Using stable isotope analysis to answer fundamental questions in invasion ecology: Progress and prospects

What makes some species successful invaders while others fail, and why some invaders have major impacts in invaded ecosystems are pivotal questions that are attracting major research effort. The increasing availability of high resolution, georeferenced stable isotope landscapes ('isoscapes'), coupled with the commercialization of stable isotope-enriched tracer molecules and the development of new analytical approaches, is facilitating novel applications of stable isotope techniques in ecology. We can now address ecological questions that were previously intractable. We review and discuss how stable isotope analysis (SIA) can complement fundamental research themes in the study of biological invasions, especially in answering questions relating to the physiological and ecological mechanisms underlying invasion processes and invader impacts. SIA was first used for simply describing the diet of invaders but, more recently, SIA-informed metrics of population and community trophic structure have been advanced. These approaches now permit the comparison of diets across space and time and provide quantitative tools to compare food webs across different stages of invasion. SIA has also been pivotal in quantifying competition for resources between native and non-native species (e.g. competition for food, water, or nutrient use). Specific questions related to modes of dispersal (e.g. origin and distance/direction travelled) and mechanisms of establishment can also be addressed using SIA in diverse taxa. An overarching goal is to highlight examples of recent studies that have used SIA in key areas of invasion ecology and use these to synthesize testable predictions where SIA could be applied to future studies. We conclude by highlighting several paths forward and describing how unresolved challenges in quantifying the rates, impacts, and mechanisms underlying invasions could potentially benefit from the use of SIA.