Little giants: a rapidly invading seagrass alters ecosystem functioning relative to native foundation species.
The spread of invasive species is a major component of global ecological change and how and when to manage particular species is a difficult empirical question. Ideally, these decisions should be based on the specific impacts of invading species including both their effects on native competitors and how they may or may not play similar roles in broader ecosystem functioning. Halophila stipulacea is an invasive seagrass currently spreading through the Caribbean, and as seagrasses are foundation species, the effects of invasion have the potential to be particularly far-reaching. To evaluate the impacts of H. stipulacea we quantified spread and potential for displacement of native seagrasses as well as the effects of invasion on multiple ecosystem processes, particularly resource support for higher trophic levels and habitat creation. Long-term monitoring suggested that H. stipulacea likely displaces some native seagrasses (Syringodium filiforme and Halodule wrightii), but not others. Halophila stipulacea had lower N and protein levels and higher C:N ratios than native seagrasses, and as such is a poorer quality resource for consumers. We also observed significantly lower consumption of H. stipulacea than the native S. filiforme but limited differences compared to Thalassia testudinum. We found H. stipulacea created a more nutrient limited environment than T. testudinum and there were significantly distinct invertebrate assemblages in native- and invasive-dominated seagrass beds, but no difference in species richness or invertebrate biomass. These results suggest that the spread of H. stipulacea would impact a variety of ecological processes, potentially restructuring seagrass ecosystems through both direct impacts on environmental conditions (e.g., nutrient availability) and indirect food web interactions.