Abstract
New Zealand mudsnails (NZMS) have spread rapidly across the western United States, but little is known about mechanisms that drive their spread within invaded streams. We used a field experiment to test if upstream movement is a potential vector of NZMS spread and how this movement is modified by flow velocity and resource availability. We found movement direction and rates were related to flow velocity, while resource availability influenced the number of individuals that moved. In slow-flow treatments, individuals moved upstream at faster rates (~ 3 m/hr) than previously recorded for this species. In fast-flow treatments, most individuals were dislodged downstream and upstream movement rates were less than 2 m/hr. In low-resource treatments, individuals were more likely to move away from their initial starting locations. We suggest that upstream movement may be important in establishing new populations within local invasions and that increases in flow velocity may be an effective means to slow the upstream spread of NZMS. The surprisingly fast movements that we recorded predict greater distribution of NZMS within invaded streams than has actually occurred, which suggests that factors in addition to NZMS movement rate may limit population spread.