The value of geohistorical data in identifying a recent human‐induced range expansion of a predatory gastropod in the Colorado River delta, Mexico

Aim Throughout the Colorado River basin, numerous dams have captured and diverted the entirety of the basin's c. 1.85 × 10 10 m 3 year −1 of water, altering the downstream delta ecosystem. In the absence of pre‐dam studies, geohistorical data can play a critical role in reconstructing ecological aspects of the past ecosystem. Here, we use geohistorical data to test the hypothesis that a shell‐drilling naticid gastropod predator, Notocochlis chemnitzii , has recently undergone a human‐induced range expansion to the Colorado River delta ( CRD ). Based on uncertainty in the literature surrounding the membership of N. chemnitzii in the CRD benthic community before the dams were built, we predicted that, for pre‐dam samples, N. chemnitzii would be absent and there would be less variance in predator size selectivity and drillhole stereotypy. Location Colorado River delta, Mexico. Methods At two localities we sampled naticid shells and collected bivalve prey from the tidal flat and pre‐dam era cheniers – dune‐like structures composed primarily of molluscan remains. Naticids were sorted and counted by species. Variation in the stereotypic placement of drillholes in the pre‐ and post‐dam construction eras was assessed by grouping drillholes into a nine‐sector grid superimposed on the prey's shell. Predator size was regressed on bivalve prey size to assess variation in predator size selectivity pre‐ and post‐damming of the Colorado River. Results As predicted, we found no N. chemnitzii in cheniers of pre‐dam age, whereas another naticid species, Neverita reclusiana , was common. Notocochlis chemnitzii and N. reclusiana were found in the actively forming chenier and on the tidal flat. As expected with the addition of another naticid species, the preferred site of drilling and size selectivity of prey by predators changed significantly between cheniers from the pre‐ and post‐dam construction eras due to variation in prey‐handling behaviour by different naticid species. Main conclusions Geohistorical evidence, from body and trace fossils, suggests that N. chemnitzii has recently expanded its geographical range to the CRD , whereas N. reclusiana has long dwelled in the delta. Geohistorical baseline data are an underutilized source of information that can provide unique ecological perspectives for restoration efforts in estuaries of dammed rivers world‐wide.