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.