Publication date: 15th July 2025

ARC’s Conservation Director, Jim Foster, outlines how evidence on a much-used field method should prompt a revision 

Understanding the quality of habitat can be very useful in conservation. For the great crested newt, it’s common to use a method known as the Habitat Suitability Index (HSI for short – though as anyone who uses it frequently will know, many computers have a frustrating tendency to “correct” this abbreviation to HIS). The great crested newt HSI was published in 2000 by Prof Rob Oldham and colleagues, based on a method developed in the USA. Since then it has become standard practice for many, volunteers and professionals alike.

The HSI allows users to derive a single value that describes the quality of a waterbody, based on assessment of ten component variables. So, an HSI score of 0.9 would indicate a high quality pond, and if newts are indeed present then one would typically expect them to be abundant there. A pond with an HSI score closer to zero would typically have a lower chance of supporting great crested newts. That said, there are various important caveats about how HSI scores should be interpreted.

Given that it’s now 25 years old, together with my colleagues Dorothy Driver and John Wilkinson we set out to explore how this method has fared, and where it’s future might lie. We spoke to a range of people with great crested newt HSI experience across a range of organisations. We also reviewed the published evidence on how effective the method can be, and reflected on our experience of using it and training others to use it.

Our findings were published as a viewpoint article last month in “In Practice”, which is produced by the Chartered Institute of Ecology and Environmental Management (CIEEM). Normally this is only available to subscribers, but CIEEM have kindly given permission for us to share our article.

We explore in particular the emerging evidence showing how some changes to the great crested newt HSI methods and guidance could help with its performance. Thanks to several researchers, there is now a useful body of evidence on where the method has weaknesses. For example, our understanding of the distribution of newts is much improved since 2000, and this will influence one element of the HSI calculation. We also comment on our observations of misinterpretation or even misuse of the method.

We conclude that it would be useful to update both the great crested newt HSI method itself and accompanying guidance, but we stop short of taking that forward pending any comments on our proposals. In particular, we set out some suggested principles behind any revisions, and ask for stakeholder views.

You can view the article below, and we welcome any comments (please submit as per the suggestion in the article).

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