An improved habitat suitability index for the great crested newt Triturus cristatus

Great crested newts Triturus cristatus are a European protected species whose conservation depends on the provision and protection of their breeding habitat. The species is in decline internationally, partly due to loss of suitable breeding habitat (European Environment Agency, 2019). A habitat suitability index (HSI) developed in 2000 is extensively used in great crested newt conservation to assess breeding habitat quality in the UK. Here, we introduce a new HSI with an improved ability to reflect T. cristatus presence/absence in UK ponds. This proposed HSI is easier to use, requires less data and predicts T. cristatus presence/absence better than the previous index. To inform the new index, we used a dual approach to identify the relative importance of environmental criteria to predict T. cristatus presence/absence. Firstly, we conducted a survey of 288 HSI users to assess the perceived strengths and limitations of the existing index. Secondly, we analysed national datasets of T. cristatus presence/absence and associated environmental data. Using these findings, we then tested various index modifications. The final modifications of the new HSI include (i) using an arithmetic (instead of geometric) mean, to reduce calculation errors and allow compensation between variables; (ii) excluding water quality and waterfowl impact, as these lacked significant power to predict T. cristatus presence/absence and were deemed inaccurate by HSI users; and (iii) changing the scoring relationship for pond area to better reflect current data and provide scores for ponds over 2000m2. We compared scores from the new and original HSIs using an independent dataset for validation, showing that the new HSI better reflects T. cristatus presence/absence (larger effect sizes and R-squared values) in comparison to the old HSI. Adopting this improved HSI will enable more effective conservation of the protected species via better-informed decision-making and monitoring.