Environmental Pressures on Top-Down and Bottom-Up Forces in Coastal Ecosystems

Global change is manifesting new and potent pressures that may determine the relative influence of top-down and bottom-up forces on the productivity of plants that undergird coastal ecosystems. Here, I present a meta-analysis conducted to assess how herbivory, nitrogen enrichment, and elevated salinity influence plant productivity according to the salinity regimes of coastal ecosystems. An examination of 99 studies representing 288 effect sizes across 76 different plant species revealed that elevated salinity negatively affected productivity across all environments, but particularly in freshwater ecosystems. Nitrogen enrichment, on the other hand, positively affected productivity. In agreement with the plant stress hypothesis, herbivory had the greatest negative impact in saline habitats. This trend, however, appears to reverse with nitrogen enrichment, with maximum losses to herbivory occurring in brackish habitats. These findings demonstrate that multiple stressors can yield complex, and sometimes opposite outcomes to those arising from individual stressors. This study also suggests that trophic interactions will likely shift as coastal ecosystems continue to experience nutrient enrichment and sea level rise.

Automated summary

Global change has complex effects on plant productivity in coastal ecosystems, with elevated salinity negatively impacting productivity across all environments and nitrogen enrichment positively affecting productivity. Herbivory has the greatest negative impact in saline habitats, but this trend may reverse with nitrogen enrichment, leading to maximum losses occurring in brackish habitats. These findings suggest that multiple stressors can have contrasting outcomes and trophic interactions may shift as coastal ecosystems continue to experience nutrient enrichment and sea level rise.