Habitat Suitability Index (HSI)

Habitat Suitability Index Models

In any housing scheme, groundworks or infrastructure upgrade, the local planning authorities will need evidence that protected wildlife habitats won’t be harmed.

A habitat suitability index (HSI) is a standard ecological tool used to evaluate water bodies, with a focus on ponds, as it commonly applies to great crested newts.

With an HSI, an ecological consultant can calculate an objective numerical score that reflects how well a body of water can sustain life for relevant wildlife species.

Instead of costly, time-consuming and possibly invasive surveys, the option of creating an index gives developers and planners an accurate snapshot of a pond’s ecological potential early in the process.

To create an HSI score, ecologists refer to one of two modelling approaches. Most planning applications use data-driven models, where a surveyor visits the site to confirm physical measurements and empirical observations. Alternatively, a surveyor can use expert-opinion models whenever field conditions are unusual or baseline metrics are limited, relying on a panel of seasoned ecologists over raw site data.

The team at 24Housing is partnered with a third-party ecological consultancy for appraising a site’s waterbodies and getting the insight you need into suitability for protected species.

History of Habitat Suitability Indexes

The concept of the index originates from North America in the 1980s, with the United States Fish and Wildlife Service coming up with the methodology in 1981 and aiming to transform environmental values into standardised and quantifiable numbers.

By using numerical values, the local authorities could quickly and directly compare the effectiveness of proposed habitat protection, land management and ecological compensation schemes.

In the UK, the technique was adapted specifically for native amphibians by Oldham et al. in 2000, publishing foundational research in a prominent herpetological journal. The work revolutionised how ecological consultants appraise aquatic environments across England and Wales.

Habitat Suitability Index Assessment

An HSI assessment can be carried out at any point in the year, making it a quick and cost-effective screening tool early in the planning process.

During an assessment, an ecological consultant will start by carrying out background desk research before conducting a physical walkover inspection of the site to review the target waterbody and the landscape surrounding it.

Although the assessment scores overall habitat quality, it doesn’t confirm animal presence or absence. Jim Foster of the Amphibian and Reptile Conservation Trust has been a consistent voice in emphasising how the habitat suitability index reflects carrying capacity rather than actual occupation.

Conservation bodies like Natural England treat HSI assessments as a vital initial feature. From there, if the waterbody presents suitable conditions, the report will help to confirm whether targeted follow-up surveys are needed to confirm great crested newt presence, such as eDNA water sampling or evening torch counts.

Habitat Suitability Index Calculator

To establish an index, an ecologist needs to submit field observations into a mathematical formula that calculates the geometric mean of the evaluated criteria:

HSI = (SI₁ x SI₂ x SI₃ x SI₄ x SI₅ x SI₆ x SI₇ x SI₈ x SI₉ x SI₁₀) ¹⸍¹⁰

Running the standard HSI calculation will result in a single decimal figure between 0.00 and 1.00.

The numerical outcomes correspond to five standard suitability bands:

• Under 0.50: Poor
• 0.50 – 0.59: Below Average
• 0.60 – 0.69: Average
• 0.70 – 0.79: Good
• 0.80 and Above: Excellent

Any ponds recording low scores indicate hostile or degraded environments where breeding is unlikely, whereas waterbodies with high HSI scores indicate optimum conditions for species survival.

Habitat Suitability Index Formula

When it comes to calculating the score, the ecologist uses ten distinct Suitability Indices, moving from SI1 to SI10.

At the point of establishing a great crested newt habitat suitability index, each variable receives a score between 0 and 1:

• SI1 – Location: Analysed using regional geographic data reflecting national population density across the UK.
• SI2 – Pond Area: The total surface area of the waterbody in square metres.
• SI3 – Pond Permanence: How reliably the waterbody holds water throughout the year.
• SI4 – Water Quality: The clarity, invertebrate diversity and absence of severe pollution.
• SI5 – Shade: The percentage of the perimeter margin shaded by trees and tall shrubs.
• SI6 – Waterfowl Impact: The presence and disturbance level caused by resident ducks or geese.
• SI7 – Fish Presence: The presence of predatory fish stocks that feed on larvae.
• SI8 – Pond Density: The number of existing ponds within a one kilometre radius of the site.
• SI9 – Terrestrial Habitat: Quality of surrounding scrub, rough grassland and sheltering ground cover.
• SI10 – Macrophytes: The proportion of the pond surface covered by beneficial aquatic plants.

Experts on the Habitat Suitability Index

The requirements for protected species matter, but addressing them shouldn’t need to hold up your development plans.

Our team works hand-in-hand with an established network of ecologists. With that, we can put you in touch with licensed specialists who have the experience, knowledge and equipment to identify whether bodies of water support great crested newts.

Based on the results of the habitat suitability index, the right steps can be decided for moving forward, and the local planning authority can get solid evidence to help with planning applications.

Reach Out for a Free Quotation

If you need to establish the environmental status of ponds on or adjacent to your site, it would be worthwhile to get in touch with our team for a high-quality referral.

Speak to 24Housing today, and make sure that you include as much information as possible about your site and project. You can do this by filling out a quote form, calling us or emailing us. Our team will then get back to you with a quote as soon as possible.