Halophila stipulacea is an invasive seagrass that has recently colonized and begun to expand through the Caribbean. Community shifts from dominance of native seagrasses to H. stipulacea are expected to have significant ecosystem level consequences because seagrasses are foundation species in these systems. However, there has been limited explicit evaluation of functional differences between native and invasive seagrasses. To evaluate the potential impacts of invasion by H. stipulacea and to identify pathways through which it could influence ecosystem functioning we conducted a series of experiments in St. John, US Virgin Islands. Firstly, we monitored seagrass composition and abundance at fixed locations to evaluate if H. stipulacea displaces native seagrasses. To evaluate potential trophic support of different species we compared chemical composition and nutritional quality between native and invasive seagrasses. To measure trophic utilization and top-down control we quantified herbivory rates on multiple seagrass species in different habitats. Lastly to assess differences in the habitat creation functions of native and invasive seagrasses we measured litter decomposition rates, nutrient limitation, and the abundance and diversity of invertebrate communities in native and invasive seagrass beds.
Results/Conclusions
Seagrass community monitoring suggested that H. stipulacea likely displaces some native seagrasses (Syringodium filiforme and H. decipiens), but not others. Our experiments also showed that H. stipulacea does not provide equivalent ecosystem services to native seagrasses. Across nearly every metric of ecosystem function we evaluated we observed some type of significant difference between H. stipulacea and some native seagrasses. Nutritional quality of H. stipulacea was lower than native seagrasses with lower N and protein levels and higher C:N ratios. We also observed significantly lower consumption of H. stipulacea than the native S. filiforme but limited differences when compared with Thalassia testudinum. When evaluating the role of seagrasses in habitat creation we found H. stipulacea had greater litter decomposition rates than T. testudinum, and created a more nutrient limited environment. When evaluating invertebrate community composition we saw significantly distinct assemblages between native- and invasive dominated seagrass beds, though not a loss in species richness or invertebrate biomass. These results taken together suggest that the spread of H. stipulacea would impact a variety of ecological processes, potentially restructuring seagrass ecosystems in major ways through both direct environmental shifts and indirect foodweb interactions.