XVI International Botanical Congess

This study examined a conceptual paradigm, the ARelative Dominance Model@, through bioassays and manipulative experiments of herbivory (i.e., top-down control) and nutrients (i.e., bottom-up control), to assess the interactive mechanisms controlling phase shifts among the major space-occupying primary producers on coral reefs. We found, as have many others, that low nutrients alone did not preclude fleshy algal growth when herbivory was reduced or even eliminated. We conclude that on undisturbed oligotrophic coral-reef habitats, the effects of top-down inhibitory controls via intense herbivory prevail, whereas, bottom-up stimulatory controls are minimal, due to lack of nutrient availability and overcompensation by grazers. However, under all combinations of experiments, coral cover decreased under elevated nutrients relative to low nutrients, such consistent coral declines concomitant with algal increases led to profound long-term effects. Changes in bottom-up controls and their interactions not only alter the dominance patterns of primary producer groups, but, hypothetically, could have even longer-term consequences mediated through structural transformations (decreased habitat heterogeneity) and chemical modifications (nighttime anoxia) to reef systems and their herbivorous fish populations.