Code and data for "Effects of multiple stressors were mixed in an experimental plant – herbivore system"

Ecological theory predicts that the effects of multiple stressors are synergistic. Yet, these effects are difficult to predict, especially the interaction of environmental and biological stressors. However, predicting these effects is critical for understanding ecological change. Spartina alterniflora (Spartina), a foundational saltmarsh plant key to coastal resilience and persistence, provides a good system in which to test this ecological theory. Spartina is subject to biological stress such as herbivory, as well as environmental stress such as chemical pollution. We experimentally tested whether the effects of multiple chemical and herbivore stressors were synergistic or not, using two chemical treatments (crude oil and dispersant) paired with two herbivores (the snail Littoraria irrorata and insects Prokelisia spp.) in a fully factorial experimental mesocosm system. We found a mixture of multi-stressor effects, with most effects being antagonistic, rather than synergistic, as expected. Fewer than half of multi-stressor effects were additive or synergistic, despite significant reductions in Spartina biomass and growth from both chemical and herbivore stressors. The effects on plant biomass and growth were detectable throughout the 8-week experiment, but the effects of chemicals and insects on plant ecophysiology (photosynthesis, Fv/Fm, qN) and stable isotope ratios (of carbon, nitrogen, and sulfur) were limited to the first 2 - 3 weeks. Overall, the mixed effects of stressors can lead to ecological surprises in both direct and indirect interactions. Understanding interactions among multiple stressors is critical for policy or governing bodies to manage for, and mitigate, the effects on ecosystem services now and in the future. Code and data for this analysis are provided here.