Identifying an Invader
The recent discovery of a well-established H. stipulacea stand near Tequesta, Florida, approximately 85 miles north of and only two years after the original Key Biscayne occurrence, may indicate that H. stipulacea has already progressed beyond the earliest stages of invasion in Florida. It raises the possibility that H. stipulacea has been present in Florida longer than previously recognized and may already be undergoing a cryptic expansion phase, as appeared to have occurred in Grenada, Dominica, St. Lucia, and Martinique. If (or when) additional populations are discovered elsewhere along Florida’s coastlines, managers may be confronted with the reality that complete eradication is no longer technically achievable and containment is unlikely, especially if recruitment from the Caribbean is persistent, as I suspect it is. This situation would parallel many previous marine invasions in which management ultimately shifted from eradication to monitoring, impact assessment, and adaptive management (e.g., lionfish).
Importantly, H. stipulacea should not be viewed merely as another introduced species. It may represent a basin-scale invasion capable of affecting all dominant seagrass groups found in Florida waters: Thalassia testudinum, Syringodium filiforme, Halodule wrightii, and native Halophila species. Its broad environmental tolerance, rapid vegetative expansion, and ability to establish across a wide depth range suggest it could become a new foundational species throughout portions of the tropical western Atlantic.
There is little doubt that H. stipulacea presents ecological risks. Caribbean studies have documented rapid expansion, formation of extensive monospecific meadows, and displacement of native seagrasses, particularly Syringodium filiforme. Surveys in Dominica observed active expansion into native seagrass habitats, while indicated substantial shifts in benthic community composition following invasion. At the same time, ecological function and ecological composition are not synonymous. A central question in modern invasion ecology above is whether management should prioritize attempts at restoration and protection of historical species assemblages or maintenance of some, albeit altered, ecosystem processes and services under rapidly changing environmental conditions. That question of prioritization, however, is probably only valid for one-off, containable introductions and not a scenario like that facing Florida, where endless source material exists just tens of kilometers to the south with the potential to arrive en masse during extreme events (i.e., hurricanes).
Weighing the Differences
Unlike Florida’s native Halophila species, H. stipulacea forms exceptionally dense rhizome networks that likely stabilize sediments and form highly cohesive benthic structures (author’s personal observation; Arabian Gulf). These rhizome mats are like those observed in dense native Halodule wrightii stands and are presumably similarly resistant to erosion, vessel wake disturbance, and storm-driven sediment mobilization. Moreover, as with all seagrasses, H. stipulacea likely provides varying degrees of the ecological functions traditionally associated with seagrass ecosystems in general, including:
- Sediment stabilization and shoreline protection;
- Carbon sequestration and organic matter storage;
- Nutrient uptake and cycling;
- Habitat provision for fishes and invertebrates;
- Enhancement of water clarity through sediment retention; and
- Rapid colonization of disturbed substrates.
These characteristics raise an important management question. In areas where native seagrass communities have already been degraded by declining water quality, dredging, vessel impacts, disease, harmful algal blooms, or climate-related stressors, should the presence of H. stipulacea automatically be viewed as a net ecological loss? It has probably not escaped the reader’s attention that the descriptions of services are all caveated (e.g., “likely”). This is because a comprehensive assessment of services has only just begun.
The answer as to whether the arrival of H. stipulacea constitutes a net ecological loss may not be straightforward. Florida has experienced substantial seagrass losses over recent decades, including major declines in the Indian River Lagoon, portions of Biscayne Bay, Florida Bay, and other estuarine systems. In some of these environments, native seagrass recovery may not currently be feasible even with restoration efforts. Therefore, if H. stipulacea becomes irrevocably established in a location, particularly a degraded one, how should this be valued?
Importantly, manatee utilization of the species may be one of the strongest points supporting a functional ecosystem perspective: if H. stipulacea proves capable of supporting manatee grazing in addition to some fish habitat, sediment stabilization, and carbon sequestration in areas where native seagrasses are failing, managers may eventually need to weigh ecosystem function against strict preservation of historical species composition. However, at present, there is only scant evidence to conclude that H. stipulacea is an equal ecological substitute for native Florida seagrass meadows as manatee habitat and forage. But if H. stipulacea were demonstrated to be capable of maintaining seafloor coverage, sediment stability, carbon storage (as it surely will), and habitat functions such as forage for manatees, especially in locations where native species fail to recover, it may serve as a functional substitute for some ecosystem services that would otherwise be lost.
The question of functional substitution also may be relevant under projected climate change scenarios. Because H. stipulacea evolved in the Red Sea and western Indian Ocean under environmental conditions that are generally warmer and often more variable than those experienced by Florida seagrasses, the species may possess physiological characteristics that confer greater resilience to increasing temperatures and environmental stress. If future coastal ecosystems become increasingly difficult for native seagrasses to occupy, H. stipulacea could potentially function as a climate-resilient successor species rather than simply an invasive competitor.
An interesting comparison of whether to “embrace or fight” a non-native seagrass may be drawn with the introduction of Zostera japonica on the Pacific Coast of North America. Zostera japonica is believed to have been introduced to the Pacific coast of North America through oyster imports from Japan during the early twentieth century and was first collected in Washington State in 1957. Initially regarded exclusively as an invasive species threatening native eelgrass (Zostera marina), subsequent research demonstrated Z. japonica altered sediment characteristics, increased faunal richness relative to adjacent unvegetated habitats, supported diverse benthic invertebrate communities, and functioned as a habitat-forming ecosystem engineer providing multiple ecosystem services—much as does the native Z. marina. As a result, management approaches in Washington and parts of Oregon evolved toward more nuanced assessments that considered ecological services alongside impacts on native biodiversity. Although California generally retained a more precautionary stance, the debate surrounding Z. japonica demonstrated that introduced seagrasses can simultaneously create ecological benefits and ecological concerns.
Management Considerations
A critical difference remains, however. Z. japonica primarily colonized upper intertidal habitats that were historically sparsely vegetated, as they were increasingly out of the vertical range of the native Z. marina and were sometimes simply unvegetated. As such, in much of their distribution, Z. japonica did not compete strongly with existing seagrass. In contrast, H. stipulacea frequently invades habitats already occupied by native seagrasses. Consequently, the risks to native seagrasses and their biota posed by H. stipulacea may be considerably greater. The consequences for changes in faunal utilization with a shift towards H. stipulacea may be significant, especially given the dramatic differences in canopy architecture among the larger-bodied Florida seagrasses that can extend up to a meter into the water column, as compared to Halophila canopies that are only a few centimeters tall. Nonetheless, this distinction does not eliminate the possibility that the species could also provide valuable ecosystem services in altered or degraded environments and may call for scenario-dependent assessment of service trade-offs.
Ultimately, given the history of inexorable spread by H. stipulacea, the long-term challenge facing Florida may not be determining whether H. stipulacea is “good” or “bad.” Rather, it may be deciding whether management should focus on attempting to preserve historical ecosystem composition at all costs, or on maintaining ecosystem function and resilience under different scenarios of environmental degradation and under climate conditions that are increasingly different from those under which Florida’s seagrass communities evolved.
As the species continues to expand throughout the state, resource managers should expect to eventually confront a difficult but scientifically legitimate question: is a dense meadow of H. stipulacea preferable to a healthy native seagrass meadow? This is almost certainly an untenable outcome in most extents of Florida’s robust seagrass ecosystems. However, in some circumstances, might it be preferable to bare sediments, chronic turbidity, recurring algal blooms, and the persistent absence of the ecosystem services once provided by historical seagrass communities? In such locations, the answer may ultimately be yes because there exists no functional or practicable alternative.
The future management of H. stipulacea in Florida may therefore become less a question of invasive species eradication and more a question of how society chooses to manage emerging novel ecosystems in an era of multiply impacted estuaries and lagoons and environmental change.
For more about CSA Ocean Sciences, visit: csaocean.com
Additional sources:
Beca-Carretero, P., Viana, I.G., Teichberg, M., Winters, G., Procaccini, G., & Reuter, H. (2020). Exploring how non-native seagrass species could provide essential ecosystem services: A perspective on the Caribbean invasion of Halophila stipulacea.
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Winters, G., Beer, S., Willette, D.A., Viana, I.G., Chiquillo, K.L., Beca-Carretero, P., Villamayor, B., Azcárate-García, T., Shem-Tov, R., Mwabvu, B., et al. (2020). The tropical seagrass Halophila stipulacea: Reviewing what we know from its native and invasive habitats, alongside identifying knowledge gaps. Frontiers in Marine Science, 7, 300. https://doi.org/10.3389/fmars.2020.00300
This feature appeared in environment coastal & offshore (eco) magazine’s 2026 issue III. Read in the magazine here.
