Applying invasion biology frameworks to predict the impacts of range-expanding predators

Global change is causing a widespread redistribution of species, and novel species are expected to impact populations in their recipient communities. Theory from invasion biology provides a framework to predict the impacts of range-shifting species. Specifically, the impacts of invasive predators are expected to be nonlinear (greatest per capita effects at low densities) and to be greater in their invaded ranges when compared to their historical ranges. For range-shifting species, we hypothesized that impacts would similarly be nonlinearly related to abundance and that impacts in the expanded range would be greater than those in the historical range (due to prey naivety and/or enemy release). Our alternative hypothesis was that impacts would be consistent between historical and expanded ranges (due to the potential for historical coevolutionary interactions with species in the expanded range). To test the applicability of this framework with range-shifting species, we conducted observational surveys and manipulative experiments in the historical and expanded ranges of two predators undergoing poleward expansions, the whelks Acanthinucella spirata and Mexacanthina lugubris. We assessed impacts on prey (acorn barnacles and mussels) abundance and community diversity and compared per capita impacts between regions. As with non-native invasive species, we found that both whelks reduced the abundance of prey species; however, our results supported a linear relationship and no decrease in per capita effect on prey with increasing density of the shifting predator, and we did not observe consistent impacts of range-shifting whelks on community diversity. Finally, impacts in whelks' expanded ranges were generally consistent with those in historical ranges, with some potential for increased impact in the expanded range. By adapting invasion frameworks, our work revealed that abundance and impacts in the historical range are indicators of range-shift impacts that could inform anticipatory management responses to range shifts.