Control of Algae Populations by Urchin Predation and its Effect on Coral Abundance

Richard Ramsundar, Roxane Javadi, Lucia Kolodiuk

Since the 1990’s, Jamaican reef environments have been recovering from a decline in the population of Diadema antillarum due to disease, natural disturbance, and persistent anthropogenic threats. D. antillarum acts as a key algae grazer in conjunction with Tripneustes ventricosis and Lytechinus variegatus. Our study assesses the effects of urchin herbivory on coral abundance through top-down predatory control of algae in reef sites close to Discovery Bay Marine Laboratory. We anticipated that coral cover would be greater in regions of high urchin density and lower in regions of reduced urchin density. Additionally, we expected to find a shift towards algal dominance in areas of low urchin density.

For our study, four reef sites were chosen haphazardly in close proximity to the lab. 12m transects were laid down at each location and photographed in a series of 30cm by 30cm quadrats. Each quadrat was evaluated using the Coral Point Count 4.1 software, which calculated percent coral cover and percent algal cover for each transect. Urchin counts were also taken for D. antillarum, T. ventricosis, and L. variegatus within a 1m radius of each transect line.

Our comparison of urchin population, coral abundance, and algal cover revealed that sizeable algae cover was present at all sites. Each site’s average coral cover ranged between 1.59 and 11.11% while algal cover ranged from 15.84 and 31.97%. As expected, there was a higher percentage of coral cover in sites with higher urchin density. There was no observable trend for the number of L. variegatus in relation to percent coral cover for each site. However, we observed increased coral cover in areas with a greater abundance of D. antillarum and decreased coral cover in areas with a lower abundance of T. ventricosis. Still, our study strongly suggests continues algal dominance within reefs surrounding Discovery Bay.

Richard laying down quadrats

 - Richard, Lucia, and Roxane