Invasion of floating fern induces regime shift and alters freshwater macroinvertebrate community structure

Ecosystems tend to maintain stability with feedback loops to resist large abrupt shifts in ecosystem structure, however, if a shift occurs, a new ecological equilibrium is established, with feedback loops to maintain the new state. Non-indigenous free-floating aquatic fern giant salvinia, Salvinia molesta Mitchell (Salviniaceae), poses a risk to stability in freshwater ecosystems through limiting light penetration, decreased submerged aquatic vegetation (SAV) abundance, and altering water quality. The loss of SAV reduces habitat complexity, resulting in a homogenous habitat. Aquatic macroinvertebrates must cope with the habitat modification, altering species interactions, potentially leading to changes in macroinvertebrate community structure. To determine whether a regime shift occurred, we conducted repeated quarterly field surveys of plant composition, nutrients, and water quality. Sampling of aquatic macroinvertebrate community allowed assessment of how their communities responded to the regime shift. Giant salvinia reduced dissolved oxygen, pH and light availability in the aquatic environment, and increased the concentration of orthophosphate and ammonium, suggesting internal nutrient loading and a regime shift had occurred. Following initial giant salvinia infestation, macroinvertebrate communities in giant salvinia resembled SAV communities, however, richness and relative abundance in giant salvinia decreased over time, resulting in a community dominated by few taxa. Communities were initially Chironomidae dominant but shifted to Amphipod dominant. Total macroinvertebrate energetic value in giant salvinia was 3.40-times lower than SAV communities. Our findings demonstrate larger ecological impacts than previously reported. Giant salvinia invasion changed habitat composition, triggered internal nutrient loading, and reduced macroinvertebrate abundance and diversity, and ecosystem productivity.