EVOLUTIONARY STRATEGIES IN A TROPICAL BARRIER REEF SYSTEM: FUNCTIONAL‐FORM GROUPS OF MARINE MACROALGAE¹

ABSTRACT Predictions of an evolutionary model were examined for 43 tropical macroalgae using a functional‐form group approach. The ranking from high to low primary producers (Sheet‐ and Filamentous‐Groups > Coarsely Branched‐ and Thick Leathery‐Groups > Jointed Calcareous‐ and Crustose‐Groups), and data from the literature, support the hypothesis that persistent forms which allocate resources for environmental resistance, interference competition or antiherbivory defenses do so at the cost of lower primary production rates. The results for percent thallus lost to fish grazing over a 24 h period support the hypothesis that members of the Thick Leathery‐, Jointed Calcareous‐ and Crustose‐Groups have evolved antipredator defenses, with a tendency for decreasing herbivore resistance toward the Sheet‐ and Filamentous‐Groups. The most heavily‐calcified species (e.g. crustose corallines) ranked among the most grazer resistant as did the thick rubbery or leathery species. The ranking of functional‐form group means for resistance to predation was as follows: Filamentous‐Group (62% lost‐24 h −1 ), Sheet‐Group (42%), Coarsely Branched‐Group (33%), Jointed Calcareous‐Group (10%), Thick Leathery‐Group (7%) and Crustose‐Group (0%), in accordance with the hypothesis. The algal groups generally showed an increase in mean penetration toughness from filaments (<200 g‐cm −2 to shear thallus) to sheets (216 g·cm 2 ), coarsely branched forms (328 g·cm −2 ) and thick leathery species (1800 g·cm −2 ) in agreement with the predictions of the model. Contrary to earlier findings, there was no consistent gradation between the first four groups (i.e. fleshy algae) based on calorific values. However, in partial support of the functional‐form model, a seven‐fold difference was noted when the mean for these groups (1.7 kcal·g −1 ) was compared with that of the Jointed Calcareous‐ and Crustose‐Groups (0.2 kcal·g −1 . The functional‐form group approach appears to have powerful capabilities in that it can indicate important morphological‐metabolic‐ecological interactions in a given community, where the macroalgae are known, without the need to examine each population in detail and without being constrained to a specific habitat or geographical region.