Predation-driven biotic resistance fails to restrict the spread of a sessile rocky shore invader
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The invasive barnacle Balanus glandula has progressively spread along the South African west coast. We used multiple approaches to assess the role of predation by indigenous whelks in regulating this expansion. B. glandula abundance and distribution were monitored annually while field observations and laboratory experiments assessed the relative predation pressure on B. glandula and the native barnacle Notomegabalanus algicola. In the mid-shore, the whelks Trochia cingulata and Burnt1pena lagenaria fed on N. algicola most often despite the alien B. glandula covering a mean of 86% of the shore at this site. Lower on the shore, the same feeding pattern was evident, although N. algicola was spatially dominant. Feeding experiments revealed that small (mean ± SD shell length: 13.9 ± 0.3 mm) and large (19.6 ± 0.5 mm) T. cingulata consumed up to 70% more N. algicola than B. glandula, displaying a significant avoidance of the alien. While small (15.5 ± 0.5 mm) B. lagenaria displayed the same pattern, large individuals (27.7 ± 0.4 mm) consumed equal numbers of the 2 barnacles. The avoidance of B. glandula may be explained by this species possessing thicker shell and opercular plates than N. algicola, while a narrow margin of vulnerable soft tissue around the circumference of the opercular plates makes the native an attractive prey choice. This study demonstrates that predation-driven biotic resistance has not contained the expansion of B. glandula along the South African coast.