Differential competitive abilities between Caribbean coral species and a brown alga: a year of experiments and a long-term perspective

The competitive replacement of corals by benthic algae is considered key to reef degradation. Such replacement could originate from direct competitive overgrowth of corals by algae or death of corals from other disturbances, followed by an increase in algal abundance. To assess the relative importance of these processes, this study experimentally tested the competitiveness of 6 Caribbean coral species against the brown alga Lobophora variegata on a fringing reef in Curacao, Netherlands Antilles. This alga has a widespread distribution and is considered particularly aggressive towards corals due to its creeping growth form. We compared the growth of transplanted algae over living and dead coral, as well as coral tissue mortality in the presence and absence of transplanted algae over a 1 yr period. Competitive trends were also related to changes in species abundance from 1973 to 2002 on the same reef. The results indicated that only 1 species, Agaricia agaricites, was competitively inferior to L. variegata and suffered more tissue mortality when exposed to the algae. Surveys of naturally occurring interactions showed that less competitive species were generally more overgrown by L. variegata, further reinforcing our results, Importantly, A. agaricites experienced the greatest decline in percent cover from 1973 to 2002 among the studied species. A large proportion of this decline occurred following the die-off of Diadema antillarum in 1983, which generally marks the onset of increased algal abundance on Caribbean reefs. We concluded that Caribbean corals have different competitive abilities against algae, highlighting the complexity and species-specific nature of coral-algal interactions. Although our data supports that prior death of corals may be generally required for algae to become established, competition with algae could play a significant role in structuring coral communities by reducing the abundance of poor competitive species. We suggest that a species-by-species approach is needed to understand the factors influencing transitions from coral to algal dominance on Caribbean reefs.