Before‐after‐control‐impact field experiment shows anti‐predator netting enhances occupancy of the threatened Hungarian meadow viper (Vipera ursinii rakosiensis)

The Hungarian meadow viper is an endangered grassland‐dwelling species, which faces high predation pressure, partially due to avian species that forage in its habitat. Predation pressure by avian predators is caused not only by abundant game species (e.g. hooded crow, Corvus cornix ) but also by protected and threatened species (e.g. short‐toed eagle, Cricaetus gallicus ; common buzzard, Buteo buteo ; roller, Coracias garrulus ) in the project area (Felső‐kiskunsági turjánvidék, Hungary). Mark–recapture data of a reintroduced viper population showed a very low, 42% yearly average apparent survival rate. To establish a strong sub‐population we applied anti‐predator netting (APN) by building a 200 × 200 × 3 m (4 ha) totally closed exclusion site with a mesh net, lateral sides boosted with a 1 m high steel field fence to exclude mammals as well as birds. To test the effect of APN we monitored viper occupancy at 50 × 50 m sampling plots in a before–after/control–intervention (BACI) design, where we randomly placed quadrats 0.25 ha (50 × 50 m) to be surveyed, n = 26 at control habitats and n = 4 below the APN enclosure. We collected data across four years (2020–2023), in each year during the spring by 10 surveys replicates in each plot resulting in 1200 surveys to record viper detection/non‐detection data. We applied a multi‐season occupancy model to estimate site occupancy changes to test the effects of the BACI design. Occupancy probabilities were increasing during the four consecutive survey years in both the control and the intervention sites, however except for the initial occupancy, the occupancy probability became significantly higher at APN sites, and the APN intervention had a significant positive effect on viper occupancy, while the distance to APN showed negative effect. Predator exclusion is an effective method to minimise predation pressure and potentially has a deterministic positive demographic outcome, however, due to the high logistical and maintenance costs, this measure can be applied at only a few sites.