Abstract: Species range limits are theorized to be set by biotic interactions in amenable environmental conditions and by physical factors in stressful environmental conditions. In accordance with this theory, the shoreward edge of seagrass beds is thought to be controlled by physical forces such as heat stress and desiccation. However, stingrays, a forager in seagrass beds, have been shown to focus their destructive feeding behavior (i.e. digging into the sediment for prey forming indentations known as pits) at the edge of seagrass systems, including their shoreward edge. We predict that this interaction may influence the extent to which seagrasses extend into the intertidal. Furthermore, nutrient availability has increased in many plant communities, especially wetlands in recent decades due to increased agricultural fertilization. Research has shown a variety of plant responses to increased nutrient availability including alterations to growth allocation and decreased resilience to stressors. There is still a gap in knowledge on how plant ecosystems, like seagrasses, respond to the coupled influence of increasing consumers (stingrays) and nutrient availability pressures and how they affect species range limits. To explore the relative impacts of stingray foraging and increased sediment nutrient availability on the shoreward extant of intertidal seagrass, we utilized two manipulative field experiments and observational surveys. Preliminarily, we found that, seagrasses planted in the upper intertidal beyond the current limit of the seagrass bed, died in open plots, but showed increased survival when planted inside stingray exclusions. Furthermore, while all treatments lost seagrass, our results suggest that nutrient addition and exposure to stingray foraging led to more loss. Finally, observational surveys suggest a negative correlation between the number of stingray foraging pits and the percent cover of seagrass in the upper intertidal. Combined, these results suggest that stingrays are having an impact of the shoreward edge limit of intertidal seagrass and that physical forces, while having an influence, are not the sole contributor to intertidal seagrass range limits, contrary to what theory predicts. With increasing populations of stingrays due to the loss of top predators like sharks, these results could be increasingly important to consider in seagrass conservation and restoration. Additionally, similar biotic interactions that influence a species range limit at the environmentally stressful edge may be more common than previously thought and could become increasingly more intense under global change factors such as nutrient enrichment and changing food web dynamics.
