30 Oct 2023

How do we define and assess irreplaceable habitats?

Heidi Tillin and Charlotte Johnson share lessons learnt from a recent trial approach for English marine habitats.

Marine spatial planning seeks to balance sustainability and conservation with socio-economic benefits from use of resources. Recently, marine specialists and advisers at Natural England found that there was a knowledge gap around marine and coastal habitat irreplaceability in English waters.

What is an irreplaceable habitat?

The Marine Biological Association (MBA) was commissioned to develop a draft definition and apply this to English marine habitats. The MBA team reviewed a wide range of literature and interviewed experts to develop the proposed definition shown in Box 1. Experts suggested that criteria for habitat irreplaceability should include recovery and ease of restoration, rarity, and uniqueness. Although the last two criteria are sometimes used interchangeably, there is an important distinction. All unique habitats are rare, but not all rare habitats occur in unique settings. This consideration is important as the uniqueness of a habitat, whether it’s due to the substratum, the local hydrodynamics, or the biological assemblage, reduces the feasibility that it could be created elsewhere.

BOX 1 – Proposed habitat irreplaceability definition

Marine irreplaceable habitats (MIH) are those which cannot be successfully restored or created, based on one or more of the following factors:

  • They are very difficult* to restore or very slow† to recover.
  • ‘Successfully created’ refers to ex situ creation of the habitat (which could be considered as replacement). Difficulties could result from availability of suitable habitats or difficulty of creation—including translocation. The assumption is that habitats that are difficult to restore are also difficult to create.
  • They may be nationally rare (based on extent, range, and distribution) and/or have an unusual or rare environmental context.

* ‘very difficult’ refers to feasibility: difficult in situ restoration would be that which requires significant technical input including maintenance and supporting infrastructure, and for which there are low rates of success.

over 25 years to recover.

Assessing the irreplaceability of England’s coastal and marine habitats

We carried out a systematic assessment of the irreplaceability of habitats defined in the UK Marine Habitat Classification (matched to the EUNIS classification) based on the following criteria:[1]

  • Natural recovery potential (years to recover).
  • Rarity of habitat (based on number of records and regional distribution).
  • The uniqueness of the environmental context (physical habitat, hydrodynamics).
  • Rarity/distinctiveness of the biological assemblage.

Categories were created to assess each criterion, with weighted scores assigned to each category that were summed to provide an overall for irreplaceability score for each habitat. Overall, the team assessed 342 habitats including all UK broad-scale habitats, 79 biotope complexes, and 225 biotopes. Sub-biotopes were only assessed if they were considered likely to score highly against the irreplaceability criteria. The assessments relied on information sources that are readily available and provide consistent, systematic assessments for a wide range of habitats. As the evidence base and scores are captured in publicly available Excel spreadsheets, the outputs are transparent and updatable.

Which habitats are more or less irreplaceable?

In general, the broad-scale habitats defined by substratum, depth, and energy, such as littoral mud or low energy infralittoral rock, are common (with the exception of deep-sea habitats) and therefore have low scores in terms of uniqueness (environmental context). Those present in areas of higher energy tend to recover quickly and those that are littoral or present in shallower areas are more readily restored. Habitats that are considered more replaceable include many intertidal sedimentary habitats that recover quickly from disturbances, are widespread, and are characterized by common species. Subtidal sedimentary biotopes are similar but are less restorable. Examples of low-scoring habitats include sublittoral mud in estuaries, shingle (pebble) and gravel shores, and strandline habitats. This doesn’t mean they are unimportant for biodiversity conservation or ecosystem function, just that they are considered more replaceable.

Higher scoring habitats that were flagged as more irreplaceable were unfeasible to restore and more restricted in spatial extent. Examples include deep-sea habitats (only found within the Western Approaches in English waters) and features such as caves. In terms of uniqueness (environmental context), all habitats that received a high score in one or more categories (physical setting, hydrodynamics, biological assemblage) scored highly overall, as these were likely to be rarer and difficult to recreate elsewhere. Rare habitats that are very restricted in extent and distribution include the unique rock habitats in the upper Tamar estuary, South Devon and Cornwall (most upper estuary reaches are characterized by muddy sediments).

Habitats that recover slowly from disturbance are typically defined by long-lived, slow growing species such as maerl, horse mussel beds (see main image), and cold-water coral reefs. These habitats are vulnerable to physical disturbance, so have tended to decline and are now rarer due to exploitation and exposure to fishing using gears in contact with the bottom.  

In conclusion

Many of the 45 habitats which have high irreplaceability scores are recognized through Marine Protected Area (MPA) designations, conservation targets, and lists of habitats and features of conservation interest. This is a positive outcome, as it shows that the habitats with the highest irreplaceability scores are being protected by UK legislation.

The project has assessed irreplaceability using a set of criteria that could be applied consistently to English marine habitats. We recognize that irreplaceability is dependent on the factors considered and the context within which assessments are made. For example, a sandbank with an abundant sand eel population lies within easy flying range of the seabirds that nest at Flamborough Head, Yorkshire. If this sandbank were further offshore, the seabirds couldn’t access it as a feeding ground. The combination of nesting site and sand eel population makes the sandbank irreplaceable for supporting birds.

The project isn’t considered to be a final iteration, but instead provides a starting point for defining MIH. Subsequent stages will, hopefully, map the MIH in English waters. The habitat evaluation is not intended to replace any aspects of marine management or licensing such as surveys and impact assessments, but is a resource to assist developers, planners, and regulators to avoid habitats that cannot be replaced or recreated elsewhere in compensation for their loss. The full report has now been published and is available online (see references) alongside the Excel scoring spreadsheet.

Heidi Tillin (hmtillin@hotmail.co.uk)

Charlotte Johnson (Charlotte.Johnson@naturalengland.co.uk), Natural England.

The MBA project team included expert input on ecology and habitats from Keith Hiscock, Harvey Tyler-Walters, and Nova Mieszkowska and was supported by Amy Watson. The project team greatly appreciate the expert input from staff of the UK Statutory Nature Conservation Agencies: JNCC, Natural England, and Natural Resources Wales, the Marine Management Organisation, Defra, and academics from the Universities of Hull, Plymouth, and Liverpool. For further information about the project, contact Charlotte Johnson.

Further reading

Tillin, H.M., Watson, A., Tyler-Walters, H., Mieszkowska, N. & Hiscock, K. 2022. Defining Marine Irreplaceable Habitats. NECR474. Natural England.


[1] The EUNIS habitat classification is a comprehensive system covering the terrestrial and marine habitat types of the European land mass and its surrounding seas.