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Ecological communities are subjected to multiple anthropogenic stressors at both global and local scales that are increasing in number and magnitude. Stressors can interact in complex ways and are classified as additive, synergistic or antagonistic; the nature of the interaction is key to predicting changes and understanding community resilience. Coral reefs are among the most impacted communities and have shifted from coral- to algal-dominated states, and overfishing, nutrient enrichment and sedimentation are local stressors that often co-occur and may support degraded algal states. Short algal turfs are abundant benthic space holders on healthy reefs that may be pushed by local stressors to long algal turfs, a more degraded state that may prevent recovery to coral dominance. We conducted a fully crossed three-factor field experiment on short algal turf communities manipulating herbivory pressure (+/−cages), nutrients (+/−fertilizer) and sediments (natural accumulation/removal). We applied stressors for 16 days, removed them and monitored turf height during and after manipulations. We found that significant pair-wise interactions between all stressors pushed the community towards a degraded state with longer algal turfs. All three types of interactions (additive, synergistic and antagonistic) were common and occurred in equal frequency, suggesting more investigations into all types are needed to accurately predict community responses to multiple stressors. For example, when herbivores were present, nutrients and sediments interacted additively, while in the absence of herbivores, nutrients and sediments interacted synergistically. All interactions broke down following termination of experimental manipulations and all effects were undetectable after 49 days, indicating that this reef may be resilient, at least when stressors are applied on a short time-scale. Synthesis. Because management of local stressors is often more tractable than global stressors, local management has been proposed as a means to offset global stressors. However, ecological communities often experience multiple local stressors simultaneously, and interactions between stressors, including synergisms and antagonisms, may be the source of nonlinear shifts in communities or “ecological surprises.” The majority of interactions in our study were both strong and nonlinear, and we suggest that, if pervasive across systems, nonlinear interactions may drive the recent global increase in “ecological surprises.”

Summary 1.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. 2.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: 1) a mesocosm experiment assessing relative nutrient limitation, 2) a field experiment comparing importance of nutrients (+/-) and herbivory (+/-) to biomass accumulation, and 3) an herbivory assay and toughness test comparing enriched and ambient thalli to assess changes to anti-herbivory defences. 3.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. 4.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. This article is protected by copyright. All rights reserved.