Marine Evidence based Sensitivity Assessment (MarESA)

General concepts

The 'concept' of sensitivity has been developed over many decades and applied in coastal and marine habitats. Numerous approaches have been developed, applied at a range of spatial scales, and to a variety of management questions (see Roberts et al., 2010).

The most common approaches define 'sensitivity' as a product of:

  • the likelihood of damage (termed intolerance or resistance) due to a pressure;

  • the rate of (or time taken for) recovery (termed recoverability, or resilience) once the pressure has abated or been removed.

Or in other words "A species (population) is defined as very sensitive when it is easily adversely affected by human activity (e.g. low resistance) and recovery is only achieved after a prolonged period, if at all (e.g. low resilience or recoverability)" (OSPAR, 2008; Laffoley et al., 2000).

Sensitivity is an inherent characteristic determined by the biology/ecology of the feature (species or habitat) in question. But it is a 'relative' concept as it depends of the degree (expressed as magnitude, extent, frequency or duration) of the effect on the feature. Therefore, sensitivity assessment uses a variety of standardized thresholds, categories and ranks to ensure that the assessments of ‘relative’ sensitivity compare ‘like with like’.  These are:

  1. standard categories of human activities and natural events, and their resultant ‘pressures’ on the environment.
  2. descriptors of the nature of the pressure (i.e. type of pressure, e.g. temperature change, physical disturbance or oxygen depletion).
  3. descriptors of the pressure (e.g. magnitude, extent, duration and frequency of the effect) termed the pressure benchmark;
  4. descriptors of resultant change / damage (intolerance/resistance) (i.e. proportion of species population lost, area of habitat lost/damaged);
  5. categories or ranks of recovery (recoverability / resilience) thought to be significant; and
  6. resultant ranks of sensitivity and/or vulnerability.

Common terms and definitions





The intolerance of a species or habitat to damage from an external factor and the time taken for its subsequent recovery.

Laffoley et al. (2000); Tyler-Walters & Hiscock (2005).


The likelihood of change when a pressure is applied to a feature (receptor) and is a function of the ability of the feature to tolerate or resist change (resistance) and its ability to recover from impact (resilience).

Tillin et al. (2010), Tillin & Hull (2003), Tillin & Tyler-Walters (2014)

Resistance (Intolerance)

Resistance characteristics indicate whether a receptor can absorb disturbance or stress without changing character.

Holling (1973)

Resilience (Recoverability)

The ability of a receptor to recover from disturbance or stress.

Holling (1973)


Vulnerability is a measure of the degree of exposure of a receptor to a pressure to which it is sensitive.

Based on Hiscock et al. 1999; Oakwood Environmental Ltd (2002).


The mechanism through which an activity has an effect on any part of the ecosystem.  The nature of the pressure is determined by activity type, intensity and distribution.

Robinson et al. (2008)


The action of a pressure on a receptor, with regard to the extent, magnitude and duration of the pressure.

Robinson et al. (2008)

Sensitivity assessment process

Sensitivity assessment involves a detailed literature review and compilation of the evidence on the effect of a given pressure on the feature (species or habitat) in question, at the pressure benchmark level of effect, on a pressure by pressure basis (Figure 1). The sensitivity assessment process 'identifies elements of the feature'  that are important for the structure and functioning of the community or characteristic of the habitat, based on the literature review. Clearly, where the feature under assessment is a single species, that species is assessed.

Habitat sensitivity assessment assumes that the sensitivity of a habitat is dependent on the physical nature of the habitat, and the sensitivity of the species that make up the community present.  In practice, communities can be composed of many tens or hundreds of species. Therefore, the species identified as important for the structure and functioning of the community or characteristic of the habitat are used to focus the assessment (Figure 1).  However, wherever possible, all component species of the habitat are considered in the sensitivity assessment.

The documented 'evidence base'

The sensitivity assessment process results in species and or habitats (features) ranked by relative sensitivity. The evidence used in the assessment is documented throughout. This resultant 'evidence base' is the ultimate source of information for the application of the sensitivity assessments to management and planning decisions.


Sensitivity assessment routinely indicates the confidence in the assessment, based on the quality of the evidence used and its applicability to the assessment of the likely effects of a pressure on a given feature (species or habitat).

Limitations and assumptions

Sensitivity assessments need to be applied carefully by trained marine biologists, for the following reasons.

  • The sensitivity assessments are generic and NOT site specific.  They are based on the likely effects of a pressure on a ‘hypothetical’ population in the middle of its ‘environmental range’.
  • Sensitivity assessments are NOT absolute values but are relative to the magnitude, extent, duration and frequency of the pressure effecting the species or community and habitat in question; thus the assessment scores are very dependent on the pressure benchmark levels used.
  • Sensitivity assessments presented are general assessments that indicate the likely effects of a given pressure (likely to arise from one or more activities) on species or habitats of conservation concern.
  • The assessments are based on the magnitude and duration of pressures (where specified) but do not take account of spatial or temporal scale
  • The significance of impacts arising from pressures also needs to take account of the scale of the features;
  • There are limitations of the scientific evidence on the biology of features and their responses to environmental pressures on which the sensitivity assessments have been based.
  • The sensitivity assessment methodology takes account of both resistance and resilience (recovery).  Recovery pre-supposes that the pressure has been alleviated but this will generally only be the case where management measures are implemented.
  • Recovery is assumed to have occurred if a species population and/or habitat returns to a state that existed prior to the impact of a given pressure, not to some hypothetical pristine condition. 
  • Furthermore, sensitivity assessment assumes recovery to a ‘recognizable’ habitat or similar population of species, rather than presume recovery of all species in the community and/or total recovery to prior biodiversity.
  • As a general rule, where resistance is ‘low’, the need for management measures should be considered, irrespective of the overall sensitivity assessment (for example where resilience is assumed to be high).
  • A rank of ‘not sensitive’ does not mean that no impact is possible from a particular ‘pressure vs. feature’ combination, only that a limited impact was judged to be likely at the specified level of the benchmark.
  • In line with the precautionary principle, a lack of scientific certainty should not, on its own, be a sufficient reason for not implementing management measures or other action.

Marine Evidence based Sensitivity Assessment (MarESA) approach (2014 onwards)

The MB0102 sensitivity assessment methodology was developed to create a pressure vs. feature sensitivity matrix to support MCZ management (Tillin et al. 2010). Due to the project MB0102 timescales the approach relied on expert judgement to create sensitivity assessments at two workshops.  The Tillin et al. (2010) methodology was modified by Tillin and Hull (2012-2013), who introduced a detailed evaluation and audit trail of evidence on which to base the sensitivity assessments. The revised methodology (hereforth termed MarESA) was subsequently applied to Ecological Groups based on species characteristic of offshore, circalittoral biotopes (Tillin and Tyler-Walters, 2014) and to biogenic habitats, that is, seagrass (d’Avack et al. 2014), Mytilus edulis beds (Mainwaring et al. 2014) and Sabellaria spinulosa reefs (Gibb et al. 2014).  Note - the MarESA approach supercedes and replaces the MarLIN approach

Sensitivity assessment involves the following stages:

  1. Define the key elements of the feature (in terms of life history, and ecology of the key and characterizing species);
  2. Assess the feature's resistance (tolerance) and resilience (recovery) to a defined intensity of pressure (the benchmark);
  3. Combine resistance and resilience to derive an overall sensitivity score;
  4. Assess the confidence in the sensitivity assessments;
  5. Document of the evidence used; and 
  6. Undertake quality assurance and peer review.

1. Key elements of the feature

In order to assess sensitivity, elements of the features must be selected as the basis of the assessment. The assessment of sensitivity should be guided by the presence of key structural or functional species/assemblages and/or those that characterize the biotope groups. The species (or assemblages) which are regarded as key structural or functional species should be identified and a full audit trail provided. The types of species that should be identified for the habitat assessments are provided in Table 1. Physical and chemical characteristics are also considered where these structure the community.

Table 1 – Types of species identified for assessment (definitions adopted from MarLIN).



Key structural species

The species provides a distinct habitat that supports an associated community. Loss/degradation of this species population would result in loss/degradation of the associated community.

Key functional species

Species that maintain community structure and function through interactions with other members of that community (for example through predation, or grazing). Loss/degradation of this species population would result in rapid, cascading changes in the community.

Important characteristic species

Species characteristic of the biotope (dominant, and frequent) and important for the classification of the habitat. Loss/degradation of these species populations may result in changes in habitat classification.

2. Resistance and resilience assessment

The resistance and resilience of the feature is assessed against each pressure using the available evidence. A standard list of pressures, pressure descriptions  and 'benchmark' levels of each pressure has been developed. The benchmarks are designed to provide a ‘standard’ level of pressure against which to assess resistance/resilience. The assessment scales used for resistance and resilience are given in Tables 2 and 3 respectively.

Table 2. Assessment scale for resistance (tolerance) to a defined intensity of pressure




Key functional, structural, characterizing species severely decline and/or physico-chemical parameters are also affected e.g. removal of habitats causing change in habitats type. A severe decline/reduction relates to the loss of 75% of the extent, density or abundance of the selected species or habitat component e.g. loss of 75% substratum (where this can be sensibly applied).


Significant mortality of key and characterizing species with some effects on physico-chemical character of habitat. A significant decline/reduction relates to the loss of 25-75% of the extent, density, or abundance of the selected species or habitat component e.g. loss of 25-75% of substratum.


Some mortality of species (can be significant where these are not keystone structural/functional and characterizing species) without change to habitats relates to the loss <25% of the species or habitat component.


No significant effects to the physico-chemical character of habitat and no effect on population viability of key/characterizing species but may affect feeding, respiration and reproduction rates.

Table 3. Assessment scale for resilience (recovery)



Very Low

Negligible or prolonged recovery possible; at least 25 years to recover structure and function


Full recovery within 10-25 years


Full recovery within 2-10 years


Full recovery within 2 years

‘Full recovery’ is envisaged as a return to the state of the habitat that existed prior to impact. However, this does not necessarily mean that every component species has returned to its prior condition, abundance or extent but that the relevant functional components are present and the habitat is structurally and functionally recognizable as the initial habitat of interest.

3. Overall sensitivity assessment

The resistance and resilience scores are combined, as follows, to give an overall sensitivity score as shown in Table 4.

Table 4. The combination of resistance and resilience scores to categorize sensitivity








Very  Low



















Not sensitive


Not Sensitive - is recorded where the habitat or species has a high resistance (and hence is likely to recovery quickly i.e. a high resilience) at the benchmark level of pressure.  It should be noted that the species or habitat may be sensitive at pressure levels higher than the benchmark (i.e. where the pressure is of greater intensity, magnitude or duration).

A sensitivity assessment is not always possible. The following terms are given to explain why.

No Exposure (NEx) – is recorded where there will be no exposure to a particular pressure.  For example, deep mud habitats are not exposed to changes in emersion.

Not Relevant (NR) – is recorded where the evidence base suggests that there is no direct interaction between the pressure and the biotope group.  NR is also used to denote fields/scored that are literally ‘not relevant’.

Not Assessed (NA) – is recorded where the evidence base is not considered to be adequate for an assessment of sensitivity to be made.

No Evidence (NEv) – is recorded where there is not enough evidence to assess the sensitivity of the specific feature/pressure combination and there is no suitable proxy information regarding the habitat (biotope) on which to base decisions.  For example, some species have a limited distribution (e.g. a few or only one locations) so that even basic physical, chemical or biological tolerances cannot be inferred.  An assessment of ‘No Evidence’ should not be taken to mean that there is no information available for features.

4. Confidence assessment

Project MB0102 (Tillin et al. 2010) provided a single confidence score based on the robustness of the underlying evidence and was developed for assessments based on expert judgement.  The approach developed by Tillin and Hull (2012-2013 Reports I-VII) was adapted for subsequent use for pressure-sensitivity assessments prepared by the project team for JNCC (Tillin and Tyler-Walters, 2014, d’Avack et al. 2014).

This approach assesses confidence in the evidence for three categories, the quality of the evidence or information used, the degree to which evidence is applicable to the assessment and the degree of agreement between evidence types (Table 5).

The confidence assessment categories for resistance and resilience are combined to give an overall confidence score for the confidence category (i.e. quality of information sources, applicability of evidence and degree of concordance) for each individual feature/pressure assessment, using Table 6.

Table 5. Confidence assessment categories for evidence.

Confidence level

Quality of evidence (information sources)

Applicability of evidence

Degree of concordance (agreement between studies)

High (H)

Based on peer reviewed papers (observational or experimental) or grey literature reports by established agencies (give number) on the feature (habitat, its component species, or species of interest).

Assessment based on the same pressures acting on the same type of feature ( habitat, its component species, or species of interest) in the UK

Agree on the direction and magnitude (of impact or recovery)

Medium (M)

 Based on some peer reviewed papers but relies heavily on grey literature or expert judgement on feature ( habitat, its component species, or species of interest) or similar features

Assessment based on similar pressures on the feature (habitat, its component species, or species of interest) in other areas.

Agree on direction but not magnitude (of impact or recovery)

Low (L)

Based on expert judgement

Assessment based on proxies for pressures e.g. natural disturbance events

Do not agree on direction or magnitude (of impact or recovery)

Table 6.  Example combined confidence assessments (based on 'Quality of evidence' assessment only).


Resistance confidence score

Resilience confidence score
















5. Documented 'evidence base' 

So that the basis of the sensitivity assessment is transparent and repeatable the evidence base and justification for the sensitivity assessments is recorded.  A complete and accurate account of the evidence that was used to make the assessments is presented for each sensitivity assessment in the form of the literature review and a sensitivity matrix that records a summary of the assessment, the sensitivity scores and the confidence levels.

6. Quality assurance and peer review

The resultant sensitivity reviews are subject to quality assurance by the MarLIN Editor and, wherever possible, subject to peer review by one or more independant experts.

Differences between MarESA and MarLIN methods

The MarESA approach to sensitvity assessment is a development of the MarLIN approach based on new scales and benchmarks introduced by the MB0102 project and subsequent refinement of the approach (see Tyler-Walters & Hiscock, 2005; Tillin et al., 2010; Tillin & Hull, 2013; Tillin & Tyler-Walters, 2014).  It represents an ongoing evolution of the sensitivity assessment process.

The approaches share the following. 

  • A defined process of literature review, documentation of evidence, and systematic assessment of the evidence to assess sensitivity (see Figure 1)
  • The identification of elements of the feature for assessment, that is, the structuring, functioning and charactersitic species
  • The definition of benchmark levels of change in the pressures (or factors) likely to affect marine species and habitats
  • The combination of an assessment of the likelihood of damage as a result of a specifc pressure (resistance or intolerance) and the time taken for the species or habitat to recover (resilience or recoverabilty) to determine sensitivity
  • The estimation of confidence in the evidence
  • Peer review where-ever possible. 

The approaches differ in the following.

  1. The list of pressures likely affect marine species and habitats has been updated and expanded in line with statutory conservation agency advice and ongoing work to support marine planning and management advice in the UK, and the needs of the Marine Strategy Framework Directive (MSFD)
  2. As a result, the benchmarks for the expanded list of pressures have also been redefined.
  3. The resistance scale has changed slightly, with clear 'quantified' and 'qualified' terms.
  4. The resilience scale has changed to a four point scale (rather than MarLIN's seven point scale) based on Holling (1973).
  5. The combined sensitivity scale is changed to a four point scale (rather than MarLIN's seven point scale).
  6. The confidence assessment has been rewritten and expanded, and now examines the 'quality', 'applicability', and 'concordance' of the evidence, based on a three point scale.  

Points 2, 4,and 5 above mean that the MarLIN and MarESA sensitivity assessment ranks are not directly comparable. However, the documented 'evidence base' remains the basis for all the assessments, and their application in a management or decision-making context.


Note all MarLIN reports are available under publications.

d’Avack, E.A.S., Tillin, H., Jackson, E.L. & Tyler-Walters, H., 2014. Assessing the sensitivity of seagrass bed biotopes to pressures associated with marine activities. Joint Nature Conservation Committee, JNCC Report No. 505, Peterborough, 83 pp. 

Gibb, N., Tillin, H.M., Pearce, B. & Tyler-Walters, H., 2014. Assessing the sensitivity of Sabellaria spinulosa to pressures associated with marine activities. Joint Nature Conservation Committee. JNCC report No. 504, Peterborough, 67 pp. 

Hiscock, K, Jackson, A. & Lear, D., 1999. Assessing seabed species and ecosystem sensitivities: existing approaches and development, October 1999 edition. Report to the Department of Environment, Transport and the Regions from the Marine Life Information Network (MarLIN). Marine Biological Association of the United Kingdom, Plymouth. [MarLIN Report No. 1.]

Holling C.S., 1973. Resilience and stability of ecological systems. Annual Review of Ecology and Systematics, 4(1), 1-23.

Holt, T.J., Jones, D. R., Hawkins, S.J. & Hartnoll, R.G., 1997. The sensitivity of marine communities to man-induced change. Nature Conservation and the Irish Sea seminar. 6th February 1997, pp. 6-23. Irish Sea Forum, Liverpool, Seminar Report No. 15.

Holt, T.J., Jones, D.R., Hawkins, S.J. & Hartnoll, R.G., 1995. The sensitivity of marine communities to man-induced change - a scoping report. Countryside Council for Wales, Bangor, CCW Contract Science Report, No. 65.

Laffoley, D.A., Connor, D.W., Tasker, M.L. & Bines, T., 2000. Nationally important seascapes, habitats and species. A recommended approach to their identification, conservation and protection, pp. 17. Peterborough: English Nature.

Mainwaring, K., Tillin, H. & Tyler-Walters, H., 2014. Assessing the sensitivity of blue mussel beds to pressures associated with human activities. Joint Nature Conservation Committee, JNCC Report No. 506., Peterborough, 96 pp. 

McMath, A., Cooke, A., Jones, M., Emblow, C.S., Wyn, G., Roberts, S., Costello, M.J., Cook, B. & Sides, E.M., 2000. Sensitivity mapping of inshore marine biotopes in the southern Irish Sea (SensMap): Final report. Report by the Countryside Council for Wales (CCW), Ecological Consultancy Services Ltd (Ecoserve), Dchas, the Heritage Service, 116 pp. [Maritime Ireland /Wales INTERREG Reference no. 21014001].

Oakwood Environmental Ltd, 2002. Development of a methodology for the assessment of cumulative effects of marine activities using Liverpool Bay as a case study. CCW Contract Science Report No 522.

OSPAR, 2003. Annex V to the OSPAR Convention. Criteria for the Identification of Species and Habitats in need of Protection and their Method of Application (The Texel-Faial Criteria). OSPAR 03/17/1-E.   13 pp. 

OSPAR, 2008. OSPAR List of Threatened and/or Declining Species and Habitats (Reference Number: 2008-6). OSPAR Convention For The Protection Of The Marine Environment Of The North-East Atlantic <>

Roberts, C., Smith, C., H., T. & Tyler-Walters, H., 2010. Review of existing approaches to evaluate marine habitat vulnerability to commercial fishing activities. Report to the Environment Agency from the Marine Life Information Network and ABP Marine Environmental Research Ltd. Environment Agency Evidence Report: SC080016/R3. Environment Agency, Peterborough  <>

Tillin, H. & Tyler-Walters, H., 2014. Assessing the sensitivity of subtidal sedimentary habitats to pressures associated with marine activities. Phase 2 Report – Literature review and sensitivity assessments for ecological groups for circalittoral and offshore Level 5 biotopes. JNCC Report No. 512B,  260 pp. 

Tillin, H.M., Hull, S.C. & Tyler-Walters, H., 2010. Development of a sensitivity matrix (pressures-MCZ/MPA features). Report to the Department of the Environment, Food and Rural Affairs from ABPmer, Southampton and the Marine Life Information Network (MarLIN) Plymouth: Marine Biological Association of the UK., Defra Contract no. MB0102 Task 3A, Report no. 22., London, 145 pp. <>

Tyler-Walters, H. & Hiscock, K., 2005. Impact of human activities on benthic biotopes and species. Report to Department for Environment, Food and Rural Affairs from the Marine Life Information Network (MarLIN), contract no. CDEP 84/5/244. Marine Biological Association of the UK, Plymouth. [View final report]

Tyler-Walters, H., Hiscock, K., Lear, D.B. & Jackson, A., 2001. Identifying species and ecosystem sensitivities. Report to the Department for Environment, Food and Rural Affairs from the Marine Life Information Network (MarLIN), Marine Biological Association of the United Kingdom, Plymouth. Contract CW0826.

Tyler-Walters, H. & Jackson, A. 1999. Assessing seabed species and ecosystems sensitivities. Rationale and user guide, January 2000 edition. Report to English Nature, Scottish Natural Heritage and the Department of the Environment Transport and the Regions from the Marine Life Information Network (MarLIN). Plymouth, Marine Biological Association of the UK. (MarLIN Report No. 4.).