Interspecific Competition Reduces Fitness in Scleractinian Corals

Résumé du livre

Interspecific competition is widely regarded as an important structuring mechanism in coral assemblages, yet there is little evidence available to determine what influence it has on the fitness of individual colonies. Fitness has been shown to decline in scleractinian corals engaged in competition with macroalgae and in intraspecific competition. However, the only attempts to investigate the costs of interspecific competition between scleractinian corals have not examined both growth and reproduction. In this paper I show, by monitoring experimental contacts, that competition between several species of scleractinian coral has consequences for colony fitness. Both Pocillopora damicornis (Linnaeus) and Acropora hyacinthus (Dana) experienced substantial declines in growth rates when competing against other corals compared to non-competing controls, although mortality was not affected. Growth in A. hyacinthus decreased from 120 to 35%, while in P. damicornis it decreased from 45 to - 16% (i.e. one group of competing colonies shrank in average size). An examination of the effects of competition on reproduction of A. hyacinthus showed that at the polyp level, there is no detectable response. This conclusion is reversed at the colony level, because the reduction in growth rate leads to smaller colonies which are less fecund. This reduction in colony fecundity due to a decrease in size is partially compensated for by a decrease in the number of non-reproductive polyps in the colony. Size-classified matrix models of cohort dynamics show that competition has a major influence on cohort survival for A. hyacinthus, but little effect for P. damicornis. This reduction in cohort survival can be directly equated to a decrease in fitness.