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Is the geographic pattern in the abundance of south African barnacles due to pre-recruitment or post-recruitment factors?
Abstract
Intertidal barnacles are significantly more abundant on the south than on the west coast of South Africa. Abundances measured at 11 sites, covering 740 km of coastline, showed that South Coast and West Coast sites
averaged 1 323 and 67 individuals.m-2 respectively. Barnacles were not replaced by other sessile organisms on the West Coast; instead the amount of bare space increased proportionately. Two general, alternative hypotheses can explain the abundance pattern: either pre-recruitment factors, those affecting the abundance of larvae, settlers and juveniles (<5 mm in basal diameter), or post-recruitment factors, those affecting the abundance of larger individuals (>5 mm in basal diameter), are significantly different on the two coasts. Measurements conducted on
adults of the two most common barnacle species, Tetraclita serrata and Octomeris angulosa, over a three-year period at a West and a South Coast site did not support the post-recruitment hypothesis. Measurements showed that adult growth and survival were not inferior on the West Coast. First, the average sizes and maximum sizes of adults of the two species were greater at the West Coast site. Second, the growth rate of Octomeris angulosa was significantly higher at the West Coast site. Third, the survival rates of adults of both species were higher at the West Coast site. Additional support for the pre-recruitment factors hypothesis was provided by the number of recruits that were present in clearings that had been left for three years; these being 70 times more abundant in the clearings at the South Coast site. The results suggest that the geographic pattern in barnacle abundance along the South African coast is produced by differences in pre-recruitment factors rather than post-recruitment
factors.
averaged 1 323 and 67 individuals.m-2 respectively. Barnacles were not replaced by other sessile organisms on the West Coast; instead the amount of bare space increased proportionately. Two general, alternative hypotheses can explain the abundance pattern: either pre-recruitment factors, those affecting the abundance of larvae, settlers and juveniles (<5 mm in basal diameter), or post-recruitment factors, those affecting the abundance of larger individuals (>5 mm in basal diameter), are significantly different on the two coasts. Measurements conducted on
adults of the two most common barnacle species, Tetraclita serrata and Octomeris angulosa, over a three-year period at a West and a South Coast site did not support the post-recruitment hypothesis. Measurements showed that adult growth and survival were not inferior on the West Coast. First, the average sizes and maximum sizes of adults of the two species were greater at the West Coast site. Second, the growth rate of Octomeris angulosa was significantly higher at the West Coast site. Third, the survival rates of adults of both species were higher at the West Coast site. Additional support for the pre-recruitment factors hypothesis was provided by the number of recruits that were present in clearings that had been left for three years; these being 70 times more abundant in the clearings at the South Coast site. The results suggest that the geographic pattern in barnacle abundance along the South African coast is produced by differences in pre-recruitment factors rather than post-recruitment
factors.
