Climatic drivers and ecological impacts of a rapid range expansion by non-native smallmouth bass

Smallmouth bass ( Micropterus dolomieu ) are a globally introduced fish species that have experienced widespread range expansions in recent decades and which can have deleterious effects on native fish communities. Rapidly assessing their expansions will aid conservation and management actions geared towards controlling their spread and mitigating their impacts. Smallmouth bass have recently experienced a rapid upstream expansion in a Great Plains river (Laramie River, Wyoming, USA), which provided an opportunity to evaluate the drivers and impacts of this expansion by using a modified before-after, control-impact (BACI) design. Our objectives were to test whether climatic drivers (temperature, precipitation, flow) were related to this range expansion and subsequent effects of the expansion on native fish communities. Smallmouth bass population size in Grayrocks Reservoir increased following a climatically extreme wet year, with statistically extreme amounts of spring-time and June precipitation creating high discharge events that coincided with the upstream expansion. Unlike previous studies highlighting the invasive nature of smallmouth bass, the modified BACI analysis revealed no declines in species richness induced by the expansion. However, there was evidence that native small-bodied minnow species (family Leuciscidae ) declined in relative abundance and that community-level and species-level trophic niches were compressed for invaded sites. Our findings provide important insight into how climatic extremes can prompt biological invasions that can alter community composition and food web structure even if local extirpations do not occur.