Humans impact natural systems in a multitude of ways, yet the cumulative effect of multiple stressors on ecological communities remains largely unknown. Here we synthesized 171 studies that manipulated two or more stressors in marine and coastal systems and found that cumulative effects in individual studies were additive (26%), synergistic (36%), and antagonistic (38%). The overall interaction effect across all studies was synergistic, but interaction type varied by response level (community: antagonistic, population: synergistic), trophic level (autotrophs: antagonistic, heterotrophs: synergistic), and specific stressor pair (seven pairs additive, three pairs each synergistic and antagonistic). Addition of a third stressor changed interaction effects significantly in two-thirds of all cases and doubled the number of synergistic interactions. Given that most studies were performed in laboratories where stressor effects can be carefully isolated, these three-stressor results suggest that synergies may be quite common in nature where more than two stressors almost always coexist. While significant gaps exist in multiple stressor research, our results suggest an immediate need to account for stressor interactions in ecological studies and conservation planning.

BackgroundSalt marshes lie between many human communities and the coast and have been presumed to protect these communities from coastal hazards by providing important ecosystem services. However, previous characterizations of these ecosystem services have typically been based on a small number of historical studies, and the consistency and extent to which marshes provide these services has not been investigated. Here, we review the current evidence for the specific processes of wave attenuation, shoreline stabilization and floodwater attenuation to determine if and under what conditions salt marshes offer these coastal protection services.Methodology/Principal FindingsWe conducted a thorough search and synthesis of the literature with reference to these processes. Seventy-five publications met our selection criteria, and we conducted meta-analyses for publications with sufficient data available for quantitative analysis. We found that combined across all studies (n = 7), salt marsh vegetation had a significant positive effect on wave attenuation as measured by reductions in wave height per unit distance across marsh vegetation. Salt marsh vegetation also had a significant positive effect on shoreline stabilization as measured by accretion, lateral erosion reduction, and marsh surface elevation change (n = 30). Salt marsh characteristics that were positively correlated to both wave attenuation and shoreline stabilization were vegetation density, biomass production, and marsh size. Although we could not find studies quantitatively evaluating floodwater attenuation within salt marshes, there are several studies noting the negative effects of wetland alteration on water quantity regulation within coastal areas.Conclusions/SignificanceOur results show that salt marshes have value for coastal hazard mitigation and climate change adaptation. Because we do not yet fully understand the magnitude of this value, we propose that decision makers employ natural systems to maximize the benefits and ecosystem services provided by salt marshes and exercise caution when making decisions that erode these services.

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Although it has long been recognized that marsh plant community composition shifts across estuarine salinity gradients, the mechanisms responsible for this species zonation have never been experimentally examined. In southern New England marshes of the United States, we investigated the relative importance of physical and biotic factors in generating estuarine species distribution patterns. Greenhouse studies revealed that all of the common plants in this system grow better in fresh water than in full-strength salt water. To test the hypothesis that the spatial segregation of these plants is driven by differential tolerance to salt stress and plant competition, we performed transplant experiments with 10 common plants in the system. When freshwater marsh plants were transplanted to salt marshes, they did poorly and generally died with or without neighbors present. In contrast, when saltmarsh plants were transplanted to freshwater marshes, they thrived in the absence of neighbors, growing better than they did in salt marshes, but when neighbors were present, they were strongly suppressed. These results suggest that the spatial segregation of plants across estuarine salinity gradients is driven by competitively superior freshwater marsh plants displacing salt-tolerant plants to physically harsh saltmarsh habitats, whereas freshwater marsh plants are limited from living in salt marshes by physical factors (e.g., high salinities). These results contribute to our understanding of the organization and assembly of tidal marsh plant communities and have important implications for understanding how marsh plant communities will respond to human modification of estuarine hydrology and climate change.