Non‐native introductions influence fish body size distributions within a dryland river

Abstract A contemporary challenge in ecology is assessing the ecosystem effects of multispecies introductions. Quantifying shifts in body sizes, a common trait with which many per capita rates of ecosystems functioning scales, provide an important way forward. Evidence suggests that freshwater fish introductions have altered species‐level body size distributions globally, but it is difficult to interpret their functional consequences because animals contribute to ecosystem functioning at the individual level at smaller spatial scales. In this study, we determine whether these macroecological patterns hold for individual size distributions (ISDs) at local scales. We use a comprehensive dataset of fish communities in a highly invaded dryland river to (1) compare the statistical moments of ISDs between native and non‐native pools and (2) relate biological and environmental covariates to the univariate moments of local ISDs and the multivariate community structure of local fish assemblages. We found that ISDs of native and non‐native pools were significantly different when data were pooled across sites. Non‐natives had smaller mean body size than natives across all guilds and within invertivore and omnivore guilds, but were significantly larger within the invertivore–piscivore guild. Moreover, differences in the variance of size distributions were often relatively greater than differences in the mean. By modeling ISDs at the site level as a function of biological and environmental covariates, we found that non‐native dominance within sites had similar effects on ISDs as expected from differences between species pools. However, neither biological nor environmental predictors explained significant community‐level variation when size and trophic structure were considered simultaneously. Shifts in size distributions can have multiple ecological consequences, and our results provide a baseline for generating size‐based predictions of non‐native species’ roles in ecosystem functioning. However, these size‐based predictions should be considered alongside other important trait distributions such as trophic structure, which had no consistent relationship with non‐native dominance in this study.