Testing population-level mechanisms of invasion by a mobile vertebrate: a simple conceptual framework for salmonids in streams

Invasion biology suffers from a lack of the ability to predict the outcome of particular invasions because of reliance on verbal models and lack of rigorous experimental data at the appropriate scale. More progress is likely to be made by considering invasions as population-level phenomena and initially focusing on specific taxa or particular categories of invasions. To this end, we propose a simple conceptual framework to motivate studies of invasion by salmonids (salmon, trout, grayling, and whitefish) in streams that emphasizes population-level mechanisms affecting native species and promoting spread by the invader. Specifically, the only direct mechanisms by which the abundance of the native species can decline are through biotic interactions which cause decreased reproductive rates or survival at specific life stages, net emigration, debilitating or fatal diseases introduced by the invader, or a combination of these factors. Conversely, abundance of the invader must increase by local reproduction, high survival, net immigration, or a combination of these factors. Review of existing salmonid invasion literature suggests that future studies could be improved by using manipulative field experiments at a spatial and temporal scale appropriate to address population-level processes, characterizing how movement affects the establishment and spread of an invader, and including abiotic context in experimental designs. Using the example of brook trout (Salvelinus fontinalis) invasion into streams containing native Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus) in the central Rocky Mountains (USA), we demonstrate how the framework can be used to design a manipulative field experiment to test for population-level mechanisms causing ecological effects and promoting invasion success. Experiments of this type will give invasion ecologists a useful example of how a taxon-specific invasion framework can improve the ability to predict ecological effects, and provide fishery biologists with the quantitative foundation necessary to better manage stream salmonid invasions.