Effects of long-term exclusion of the limpet Cymbula oculus (Born) on the distribution of intertidal organisms on a rocky shore

Abstract

Zonation patterns on rocky shores are typically as a result of both physical factors and biological interactions. Physical factors generally set the upper limits of species distributions whereas biological interactions generally set their lower limits. However, recent research has shown that biological factors can also influence the upper limits of species' recruitment and colonisation. Whereas such evidence has been shown for rocky shores internationally, little experimental evidence exists for South African shores. This study provided such evidence for the biological effects of long-term exclusion (for the period 2003-2008) of the South African herbivorous limpet Cymbula oculus (Born), on the community structure of the Kalk Bay rocky intertidal zone on the South African south-western Cape coast. To demonstrate this, a herbivore exclusion experiment was set up in the mid-eulittoral zone in May 2003. Initially, all herbivores were removed from the exclusion plots; after one year, only C. oculus individuals recruiting into the plots were continually removed. Algal recruitment (percentage cover abundance) was determined monthly for the first 12 months and then annually thereafter, whereas invertebrate recruitment (density) was monitored only annually. The results showed that the natural density of C. oculus had increased markedly during the first year (from 4.87 ± 1.09 individuals m-2 to 12.35 ± 1.70 individuals m-2, p = 0.001), and that grazing by C. oculus is the primary biological factor preventing the recruitment and colonisation of macroalgae and some invertebrates onto the Kalk Bay intertidal zone. Not only does herbivory by C. oculus prevent recruitment and colonisation, but it also decreases algal diversity and prevents algal succession. Within the mid-eulittoral zone of the Kalk Bay rocky intertidal zone, herbivory by C. oculus is thus more important in structuring this marine community than physical factors associated with desiccation stress.