Habitat Suitability Index (HSI)

Applicable to a handful of different wildlife species, a Habitat Suitability Index (HSI) is an impactful method of analysing the acceptability and effectiveness of an animal’s home on a site, particularly in the case of great crested newts. The results can then play an important role in planning development projects, not only by avoiding harm from coming to protected species, but also through gauging whether further ecology surveys are needed.

What is Habitat Suitability Index?

A Habitat Suitability Index (HSI) is a numerical representation of a wildlife dwelling that determines whether or not it has the necessary capacity to support the relevant species occupying it. Through developing an HSI, it will also be possible to formulate a pivotal map of species and habitats in the chosen areas, detail current species distributions and relative abundances, predict potential future changes to species distribution, and produce insightful data that could support other ecological surveys.

In order for an HSI to be calculated, an ecologist would need to undertake an assessment, including a desk study into core considerations about the local area and a physical inspection of the site to evaluate the factors that will decide the correct HSI values used to reflect the overall suitability of the habitat.

How Are Habitat Suitability Indexes Developed?

Between the desk study and the physical inspection of the study area on the site, the ecologist should have all the information they need to create a Habitat Suitability Index. The ecological consultant will consider primary factors about the dwelling in regards to habitat quality, size, comfort and potential hazards before giving each element a number based on the value and quality for housing the animal species. Data from the assessment will then be entered into a spreadsheet containing a formula to calculate the correct HSI based on the nature of the habitat.

Observations from the ecologist could also lead to the need for further ecology surveys on the specific sites such as protected species surveys, particularly if the HSI indicated that a proposed development ran the risk of posing a hazard to animals and habitats present.

Habitat Suitability Index Models

Commonly, ecologists will use data-driven methods to undertake an HSI assessment as it means utilising information they’ve retrieved from the site to form sound conclusions. Alternatively, the ecologist has the option of moving away from data-driven HSI models in favour of what are known as expert methods.

If they choose this other route, they will opt to refer to a panel of experienced ecologists, making a decision on the HSI by analysing each factor based on their own knowledge and expert opinion ahead of data collected first-hand from the site using such techniques as statistical analysis.

History of Habitat Suitability Indexes

Originally an American concept, the United States Fish and Wildlife Service et al announced and created the first HSI model in 1981 to turn the value of habitats into a numerical value.

By doing this, it was easier to see alternative environmental management methods for conservation and restoration projects that focused on given species of animal.

Period for Undertaking a Habitat Suitability Index

Unlike many other forms of ecology survey, an assessment for developing Habitat Suitability Indices can be conducted any time throughout the year. A factor that can alter the applicable times for undertaking an assessment, however, is the animal species in question.

That said, the majority of animals that could be subject to an HSI based on their habitats – such as great crested newts, for instance – are often unlikely to hibernate, meaning that an assessment could be carried out any time in the year.

Habitat Suitability Index Formula

Once all pieces of relevant environmental data have been retrieved from the site, they will be given numerical figures and calculated together to work out the HSI of surveyed habitats. Every element for an animal habitat will vary based on a number of factors such as condition, location and quality, as well as the animal species and the habitat requirements they need to live safely in the given habitat.

In a swamp, for example, suitable habitat variables could include stand maturity, stand structure, water column, water depth, water regime, water temperature and the average salinity in the water, and depending on the wildlife species, the list of potentially many variables could be ranked as more or less important than another, impacting the eventual HSI calculation. When each of the values have been decided, they can be combined in an HSI formula to determine the numerical index.

Habitat Suitability Index Calculator

The spreadsheet used to calculate the HSI will feature all of the elements and other variables taken from an on-site assessment.

As an example, a HSI for the habitat of a great crested newt would likely feature the elements below:

1. Fish presence
2. Geographic location
3. Macrophyte cover
4. Pond area
5. Pond density
6. Pond permanence
7. Shade
8. Terrestrial habitat
9. Water flow effect
10. Water quality

Each element will have been scored between 0 and 1, labelled as a Suitability Index Variant (SIv), and given a number to differentiate from each element and all additional variables. The elements will then be multiplied by each other to find a geometric mean, and the resulting figure will be divided by 10 to make the HSI.

In this case, the HSI calculation would be written as:

HSI = (SIv1 x SIv2 x SIv3 x SIv4 x SIv5 x SIv6 x SIv7 x SIv8 x SIv9 x SIv10)^1

HSI scores range from 0 to 1, with 0 representing low scores and 1 representing high scores:

• 0.50 or below = Poor
• 0.50 to 0.59 = Below average
• 0.60 to 0.69 = Average
• 0.70 to 0.79 = Good
• 0.80 or above = Excellent

Habitat Suitability Index Assessment

As Arbtech is established as an effective, reliable and experienced ecological consultancy, we are ready and able to undertake an HSI assessment for animal habitats present on your site. A qualified, trained and licensed ecologist can then attend your site, conduct the assessment and provide you with a score to give you an accurate representation of the habitat’s viability.

Upon completion of the assessment, the ecological consultant will create a Habitat Suitability Index report as demonstratable evidence that the survey work has been conducted correctly by a registered ecologist. It will also contain the results of the assessment, along with findings that could indicate the need for further ecological surveys on the site.

For a free quote, simply send across details of your site and project by visiting our contact page, completing an online quick quote form or calling us directly.