Informing spread predictions of two alien snails using movement traits

Invasive alien species are a growing global problem, and aquatic ecosystems have been regarded as particularly vulnerable. Biological invasions can alter ecosystem functioning, threaten native biodiversity and burden the global economy. Understanding alien species ability to disperse via locomotion following arrival to new environments is critical for prediction of spread rates. Here, we quantified in-field densities and compared movement traits between two widespread invasive alien snails, Tarebia granifera and Physa acuta. We measured the: (i) net distance and velocity to determine dispersal potential; and (ii) turning angles (both absolute and relative) and straightness index as proxies for exploratory behaviour. Tarebia granifera exhibited a significantly greater velocity and covered a significantly larger net distance (i.e., greater spread rate) than Physa acuta. In-field densities were marked for both species (T. granifera: mean 351 individuals m−2; P. acuta: mean 235 individuals m−2), but differed spatially. The exploratory behavior (i.e., mean or absolute turning angles and straightness index) did not differ significantly between the two alien species; both species showed a slight tendency to turn counterclockwise. The present study suggests a more rapid capacity to self-disperse in T. granifera than P. acuta, which could facilitate rapid spread within and between aquatic systems. Thus, this current study highlights the often-overlooked role of animal behaviour in promoting invasion; this autecological information can help inform predictive models for the spread of alien snails within freshwater ecosystems