For decades, scientists have debated the mechanisms preserving ecosystem balance and plant-grazer coexistence. Originally, salt marsh systems were understood to be controlled by bottom-up forces. Recently, top-down pressure has proven to be an important driver in these systems, protecting plants against herbivory. However, it is both untested and the goal of this research to better understand whether a predictable balance exists between top-down forcing and plant defense production in salt marshes. To investigate this relationship we (1) deployed an observational survey along a naturally existing predator gradient in a Southeastern Spartina alterniflora marsh and (2) designed a factorial experiment with increased predator pressure across three treatments in short-form S. alterniflora in Atlantic Beach, North Carolina. This experiment ran over the course of two years. In that time, we identified a strong trophic cascade between the experimental predator Panopeus herbstii, grazer marsh periwinkle snail Littoraria irrorata, and saltmarsh cordgrass S. alterniflora (p=2.43e-3). Our experiment revealed that as predator pressure increased, plant defenses waned. After two seasons, the tensile strength of S. alterniflora in predator-present treatments was significantly lower than in predator-absent treatments (p=0.04). A palatability trial resulted in similar findings, indicating grazer preference for S. alterniflora samples from predator-present treatments over predator-absent treatments (p=0.01). Our findings provide evidence of plasticity in defense production of S. alterniflora in response to predator pressure. This research has exciting implications to better understand predictability in ecosystem balance not only in model ecosystems, but large-scale systems worldwide.
