Competitive dominance by tabular corals: an experimental analysis of recruitment and survival of understorey assemblages.

Tabular and staghorn corals of the genus Acropora often form low-diversity stands on shallow coral reefs, presumably due to their rapid growth rate and ability to outcompete understorey assemblages. Coral cover underneath the abundant Indo-Pacific tabular coral, Acropora hyacinthus, was four times lower than on the adjacent substratum on the reef crest at Lizard Island on the northern Great Barrier Reef. We investigated the effect of A. hyacinthus on patterns of recruitment and mortality by placing experimental panels and coral fragments underneath large colonies of A. hyacinthus. After 8 weeks, recruitment of corals, filamentous algae and crustose coralline algae (CCA) underneath A. hyacinthus was 96, 85 and 50% lower, respectively, compared to panels placed in the open. In contrast, recruitment by bivalves and polychaetes was uniform among treatments, while bryozoans recruited four times more abundantly under A. hyacinthus than in the open. Consequently, the low rate of recruitment by corals beneath A. hyacinthus does not appear to be due to a reduction in the delivery of larvae underneath tables. Instead, the disparity between phototrophic and heterotrophic taxa suggests that diminished light levels under A. hyacinthus are partially responsible for the divergence in recruit assemblages. To test the effect of A. hyacinthus on early mortality and growth of established organisms, recruitment panels were placed on the open for 9 weeks then transplanted underneath A. hyacinthus for a further 8 weeks. The survivorship of juvenile corals underneath tables was less than half that of those on control panels on the unshaded reef crest. Furthermore, the abundance of algal turfs and CCA was sharply lower on transplanted panels. In contrast, heterotrophic organisms increased in cover, regardless of treatment. Experimental branch fragments of Acropora intermedia and Pocillopora damicornis also survived poorly following transplantation underneath A. hyacinthus, compared to adjacent, unshaded controls. We conclude that A. hyacinthus is a formidable competitor which can kill neighbouring corals by overgrowing them, and pre-empt future competition by reducing coral recruitment.