Seventy years of stream-fish collections reveal invasions and native range contractions in an Appalachian (USA) watershed

Aim: Knowledge of expanding and contracting ranges is critical for monitoring invasions and assessing conservation status, yet reliable data on distributional trends are lacking for most freshwater species. We developed a quantitative technique to detect the sign (expansion or contraction) and functional form of range-size changes for freshwater species based on collections data, while accounting for possible biases due to variable collection effort. We applied this technique to quantify stream-fish range expansions and contractions in a highly invaded river system. Location: Upper and middle New River (UMNR) basin, Appalachian Mountains, USA. Methods: We compiled a 77-year stream-fish collections dataset partitioned into ten time periods. To account for variable collection effort among time periods, we aggregated the collections into 100 watersheds and expressed a species' range size as detections per watershed (HUC) sampled (DPHS). We regressed DPHS against time by species and used an information-theoretic approach to compare linear and nonlinear functional forms fitted to the data points and to classify each species as spreader, stable or decliner. Results: We analysed changes in range size for 74 UMNR fishes, including 35 native and 39 established introduced species. We classified the majority (51%) of introduced species as spreaders, compared to 31% of natives. An exponential functional form fits best for 84% of spreaders. Three natives were among the most rapid spreaders. All four decliners were New River natives. Main conclusions: Our DPHS-based approach facilitated quantitative analyses of distributional trends for stream fishes based on collections data. Partitioning the dataset into multiple time periods allowed us to distinguish long-term trends from population fluctuations and to examine nonlinear forms of spread. Our framework sets the stage for further study of drivers of stream-fish invasions and declines in the UMNR and is widely transferable to other freshwater taxa and geographic regions.