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Rarity criterion in marine biological evaluation

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This article formed part of discussions centred around valuing the marine environment at a workshop held from 6 to 8 December 2006 at Ghent (Belgium). The workshop was a joint venture of the EU CA ENCORA and the EU NoE MARBEF. Both Theme 7 within ENCORA and Theme 3 within MARBEF deal with marine/coastal biological valuation and the workshop aimed to reach a consensus on this topic. The workshop report can be downloaded at (http://www.marbef.org/documents/Theme3/GhentWS/report.pdf) It specifically considers the use of 'rarity' as a criterion in assessing marine nature conservation features.


The ‘rarity’ criterion is defined[1] as the degree to which an area is characterized by unique, rare or distinct features (landscapes/habitats/ communities/species/ecological functions/ geomorphological and/or hydrological characteristics) for which no alternatives exist.[2][3][4][5][6]

Application of the criterion

Rarity can be assessed on different scales, e.g. national, regional, global.

In order to be able to assess the rarity of marine species or communities on a regional or global scale, international lists of rare species, habitats or communities are needed. Unlike the terrestrial environment, however, very few marine species are included in Red Data Books, like the IUCN Red Lists or the appendices of CITES, CMS (RAMSAR COP 7, 1999) and the Bern Convention (1979). This is due to the lack of systematic assessment and study of marine species at a regional scale (Sanderson 1996a, 1996b, Ardron et al. 2002). It should be noted that most species or communities that are mentioned on lists as mentioned above are ‘rare’ because their numbers have been depressed by human actions, while other species or communities are just not numerous. For the purpose of this paper both types of rare species/communities are considered.

If such rare species lists on a local or regional scale are not available, species rarity within a subzone can still be assessed if data on their population size (at a national or regional scale) and trends are available.

Population data are frequently lacking, which only leaves the ‘area of occupancy’ concept as a proxy to assess the number and location of rare species within a study area (Sanderson 1996a, 1996b, Connor et al. 2002). The application of this concept is shown in Table 2. This approach has been adopted for the UK’s Review of Marine Nature Conservation (DEFRA 2004, Golding et al. 2004, Vincent et al. 2004, Lieberknecht et al. 2004a) and the UK Biodiversity Action Plan for marine species and habitats (UK BAP 2005), both in combination with other criteria. A species described by the method of Sanderson (1996a, 1996b) as nationally rare or scarce, is not necessarily regionally or globally rare or scarce: it may simply have been reported at the edge of its range; or else this designation may indicate subtle adversity such as stress caused by human activities in the study area. However, it could also be important to give a high value to subzones containing species at the margins of their range, because these sites could host important genetic stocks of a species. Also, populations of sessile southern or northern species have a poor capacity for recovery and recruit slowly at the northern, respectively southern, margins of their distribution and are therefore particularly vulnerable to even the most minor, infrequent impacts (Sanderson 1996a, 1996b). Nationally rare or scarce species may also be restricted to specific habitat types that themselves may be rare in the study area and need to be given a high value (e.g. the rocky island habitats of Helgoland in the sedimentary southern North Sea). A disadvantage of rarity assessment as discussed in Table 2 is that it may overlook local densities. Locally abundant species (in one or several subzones of a study area) which are restricted in their range might be considered to conflict with assertions made about national rarity, should population-based methods of assessment ever be used (Sanderson 1996a, 1996b). The value of rarity as a criterion for marine biological assessment was also discussed.

Uniqueness and distinctiveness (Roff & Evans 2002) are also considered under this criterion and to assess the number and location of unique or distinct features/genetic stocks/species/communities within the study area, information on their occurrence is needed.


These paragraphs are based on the paper of Derous et al. (2007). A concept for biological valuation in the marine environment. Oceanologia 49 (1). See FLANDERS MARINE INSTITUTE web site at [1] for more information and to download a copy of the paper.


  1. Derous S., Agardy T., Hillewaert H., Hostens K., Jamieson G., Lieberknecht L., Mees J., Moulaert I., Olenin S., Paelinckx D., Rabaut M., Rachor E., Roff J., Stienen E.W.M., van der Wal J.T., Van Lancker V., Verfaillie E., Vincx M., Weslawski J.M., Degraer S. (2007). A concept for biological valuation in the marine environment. Oceanologia 49 (1).
  2. DFO (2004). Identification of ecologically and biologically significant areas. DFO Can. Sci. Adv. Sec. Ecosystem Status Report 2004/006.
  3. Rachor E., Günther C.-P.(2001). Concepts for offshore nature reserves in the southeastern North Sea. Senckenb. Marit. 31 (2), 353-361, modified and complemented after Salm R.V., Clarke J.R. (1984). Marine and coastal protected areas: a guide for planners and managers. Gland (IUCN).
  4. Salm R., Price A. (1995). Selection of marine protected areas. In: Marine protoected areas: principles and techniques for management. S. Gubbay (ed.), Chapman & Hall, London, 15-31
  5. Kelleher G. (1999). Guidelines for marine protected areas. IUCN, Gland, Cambridge.
  6. UNESCO (1972). Convention concerning the protection of the world cultural and natural heritage. UNESCO meeting in Paris from 17 October to 21 November 1972, at its seventeenth session.