A revised system for interpreting great crested newt habitat suitability indices

A widely used system for assessing habitat for the great crested newt uses five categories ranging from ‘poor’ to ‘excellent’ based on thresholds for the Habitat Suitability Index (HSI). However, how these categories relate to pond occupancy, at an England-wide scale, is unknown. Equally, the Habitat Suitability Index system has so far only been validated using traditional direct observation methods rather than environmental DNA protocols that are becoming commonplace. Without further validation on a national scale, misleading decisions may be made concerning the likely presence or likely absence of great crested newts. Using environmental DNA data collected from over 5300 ponds distributed across much of England, we show that the existing scoring system underestimates pond occupancy in the lower categories and overestimates pond occupancy in the higher categories, while the median habitat suitability index value was found just within the 'good' category. We found that the median habitat suitability index for occupied ponds was 0.7,confirmingthis value as a target to aim for when creating or restoring ponds for great crested newts. We suggest a revised system based on the median occupied pond HSI score, whereby the two extreme 'poor' and 'excellent' categories each contain just 10 % of occupied ponds; the 'below average' and 'good' categories each contain 20 % of all occupied ponds, and the 'average' category contains the central 40 % of occupied ponds. Although regional variation in estimated pond occupancy rates using this system may need to be accounted for when interpreting HSI scores, the revised scoring system is generally robust across England. Both the existing and revised HSI scoring systems are no substitute for surveys, and caution is needed when interpreting absence of newts based on habitat suitability data only.