Rare Species as Bioindicators in Marine Monitoring

Abstract

Soft-bottom invertebrates commonly form the core of marine moni- <br /><br />toring studies. Accordingly, those species which occur frequently and abundantly <br /><br />are favored in analyses, and rare species are normally excluded from consider- <br /><br />ation. Since rare species commonly occur at their geographic limits under sub- <br /><br />optimal conditions, it has been postulated that they would be vulnerable to natural <br /><br />and anthropogenic stresses. We examined the potential of rare species as bioin- <br /><br />dicators in monitoring a major marine outfall on the San Pedro Shelf, California. <br /><br />A rare species was operationally defined as occurring only once per sample (0.1m 2 <br /><br />Van Veen Grab). Based on 780 quantitative benthic samples, distributed over <br /><br />thirteen 60 m (outfall depth) stations, and 12 years, we concluded that the number <br /><br />of rare species generally declined towards outfall stations. We suggested that a <br /><br />more conventional definition of rarity based on actual geographic range might <br /><br />provide an increased degree of sensitivity to identify bioindicators.