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.