A newly arrived fern makes a splash in freshwater ecosystems

Primary Source: Wahl, C., Kaller, M., and Diaz, R. 2021. Invasion of floating fern alters freshwater macroinvertebrate community structure with implications for bottom-up processes. Hydrobiologia 848:2523-2537. https://doi.org/10.1007/s10750-021-04571-4

A global society- not just for humans

Although our world may seem large, it is incredibly interconnected. COVID-19, unfortunately, has given us a clear demonstration of this fact. Within just two months of the virus’ first case, COVID-19 was declared a global pandemic.

Plants, animals, and wildlife diseases can travel quickly, too. Some are unable to survive in the new environments in which they find themselves, but others thrive in these novel conditions. In some cases, these non-native species become so abundant that they negatively affect other species in the ecosystems. The invaded ecosystem may contain no natural predators or competitors to limit the invader’s growth, and the invader’s new prey and/or competitors have likely not co-evolved defenses against them. This can give non-native species a leg up on the native residents.

New fern on the block

One such invasive species is the aquatic fern, Salvinia molesta, commonly named giant Salvinia. This species native to Brazil has become established in freshwater marshes in U.S. states along the Atlantic and Gulf coasts, growing to form thick mats on the water’s surface. Freshwater marshes provide habitat for animals, natural filtration of pollutants and excess nutrients, and flood mitigation. Native species of submerged aquatic vegetation are important members of these marshes. They trap energy in the form of carbon and provide habitat and food for waterfowl, insects, small crustaceans, and fish.

As giant Salvinia has entered freshwater marsh ecosystems, it has, in some cases, taken over the role of native submerged vegetation. A team of scientists based in Louisiana State University aimed to determine the newcomer’s effects on the freshwater marsh ecosystem. They focused on macroinvertebrates, a group that includes insects and small crustaceans. Macroinvertebrates act as a link within the ecosystem, through which energy can travel from primary producers (e.g., giant Salvinia and native species of submerged vegetation) to other, larger consumers (e.g., fish and waterfowl). Specifically, the researchers aimed to investigate giant Salvinia’s impact on environmental conditions, macroinvertebrate abundance and species, and the total energy of the macroinvertebrate community.

The experiment

            The research group chose 6 sampling sites within a privately owned parcel of land in Louisiana. Three of the sampling sites were dominated by giant Salvinia, while three were dominated by native species of submerged aquatic vegetation. Sampling was conducted once per season to measure physical and chemical characteristics of the water, and to measure the Salvinia mat thickness and root length. Additionally, plant traps and nets were used to collect native vegetation and giant Salvinia. The collected plants were brought to the lab, and macroinvertebrates were rinsed off. Those macroinvertebrates were then counted and identified. Previously published caloric values of the macroinvertebrate types were used to calculate the total potential caloric energy value of the macroinvertebrate community at each sampling site.

Giant Salvinia has giant ecosystem effects

Results showed that, compared to sites with native vegetation, giant Salvinia sites housed fewer macroinvertebrate species with lower overall biomass and energy. While native vegetation housed mostly insects, giant Salvinia housed small crustaceans. Insects are an important food source for fish and waterfowl, and the fish (dependent on insects) are commonly eaten by wading birds. Consequently, the lack of insects in Salvinia-dominated marshes could have effects that ripple throughout the food web.

While this study did not directly address the specific reasons for the observed shifts in macroinvertebrate community size and structure, the researchers were able to infer some potential mechanisms. Because Salvinia forms thick mats, they may block air-breathing aquatic insects from reaching the surface. Additionally, the habitat structure created by Salvinia is less complex than that created by native submerged vegetation. This may mean that macroinvertebrates in Salvinia-dominated marshes may have to compete more for less space compared to macroinvertebrates inhabiting marshes dominated by native vegetation.

The results of this study suggest that Salvinia may have far-reaching impacts on freshwater ecosystems. Further research would be useful to determine the likelihood and timeline for macroinvertebrate communities to recover after Salvinia removal, and to examine effects of Salvinia on other communities within freshwater marsh ecosystems.

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Sarah Shainker

Sarah is a Phd student at the University of Alabama in Birmingham interested in evolutionary ecology, population genetics, citizen science, and macroalgae. Before beginning grad school, she worked as an outdoor educator in the north Georgia mountains and as a coastal resource management volunteer for Peace Corps Philippines. Twitter: @SarahShainker

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