Interaction of top-down and bottom-up processes facilitate the rapid range expansion of Turbinaria ornata in the South Pacific

Background/Question/Methods

Recent ecological research has focused on top-down and bottom-up processes controlling primary producer biomass and their role in ecosystem shifts, as anthropogenic alterations increasingly impact their overall and relative importance.  In marine systems, bottom-up controls are thought to be weak and not to interact strongly with top-down processes. However, recent evidence suggests complex interactions between the two may control tropical algal communities. We hypothesized that a dramatic increase in the range and abundance of a brown macroalgae, Turbinaria ornata, in the South Pacific since the 1980s may be caused by such interactions. There is currently little evidence of control of T. ornata by top-down or bottom-up processes as it is a slow-growing persistor species and with chemical and structural antiherbivore defenses. This study directly investigates the  role of nutrients and herbivory, and considers how strong interaction between these two factors may control biomass accumulation of T. ornata through: 1) a nutrient addition (+/-N. +/-P) experiment assessing relative nutrient limitation, 2) a field experiment manipulating nutrients (+/-) and herbivory (+/-cages) to compare the importance of nutrients and herbivory and their interactions, and 3) an herbivory assay in which physical defenses were removed to assess palatability of nutrient enriched and ambient thalli.

Results/Conclusions

Our results suggested that Turbinaria ornata biomass accumulation on reefs may be caused by an interaction between nutrients and herbivory as nutrients limited the force of herbivory by increasing T. ornata’s physical defenses. Nutrients did not directly limit algal biomass accumulation as nutrient-enriched algae did not grow faster than ambient. The field experiment confirmed T. ornata did not respond directly to nutrients. However, nutrient-enriched T. ornata thalli were less palatable to herbivores as they were consumed significantly less than those in ambient nutrients. When the physical toughness of the algal thallus was removed, enriched algae were strongly preferred, with consumption rates 25-fold those of unenriched algae, suggesting that physical not chemical defenses were bolstered by nutrient enrichment. Taken together, these results suggest that under nutrient enrichment, T. ornata may be released from herbivore control and, hence, were able to increase dramatically in abundance dramatically. Thus, the unprecedented increase in T. ornata on reefs in the South Pacific may be explained by a stabilizing force cause by increased nutrient levels, the resulting decrease in herbivory and, consequently, increased T. ornata biomass, potentially expanding their habitats to those usually subjected to much higher herbivory pressure.