Bolstered physical defences under nutrient‐enriched conditions may facilitate a secondary foundational algal species in the South Pacific

Summary Humans have a long history of changing species' ranges and habitat distributions, making studies of the ecological processes that may facilitate these changes of key importance, particularly in cases where a primary foundation species is replaced by another, less desirable species. We investigated the impact of nutrients and herbivory on Turbinaria ornata , a secondary foundational macroalga that depends on and likely competes with coral, the primary foundational community. T. ornata is also rapidly expanding in range and habitat across the South Pacific. We conducted (i) a mesocosm experiment assessing relative nutrient limitation, (ii) a field experiment comparing importance of nutrients (+/−) and herbivory (+/−) to biomass accumulation, and (iii) an herbivory assay and toughness test comparing enriched and ambient thalli to assess changes to anti‐herbivory defences. We found no evidence of growth being nutrient limited in T. ornata ; rather than stimulating growth, nutrient addition deterred herbivores. However, when physical toughness was removed, enriched algae were preferred, with consumption rates 25‐fold those of unenriched algae. Additionally, enriched thalli were tougher than ambient thalli, suggesting physical defences were bolstered by nutrient enrichment. Synthesis . We found a unique interaction where nutrients inhibit herbivory and facilitate Turbinaria ornata biomass accumulation. While concern is often placed on degradation of foundation species via anthropogenic change, instead here we show that anthropogenic change can facilitate secondary foundation species. This facilitation may allow a secondary foundation species to better compete with primary foundation species.