Area of occupancy, as a proxy for assessing rarity in the marine environment
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As part of the Encora project a workshop was held on marine biological valuation, from 6 to 8 December 2006 at Ghent (Belgium). The workshop was a joint venture of the EU CA ENCORA (http://www.encora.org) and the EU NoE MARBEF (http://www.marbef.org). This article reflects the discussions on the concept of 'Area of occupancy' for marine species.
Assessing the number and location of rare species
This approach has been adopted for the UK’s Review of Marine Nature Conservation and the UK Biodiversity Action Plan for marine species and habitats, 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. 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 the table 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.
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  for the full citation and to download a copy of the paper.
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