Despite the ubiquity of multiple plant invasions, the underlying mechanisms of invasive-invasive interactions remain relatively unknown. Given the importance of plant–soil feedback (PSF) in contributing to single species invasions, it may be an important factor influencing invasive–invasive species interactions as well. PSF between multiple invaders has rarely been examined, but could inform the nature of invasive–invasive interactions and advance understanding of how multiple invaders impact plant communities. Alternative mechanisms of plant invasions include novel weapons and enemy escape. We develop graphical PSF predictions based on these mechanisms and other possible invasive–invasive dynamics. Comparing these predictions to observed results is a first step in interpreting PSF among co-occurring invasive species. We illustrate this with a case study of net pairwise PSF among three common invaders of tallgrass prairie: Lotus corniculatus
(birdsfoot trefoil), Phalaris arundinacea
(reed canarygrass), and Cirsium arvense
(Canada thistle). We found that feedback among all pairwise combinations of these invasive species was neutral. Neutral feedback can arise from a mutual lack of soil borne pathogens, consistent with the enemy escape hypothesis, although we cannot rule out shared benefit from generalist mutualists. While both facilitative and competitive interactions among these three species have previously been shown, our data suggest that such interactions are unlikely to operate through a legacy effect of PSF. Our results inform follow-up PSF experiments that would help to confirm the existence and nature of PSF interactions among these species.
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