Salt marsh plant communities have long been envisioned as dynamic, resilient systems that quickly recover from human impacts and natural disturbances. But are salt marshes sufficiently resilient to withstand the escalating intensity and scale of human impacts in coastal environments? In this study we examined the independent and interactive effects of emerging threats to New England salt marshes (temperature increase, accelerating eutrophication, consumer-driven salt marsh die-off, and sea level rise) to understand the future trajectory of these ecologically valuable ecosystems. While marsh plant communities remain resilient to many disturbances, loss of critical foundation species and changing tidal inundation regimes may short circuit marsh resilience in the future. Accelerating sea level rise and salt marsh die-off in particular may interact to overwhelm the compensatory mechanisms of marshes and increase their vulnerability to drowning. Management of marshes will require difficult decisions to balance ecosystem service tradeoffs and conservation goals, which, in light of the immediate threat of salt marsh loss, should focus on maintaining ecosystem resilience.

Intertidal wetlands provide vital habitat for numerous species by serving as a basal resource and refuge for shelter-seeking fauna. These environments are particularly important for transient nekton that trophically link marine and terrestrial food webs via migration into the intertidal during the flooding tide. Changes in coastal vegetation due to a warming climate, such as the replacement of temperate saltmarshes with tropical mangroves, may alter habitat use by transient species due to differences in food provisioning and the structural attributes of mangrove and saltmarsh vegetation. In northeast Florida (USA), Fundulus heteroclitus (mummichog) and F. majalis (striped killifish) are abundant, trophically important, transient fishes that may exhibit behavioral changes with coastal habitat shifts. We experimentally manipulated predator presence in a large-scale mesocosm containing an array of 4 habitats found at the mangrove-saltmarsh ecotone: marsh vegetation (smooth cordgrass Spartina alterniflora), 2 species of mangroves (red mangrove Rhizophora mangle and black mangrove Avicennia germinans), and sand, and measured prey fish habitat use and schooling dynamics. Mummichogs utilized black mangrove habitat to the same degree as cordgrass regardless of the presence of a predator, indicating that mummichogs perceive both habitats as providing equivalent protection. Killifish exhibited little affinity for any particular habitat and exhibited schooling behavior over seeking refugia for predator avoidance, suggesting that vegetation changes may have little effect on killifish behavior. Further research on how such patterns of habitat use combined with habitat-specific dietary constituents of fundulids will help to predict how saltmarsh loss at the ecotone will influence nutrient and energy flows in coastal food webs.

Foundation species traits that structure communities are rarely experimentally examined; thus, a predictive understanding of their functions lags behind patterns of observed species associations. Red mangrove Rhizophora mangle roots form complex living habitats that support diverse epibiont communities, making them a model system for testing links between variation in foundation species traits and associated biodiversity. Here, we compared epibiont community composition between living and non-living mangrove roots, as well as root mimics, to test how foundation species traits affect community structure. We also quantified the community structure of associated mobile invertebrates to examine their relationship with secondary foundation species (e.g. sponges, bivalves) that grow on the roots. After 14 mo of colonization and succession, substrate composition (i.e. mangrove, wood, PVC) had significant effects on community composition, richness, and abundance of sessile epibionts and mobile invertebrates. Non-living mangrove roots were 5 times more likely to deteriorate, and consequently had the lowest epibiont richness and abundance. We found strong positive relationships between mobile invertebrate richness and the abundance, measured as biomass, and richness of sponges and bivalves, suggesting that variation among roots in secondary foundation species play an important role in mediating mobile invertebrate community composition. This study highlights the functional role of habitat structure and how rapidly that function can be lost without biogenic maintenance. Our results indicate the importance of facilitation cascades in fostering diverse mobile invertebrate communities and highlight both advantages and limitations in using artificial structures in restoration programs.