Sigmoidal curves reflect impacts and dynamics of aquatic invasive species

Identifying general patterns and trends underlying biological invasion population dynamics and impacts has proven elusive for scientists. Recently, the impact curve was proposed as a means to predict impacts over time from invasive alien species, characterized by a sigmoidal growth pattern in cumulative abundances. While the impact curve has been empirically demonstrated with monitoring data of a single invasive alien species (New Zealand mud snail, Potamopyrgus antipodarum), the broadscale applicability of this trend remains to be tested for other taxa. Here, we examined whether the impact curve can adequately describe the invasion dynamics of 13 other aquatic invasive alien animal species (spanning over different taxa: Amphipoda, Bivalvia, Gastropoda, Hirudinea, Isopoda, Mysida, and Platyhelminthes) at the European scale, employing multi-decadal time series of macroinvertebrate abundances from regular benthic monitoring efforts. For all except one tested species (the ‘killer shrimp’, Dikerogammarus villosus), the sigmoid impact curve was strongly supported (R2 > 0.95), characterized by an exponential increase in cumulative abundance, followed by a subsequent decline in the rate of accumulation and eventually approaching a saturation level in the long term, where impact was maximized. For the D. villosus exception, the impact had not yet saturated, likely reflecting the early phase of an ongoing European invasion. The impact curve further allowed estimation of introduction years and lag phases, as well as parameterisation of growth rates and carrying capacities, providing strong support for the boom-bust dynamics typically observed in several invader populations. These findings suggest that impact can grow rapidly before saturating at a high level, with timely monitoring often lacking for the detection of invasive species post-introduction. We further confirm the applicability of the impact curve to determine trends in invasion stages, population dynamics, and impacts of pertinent invaders, ultimately helping inform timing of management interventions. We hence call for improved monitoring and reporting of invasive alien species over time to permit further testing of large scale impact consistencies across various habitat types.