Biodiversity & Conservation

Benchmarks for the Assessment of Sensitivity and Recoverability


The sensitivity of a species (or community) is an estimate of its intolerance to damage from an external factor and is determined by its biological and physical characteristics. Sensitivity must be estimated (assessed) in response to a change in a specific environmental factor and to the magnitude, duration, or frequency of that change.

Marine organisms may be affected by a number of human activities and natural events. The effects of an activity (or event) are dependant on the receiving environment. The same activity (or event) in different locations may have different effects. For example, an activity that markedly increased siltation may have little effect in a turbid estuary whereas it would probably have significant effects in a sheltered embayment. Therefore, the effects of an activity and the resultant change in environmental factors are site specific and cannot be generalised.

Hence, the magnitude, duration, and frequency of change in an environmental factor, are dependant on both the nature and scale of the human activity or natural event, as well as the location or site at which the activity or event occurs. Therefore, it was necessary to set standard 'benchmarks' to enable the assessment of sensitivity relative to a specified change in an environmental factor.

The use of a standard benchmark level of change in an environmental factor ensures that the sensitivity of different species or communities is assessed with respect to the same level of change or perturbation. In addition, standard benchmarks allow the relative sensitivity of different species and communities to be compared.

Derivation of benchmarks


The standard benchmarks were derived from a review of relevant literature. Key references are listed below. In many cases, the available information did not allow ‘quantified’ benchmarks to be set. Therefore, it was necessary to adopt a mixture of approaches to derive the benchmarks, depending on the environmental factor. The following approaches were used.

The chosen magnitude and duration of each benchmark reflects the reported or likely change in the factor because of relevant maritime activities or natural events, unless otherwise stated, and represent a hypothetical ‘average’ level of effect. It was necessary to avoid negligible or extreme levels of effect, as these would under or over estimate sensitivity respectively.

To assess sensitivity or recoverability a hypothetical ‘average’ population is considered. A hypothetical ‘average’ population may be thought of as a population in the middle of its habitat preferences with respect to, for instance, its biological zone, temperature or salinity tolerances, wave exposure tolerances, or geographical distribution. Populations at the extremes of their habitat preferences (or range) are likely to be exposed to environmental conditions close to their physiological tolerances limits and are, therefore, likely to be more sensitive. In addition, where appropriate, increases or decreases in an environmental factor are assessed separately.

Note: The benchmarks are intended to:

The chosen benchmark levels of change in environmental factors are likely to affect different marine species to different degrees. Therefore, the benchmarks are considered precautionary in nature (sensu ‘the precautionary approach’).

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Duration of change


In addition to a magnitude (or level of effect), the benchmarks specify a duration wherever possible. The magnitude or duration of changes in environmental factors include:

Where activities are likely to cause more than one type of change, separate benchmarks are given for short term acute or long term chronic changes. Where there was clear evidence on the known sensitivity or effect of activities on a particular factor, representative time frames were used. For example, Crisp ed. (1964) reported mortalities for a wide range of marine species resulting from a drop in temperature of 5-6 ºC.

However, in most cases, ‘short-term’ was defined arbitrarily as 'one month' and a period of one year was chosen arbitrarily to represent 'long term' change since this period accommodated the life cycle of many marine species.

In all cases, the rationale behind the chosen benchmarks, together with a definition of the term is provided under further details.

The interactions between an activity and its effects are extremely complex and the benchmarks should not be considered perfect. The MarLIN team would welcome any comments or additional guidance.

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Interpretation of benchmarks


Short term acute and long term chronic change were chosen because they represented the most likely effects of maritime activities. The benchmarks are only a 'starting point' and sensitivity assessments can be interpolated if the known or predicted change is greater or less than the benchmark. For example:

However, the frequency of change should be compared with the species or communities recoverability. If the species or community is likely to recover between the impacting events then it may not exhibit an increased sensitivity.

Activities that result in incremental long term change, such as climate change, are difficult to assess since the given level of change varies with time. These effects have NOT been addressed within the present sensitivity assessments. However, benchmarks could be compared to the predicted level of change at specific time intervals.

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PLEASE NOTE:

Sensitivity assessments are indicative qualitative judgements based on the best available scientific information. They represent the most likely (probable) result of a given change in a factor. They do not allow quantitative analysis. The sensitivity assessments should be used in conjunction with the key information provided with each species. In all cases, the rationale (explanation) behind each sensitivity assessment, the relevant key information and references are highlighted.

The benchmarks that follow were revised in March 2003, based on the experience gained after three years research on the biology and sensitivity of marine species and biotopes. The benchmarks used prior to March 2003 are published in Tyler-Walters et al. (2001), which may be viewed in the ‘Reports’ section of the MarLIN Website.

References:

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Physical factors


Substratum loss

All of substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details.

Smothering

All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details.

Changes in suspended sediment

An arbitrary short term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for 1 month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details.

Desiccation
  1. A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for 1 hour.
  2. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details.
Changes in emergence

A 1 hour change in the time covered or not covered by the sea for a period of 1 year. Further details.

Changes in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year. For example from moderately strong (1-3 knots) to very weak (negligible). Further details.

Changes in temperature
  1. A short term, acute change in temperature; e.g., a 5 ° C change in the temperature range for 3 consecutive days. This definition includes ‘short term’ thermal discharges.
  2. A long term, chronic change in temperature; e.g. a 2 ° C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or communities. Further details.

Changes in turbidity
  1. A short term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
  2. A long term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details.
Changes in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of 1 year. Further details.

Noise

Underwater noise levels e.g., the regular passing of a 30 metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for 1 month during important feeding or breeding periods.

Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for 1 month during important feeding or breeding periods. Further details.

Visual presence

The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details.

Physical disturbance or abrasion

This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest.

Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment.

Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details.

Displacement

Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details.

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Chemical factors


Changes in levels of synthetic chemicals

Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example:

  • evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity;
  • evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity;
  • evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity.

The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details.

Changes in levels of heavy metals
Changes in levels of hydrocarbons
Changes in levels of radionuclides
Changes in levels of nutrients
Changes in salinity
  1. A short term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
  2. A long term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.
Changes in oxygenation

Exposure to dissolved oxygen concentration of 2mg/l for 1 week. Further details.

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Biological factors


Introduction of microbial pathogens and parasites

Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details.

Introduction of alien or non- native species

Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details.

Specific targeted extraction of this species

Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly.

Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Specific targeted extraction of other species

A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed.

Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

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