The Habitat Suitability Index (HSI) assessment is the first step in evaluating whether a waterbody would be able to support great crested newts. This is based on work by Oldham et. al (2000), where a correlation between habitat quality and population size was found.
The HSI is a numerical index where scores between 0 and 1.0 are recorded for each habitat variable. These abiotic variables include:
- pond area;
- geographic location;
- permanence (whether the pond dries out);
- water quality (invertebrate diversity);
- presence of water fowl;
- presence of fish; and
- the quality of the surrounding habitat.
The HSI for the waterbody is then calculated from the scores. It is important to note however that some ponds with poor HSI scores have been found to be inhabited by great crested newts and some with high HSI scores don’t always support great crested newts. Therefore HSI is not a substitute for presence/absence surveys.
In general, great crested newts prefer small to medium sized breeding ponds with aquatic vegetation for egglaying. Great crested newts require both aquatic and terrestrial habitat. A well established, mid-succession pond, with some open and less vegetated areas, is commonly considered ideal for great crested newts.
The results from HSI assessment are used to inform the likelihood of finding great crested newts at the site and whether further survey works would be required.