Cooperative Fish and Wildlife Research Units Program: Wyoming
Education, Research and Technical Assistance for Managing Our Natural Resources

Smallmouth bass (Micropterus dolomieu) are an invasive fish species that have experienced widespread range expansions in recent decades and can have deleterious effects on native fish communities. Rapidly assessing their invasions 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 determine the drivers and impacts of this expansion by using a modified before-after, control-impact (BACI) design. Our objectives were to determine whether climatic drivers were responsible for this range expansion and subsequent effects of the expansion on native fish communities. Smallmouth bass abundance in a local reservoir spiked 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 Cyprinidae) 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 and how some invasions may not produce immediate negative effects on community diversity.