Scottish seabird conservation action plan: vulnerability report

Approach To Determining Species Vulnerability To Pressures

For the purpose of this report, species vulnerability is measured as the risk of impact to the Scottish population of each of the named seabird species from a known human-induced pressure acting in Scottish waters. Vulnerability is considered to be a function of the sensitivity of a species to a particular pressure combined with the level of potential interaction (exposure) the species has to that pressure. A three-step approach was used to determine species vulnerabilities:

Step 1: Assess species sensitivity to all marine pressures identified in the Feature Activity Sensitivity Tool (FeAST).

Step 2: Assess the level of exposure for each species to each pressure by overlapping seabird distribution data with mapped information on activities associated with pressures assigned a high or medium sensitivity score.

Step 3: Combine the sensitivity and exposure scores to determine the species vulnerability score to each species-pressure combination (see also Figure 3).

Figure 3: Diagram explaining how an activity results in a pressure that affects a species. The species tolerance and recovery to that pressure will determine their sensitivity to it, and sensitivity alongside exposure to the pressure will result in the species level of vulnerability to that particular pressure.

Step 1: Assessing Species Sensitivity

The sensitivity assessments of each species to each pressure, except bycatch (see below), are taken from Rogerson et al. (2021). Rogerson et al. completed a detailed literature review to provide an up-to-date evidence base for each species to each marine pressure listed in FeAST. Using this evidence base, Rogerson et al. followed the FeAST methodology to determine the sensitivity of each seabird species to pressures.

FeAST is a web-based application which allows users to investigate the sensitivity of marine features (habitats, species, geology, and landforms) in Scottish seas, to pressures arising from human activities. It has been used to underpin assessments in Scotland’s Marine Assessment 2020, and for discussions on management requirements for Marine Protected Areas and Priority Marine Features.

FeAST includes 36 human induced pressures, each with a clear definition and impact benchmark (where possible) at which feature sensitivities are assessed. The benchmarks define the intensity at which the pressure is applied for the purpose of the assessment, e.g. it is being assessed if the sensitivity of a population to disturbance is high, medium or low at a defined benchmark level of disturbance. This ensures the assessments are consistent across all marine habitats and species. FeAST’s marine pressures list is adapted from an inventory and prepared and agreed by the OSPAR Joint Assessment Monitoring Programme.

The FeAST assessment approach is based upon the methods originally developed by MarLIN and later adapted for use with highly mobile species by Pérez-Domínguez et al. (2016). In this method, sensitivity is defined as a combination of initial resistance (known as ‘tolerance’ in the FeAST) of a feature to a pressure, and its resilience (known as ‘recovery’ in the FeAST) from the impact of a pressure, both based on the perceived impact to the species population.

Species tolerance and recovery are assigned ‘low’, ‘medium’, or ‘high’ using standard criteria (Table A1.1 and A1.2) and based on available evidence. Sensitivity scores were then derived using the matrix shown in Table 2 combining tolerance and recovery scores. Further details on the criteria used by Rogerson et al. (2021) to assess tolerance, recovery and confidence (Table A1.3) in the overall sensitivity score are provided in Annex 1.

Standard criteria were also used to determine the confidence in the overall sensitivity score (Annex 1) which is a combination of the confidence in both the tolerance and recovery scores with the lower of the two assigned as a precautionary approach. As a result, the seabird sensitivity assessments reflect the different degrees in confidence associated with the overall tolerance score confidence because all species recovery confidence scores were assigned a ‘high’ level of confidence.

While the sensitivity to most pressures was taken from Rogerson et al. (2021), seabird vulnerability assessments to bycatch in UK waters were taken from Bradbury et al. (2017) to ensure consistency in the use of vulnerability assessments that were already available. Bradbury et al. (2017) calculated a Seabird Sensitivity Index (SSI) by scoring known traits of conservation status, demography/ecology and behaviour. Each trait was scored by panel of nine experts and the median scores across experts were used to provide an overall index.

Those pressures with a sensitive or low sensitivity score, or where they could not be assessed due to lack of data (Table A1.4) were not considered further for assessment as actions to address activities associated with these pressures were considered not the most important and/or beneficial. All high and medium sensitivity scores resulting from the analysis conducted by Rogerson et al. (2021) were further sense checked through expert judgment to ensure the activities associated with these pressures were appropriate for focussed marine and/or coastal seabird conservation actions. Six additional pressures were excluded from further assessment at this stage due to the main source of pressure not being associated with coastal and/or marine activities, and/or birds experience the pressure only at a small scale in their extent (Table A1.5). These pressures were not considered to be the highest risks to seabirds in Scotland, and therefore were not considered further.

All remaining pressures resulting in high or medium seabird sensitivities were identified as the main pressures impacting seabirds in Scottish water and at coastal colonies. These were taken forward to the exposure assessment.

Step 2: Assessing Exposure

To assess exposure, the distribution of activities associated with the main pressures resulting from the sensitivity assessments were compared to the breeding colonies and at sea distributions in Scotland for each of the 22 seabird species covered by the Vulnerability Report.

Pressure-activity associations i.e. human-induced activities that are associated with producing a pressure were adopted from FeAST.

Data on Distribution

Seabird breeding colony locations were taken from the Seabird Monitoring Programme^[2] prior to publication of Seabirds Count^[3] census. Seabird foraging distributions were mapped using data from Waggitt et al. (2019).

The pressure-activity associations helped to narrow-down appropriate sources of data to determine the extent of pressure distributions. There was substantial variation in the information available for distribution, scale, intensity and/or duration of each pressure. The sources for pressure-activity distribution information on each pressure/threat are listed in Table A2.1.

Assessing Seabird Distribution

Seabird species distributions in Scottish waters were categorised as having widespread, restricted or localised distributions through expert judgment. The criteria used to assign each species to a category were a combination of:

• at sea distributional maps (for example see Figure A2.1); and
• known breeding colony locations.

The quality of data available for analysing seabird species distributions was variable and therefore confidence in the species distribution scores was categorised as ‘low’, ‘medium’, or ‘high’ through expert judgment. The criteria used to assign data confidence to a category were a combination of:

• the extent of regular breeding survey effort across the species range, along with the detectability of nests,
• how detectable the species is during at-sea or from digital aerial surveys, and;
• the amount of tracking data available and number of colonies targeted.

The results of the species distribution analysis and the data confidence scores are listed in Table A2.2.

Assessing Pressure Distribution

The distribution of key pressures resulting from the sensitivity assessments were analysed using distribution maps for activities associated with the main pressures (Figures A2.2, A2.3, A2.4, A2.5), where this information was available. Where information on the spatial distribution of a pressure was not available (e.g. climate change, marine litter) a qualitative judgement utilising expert opinion was made on the likely distribution of the pressure.

The distribution of the activities and pressures for each threat/impact were categorised as ‘widespread’, ‘restricted’ or ‘localised’ following the same method as species distribution.

The quality of pressure data available for analysis was variable and therefore confidence in these scores was also categorised as ‘low’, ‘medium’, or ‘high’ through expert judgment. The criteria used to assign data confidence to a category were a combination of:

• whether data is available for all the main activities leading to a pressure;
• the spatial coverage of any data, whether it covers the whole of Scotland or certain regions only; and
• the relevance of the data available to the impact pathway identified between the relevant activities and seabird species.

The results of the pressure distribution categorisations and the data confidence scores are listed in Table A2.3.

Combining Species Distribution & Pressure-Activity Distribution Scores

A matrix (Table 3) was used to combine the respective results from the species and pressure-activity distribution analysis. Different combinations were assigned a relative score of ‘high’, ‘moderate’, ‘low’ or ‘negligible’ to identify the potential level exposure of each species to each potential impact/threat. These scores were predominantly based on the approximate spatial extent of the overlap between a species and a pressure, although where possible the intensity of the pressure was also considered.

The exposure scores were then sense checked through expert judgment, and where required, scores were revised to take account of the known or perceived degree of overlap between the distributions of each species and each pressure/activity (Table A2.4).

Where a species was considered to have no or negligible exposure to a pressure, these were classified as ‘negligible’ (e.g. little gull (non-breeding) and habitat loss & mortality from invasive predatory mammals).

Confidence in the exposure assessment was taken as the lowest score of confidence in either the species or pressure distribution data, according to the matrix in Table 4.

Step 3: Assessing Vulnerability

To assess vulnerability of a species to a threat/impact, another matrix (Table 5) was used to combine the respective results from the species and pressure-activity distribution analysis. Different combinations of the sensitivity scores from step 1 and the exposure scores from step 2 were assigned a relative score of ‘high’, ‘moderate’, ‘low’ or ‘negligible’ to identify the potential level of vulnerability of each species to each potential threat/impact.

Confidence in the vulnerability assessment was taken as the lowest score of confidence from the sensitivity and exposure confidence scores (Table 6).