodiversity in these ecosystems is thus crucial to recommend effective management actions, in particular for species with limited dispersal capacity. This study investigated the presence of the threatened spider Dolomedes plantarius using a dynamic occupancy approach during spring to summer, a period of high adult activity. From May to July 2022, we censused adult presence–absence during six visits of 103 transects located in various fen habitats of the Sacy wetlands in northern France. Species detection probability was low (mean p = 0.21) and maximal at temperatures from 18 to 19°C. Initial occupancy probability (mean psi = 0.53) was high, positively influenced by water and Cyperaceae cover and negatively by vegetation height. As the season progressed, the probability of colonization of new transects was low ( gamma = 0.09), whereas the probability of extinction in previously occupied transects was high ( epsilon = 0.42). This epsilon decreased with maximum litter thickness and sedge tussock cover: these two factors probably protect individuals from heat and drought. Thus, while hydrological restoration actions of this threatened ecosystem should focus on maintaining high water levels, we also recommend targeting Cyperaceae species, including areas with low vegetation height and maintaining litter and sedge tussocks by extensive grazing or rotational mowing to mitigate drought effects on Dolomedes plantarius .