Invasive Floating Fern Limits Aerial Colonization and Alters Community Structure of Aquatic Insects

Aquatic insects are important to the transfer of energy in freshwater wetlands. They link primary producers and organic matter to higher trophic levels, and pass energy from the aquatic environment to the terrestrial ecosystem via predation of adults. Introduction of the invasive free-floating aquatic fern giant salvinia, Salvinia molesta Mitchell, threatens aquatic insect communities and life cycle completion, specifically colonization, through mat formation and habitat changes. We examined how aerial colonization and assemblage structure of aquatic insects were affected by giant salvinia coverage. Mesocosms were established with three giant salvinia coverages, 25%, 50%, and 100%, and a native submerged macrophyte, 0% coverage; and environmental conditions and community structure were sampled monthly, over four months. After four weeks, all giant salvinia coverages reduced dissolved oxygen, pH, and light availability in the aquatic environment. In the 100% cover treatment, aquatic insect colonization was impeded, resulting in communities with lower abundance and richness, which were dominated by a coleopteran. The 25% and 50% giant salvinia coverages changed habitat complexity, which yielded different aquatic insect communities from the 0% and from 100% coverages. Regardless of the coverage, giant salvinia changed environmental conditions and putatively the 100% coverage disrupted the aquatic insect life cycle by limiting aerial colonization. This inability to complete their life cycle means aquatic insects cannot replenish themselves, leading to a diminished species pool and reduced energy potential. Our results highlight new, previously unreported, negative impacts from giant salvinia on ecosystem structure and function.