The loss of coral cover, frequently driven by anthropogenic disturbances, can result in a phase shift to dominance by fleshy macroalgae, many of which contain anti-herbivore defenses. Using field surveys, a mesocosm experiment, and field experiments, we evaluated whether 2 human impacts — nutrient enrichment and reduction in herbivory — affected production and maintenance of thallus toughness, a physical defense of the brown macroalga Turbinaria ornata that has recently expanded across the South Pacific. In contrast to our expectations, there was a weak negative relationship between herbivorous fish abundance and thallus toughness. This relationship was driven by greater toughness in algae collected at the more eutrophic sites, which also had lower herbivore abundances. A mesocosm experiment confirmed a positive relationship between nutrients and thallus toughness with no measurable cost to growth. Mechanical damage simulating herbivory maintained thallus toughness in cages, with a significant trade-off in growth. In addition, reduction of herbivory through caging in a transplant experiment resulted in a reduction in thallus toughness; however there was no measurable benefit to growth, possibly due to a concurrent change in environmental context from the transplant. While reduction in herbivory via overfishing allows this alga to trade energy normally spent on defense for increase growth, nutrient enrichment provides T. ornata with additional resources to increase defenses. As anthropogenic impacts become increasingly prominent in coral reef systems, it is critical that we understand the processes that may facilitate the expansion and dominance of coral reef algae, especially those with inducible anti-herbivore defense