Mobile consumers influence the shoreward edge of intertidal seagrass ecosystems

Habitat edges are often considered environmentally stressful areas, and as such, research has largely focused on the impacts of physical factors in shaping these edges. However, less is known about the relative importance of biotic disturbance agents and bottom-up drivers in shaping habitat edges. Here, we used intertidal seagrass beds as a model system to test how the independent and combined effects of stingrays-a disturbance-generating forager in seagrass beds-and nutrient addition affect the upper elevation edge of seagrasses. A two-season long manipulative experiment with stingray exclusion × nutrient addition revealed that shoreward seagrass edges experienced heightened loss in percent cover when exposed to stingrays (p = 0.037) but were not impacted by nutrient additions to marine sediments (p = 0.13). Additionally, transplant experiments designed to test whether stingrays could limit intertidal seagrass establishment in higher elevation found that transplanted seagrass had a higher chance of survival when stingrays were excluded (p < 0.01), suggesting that seagrass could live higher in the intertidal and that stingrays may limit shoreward expansion. Finally, a multi-site observational survey demonstrated that stingray pit abundance was a strong predictor of the distance between seagrass edge and shoreward habitats. Combined, these results challenge current understanding in plant ecology that seagrass edges are controlled mainly by physical factors and instead suggest that the structure of the seagrass shoreward edge is controlled by both physical and biotic drivers. Our results also indicate that the relative effects of consumer disturbance and physical factors in controlling edge limits are likely predicated on consumer density: in areas with higher densities of large consumers, biotic forcing is likely to be more important. Furthermore, these results could have implications for restoration in areas with high densities of disturbance-generating foragers and align with calls for greater inclusion of animal impacts into restoration schemes. Biotic drivers along environmentally stressful edges are likely not limited to seagrasses and the generality of biotic control of habitat edges deserves further exploration across diverse ecosystems.