| Title | Experimental manipulations of the density of Cymbula granatina on rocky shore community |
|---|---|
| Authors |
Ndiviwe Baliwe |
| Publisher | Department of Forestry, Fisheries and the Environment (2024) |
| Contributors |
Project Member: George Branch Project Member: Maya C. Pfaff Contact Person: Ndiviwe Baliwe |
| Abstract | Densities of a commonly-harvested intertidal limpet Cymbula granatina were manipulated at two sites within a no-take marine protected area on the west coast of the Cape Peninsula in South Africa, using four treatment levels ranging from zero to maximum natural densities, together with control plots, to evaluate the effects of different harvesting intensities on rocky shore community composition. Following removal or thinning of C. granatina, community composition changed: cover of corticated and ephemeral algae increased and recruitment of C. granatina decreased. These outcomes were, however, dependent on the time frame considered, as algae underwent an annual cycle, and the effects of limpet removal were evident predominantly during the summer upwelling season when the algae proliferated. The management implications of the results of this experiment are discussed. |
| Methods | To evaluate the effects of variable densities of C. granatina on rocky shore communities, a herbivore exclusion experiment was set up in November 2017, in which this limpet was excluded or thinned to fixed proportions of normal densities inside 30 x 30-cm plots. To simulate different levels of exploitation and control for potential side effects arising from the experimental procedure, the experiment had three types of exclusion treatments: a cage treatment (C), a semi-fenced treatment (SM) and an unfenced treatment (U), with treatments having four density levels that were each replicated four times (Fig. 1). The levels were: (a) 100% of natural densities (>12 individuals/plot, control) where no reduction of density was undertaken, (b) 50% (7-9 individuals/plot, lightly harvested), (c) 10% (single limpet/plot, heavily harvested) and (d) 0% (no limpets/plot, depleted). These levels are henceforth referred to as C100, C50, C10 and C0, respectively, for the cage treatment. Equivalent densities were established for the unfenced treatment. The semi-fenced (SM) treatment had two density levels only, 100% (SM100) and 0% (SM0) of natural densities of C. granatina because of time constraints on sampling during low tide and because we assumed that examining the extremes of density would be sufficient to test for caging effects. The total number of experimental units was thus 40 per site, for 10 treatment/density combinations. The use of three exclusion methods allowed me to control for possible caging effects, such as any influence of the caging materials and of wave reduction. Specifically, cages vs. semi-fences were compared to test for wave-reduction effects, and semi-fences vs. unfenced to test for cage-material effects. |
| Data | |
| Temporal extent | 27 Feb 2017 – 11 Feb 2020 |
| Geographic extent |
SOUTH ATLANTIC OCEAN
North: -34.205 |
| Keywords | density, no-take areas, rocky shores, SDG 14.2.1 Number of countries using ecosystem-based approaches to managing marine areas, shell length, SOUTH ATLANTIC OCEAN, Table Mountain National Park MPA |