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 an d ability to outcompete understorey assemblages. Coral cover underneath the abundant Indo-Pacific tabular coral, Acropora hyacinthus, was four times l ower than on the adjacent substratum on the reef crest at Lizard Island on the northern Great Barrier Reef. We investigated the effect of A. hvacinthl ts on patterns of recruitment and mortality by placing experimental panels and coral fragments underneath large colonies of A. hyacinthus. After 8 wee ks, recruitment of corals, filamentous algae and crustose coralline algae ( CCA) underneath A. hyacinthus was 96, 85 and 50% lower, respectively, compa red to panels placed in the open. In contrast, recruitment by bivalves and polychaetes was uniform among treatments, while bryozoans recruited four ti mes more abundantly under A. hyacinthus than in the open. Consequently, the low rate of recruitment by corals beneath A. hyncinthus 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 d iminished Light levels under A. hyacinthus are partially responsible for th e divergence in recruit assemblages. To test the effect of A. hyacinthus on early mortality and growth of established organisms, recruitment panels we re placed on the open for 9 weeks then transplanted underneath A. hyacinthu s for a further 8 weeks. The survivorship of juvenile corals underneath tab les 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 i ntermedia and Pocillopora damicornis also survived poorly following transpl antation underneath A. hyacinthus, compared to adjacent, unshaded controls. We conclude that A. hyacinthus is a formidable competitor which call kill neighbouring corals by overgrowing them, and pre-empt future competition by reducing coral recruitment. (C) 2000 Elsevier Science BN. All rights reser ved.