Action Summary:
Researchers removed sea urchins (Centrostephanus rodgersii) in sites to create four urchin density treatments; 0% (Removal), 33%, 66% and 100% (Control) of the original population. Treatments were replicated in four discrete barren patches 3-4m2 in diameter. 16 patches were selected, and treatments were randomly assigned. The percentage cover of crustose, filamentous and foliose algae (primary cover only), the density of Sargassum spp., the density of limpets (Cellana trarnoserica, Patelloida alticostata and l. mufria) and the area of the manipulated patches of barrens habitat were measured.
Lessons Learned:
Coralline algae declined with urchin removals while filamentous and foliose algae increased. Effects varied with urchin density, but there was a non-linear relationship between barren size and urchin density. It was suggested the size and shape of urchin home crevices influences barren size.
Project Outcomes:
Partial and total removals of urchins resulted in the loss of crustose coralline algae and an increase in filamentous algae in barren patches. The removal of all large C. rodgersii caused an increase in the percentage cover of foliose algae. After 18 months, foliose algae covered a mean of 49.1 % of the substratum but never covered more than 5% of the substratum in the partial removal and Control treatments. The species composition of foliose algae differed greatly among patches in the Removal treatments. Sargassum spp. also recruited to patches with partial removals of sea urchins and were abundant in both 33% and 66% treatments in March 1986. Densities declined thereafter and Sargassum spp. were either rare or absent from patches at the end of the experiment. No Sargassum spp. were recorded in Control patches.
