Co-occurring invasive plants are an understudied yet common phenomenon, and likely to become even more frequent due to climate change-driven range shifts. Invasive-invasive interactions can be competitive, facilitative, symmetric, or asymmetric, and their impacts on natives can be additive or non-additive. If the impacts of co-occurring invasives are non-additive, the species could potentially reduce or enhance each other’s effects non-linearly, having profound implications for invasive management and native restoration. We investigated interactions among the co-occurring invasive species Cirsium arvense (Canada thistle), Lotus corniculatus (birdsfoot trefoil), and Phalaris arundinacea (reed canarygrass) and the effects of such interactions on communities of native prairie species. Employing greenhouse mesocosms, we isolated invasive-invasive interactions by growing each invasive species alone, in pairs, and in three-way combination. To assess impacts of co-occurring invasives, native prairie communities were grown with each single invader, all possible pairs of invaders, or all three invaders together. We hypothesized that invasive species from dissimilar functional groups would non-additively facilitate each other, resulting in greater negative impacts on native communities than single invasions.
Results/Conclusions
We observed both invasive-invasive facilitation and competition when invasive species were grown separately from native communities. Based on aboveground biomass, Canada thistle and reed canarygrass were facilitated by birdsfoot trefoil, whereas birdsfoot trefoil was unaffected by invasive context. These invasive responses demonstrate asymmetric invasive facilitation. Additionally, Canada thistle was inhibited by reed canarygrass while reed canarygrass was unaffected by Canada thistle. These interactions demonstrate asymmetric invasive competition. In contrast with expectations, the impact of invaders on the native community was similar among invasion treatments and less than would be expected, even under additive conditions. Invasive-invasive interactions within native communities were similar in pattern when compared to invasive-invasive interactions in invasive-only mesocosms, yet diminished in magnitude. These results suggest that interactions with native species modify invasive-invasive interactions. We discuss a functional group approach as a useful next step in the analysis of co-occurring invasion dynamics.