Nutrient additions change the agonistic and grazing behavior of an ecosystem engineer on tropical reefs.

As human impacts on ecosystems accelerate, understanding how shifts in resource quality alter animal behaviors that structure communities are becoming increasingly important. Nutrient pollution is especially important for oligotrophic ecosystems such as coral reefs, where small changes in nutrient inputs can impact production and biotic interactions. Here, we conducted a before-after control-impact experiment to examine the effects of short-term nutrient addition on the behavior of the damselfish Stegastes nigricans, a common ecosystem engineer on Pacific coral reefs. We added fertilizer to treatment versus control territories of this fish and quantified changes in territory size, aggression, and grazing behavior. After two weeks, fertilizer-enrichment decreased territory size by 18 ± 5%; control territories remained unchanged (-3 ± 4%). Fertilizer addition did not affect the number of resident fish within a territory, but grazing rates of both residents and roving herbivorous fishes increased by 55% and 87%, respectively. Aggression towards heterospecifics increased by ~ 54% in fertilized replicates, while aggression towards conspecifics decreased by ~ 74%, suggesting changes in damselfish social structure. Thus, shifts in the quality or growth rate of algal resources can rapidly impact agonistic and grazing behavior of damselfish. Given that damselfish territories cover up to ~ 70% of some reefs, our results highlight that nutrient pulses can modify resource quality and rapidly alter the behavior of an important and common ecosystem engineer.