Long-term monitoring for adders: an evolving methodology

Currently, there is no recommended methodology for long-term population monitoring of European adders (Vipera berus). To open a debate on a preferred methodology, we describe an approach based on 10-years’ experience of monitoring in a chalk grassland reserve. The main elements are: 1) selection of a site with areas offering contrasting environmental conditions; 2) detection of adders along standard survey paths (transects) combined with paired artificial refuges of corrugated iron and roofing felt that are essential for detecting immature stages; 3) recognition of individual adders based on head-scale and neck patterns; 4) frequent site visits throughout the reptile active season; and 5) adoption of an Encounter Index (E.I.) that combines data from standard paths and refuges and normalises them for variations in survey effort and for shifts between years in the encounter rates along paths and at refuges. E.I. values correlate strongly with the numbers of known adders in the reserve but in some years E.I. values have been disproportionally high. Future objectives of the project are to explain variations in detectability and to estimate adder detectability associated with the current monitoring approach. Effective long-term monitoring is achievable by deploying “sufficient” refuges and, within practical limits, maximising path lengths and site visits. Future analysis of our own results will likely confirm our methodology as a “rule of thumb” for adder monitoring on, at least, chalk grassland.