Co-occurring invasive plant interactions do not predict the impacts of invasion in experimental tallgrass prairie communities

Abstract

Co-occurring invasive plants are an understudied yet common phenomenon, and likely to become even more frequent with climate change-driven range shifts. Invasive–invasive interactions can be competitive, neutral, or facilitative; symmetric or asymmetric; and their impact on natives can be additive or non-additive. Non-additive impacts, wherein co-occurring invasives either reduce or enhance one another’s effects, have heightened implications for invasive management and native restoration. We investigated interactions among three invasive species—the legume Lotus corniculatus, the forb Cirsium arvense, and the grass Phalaris arundinacea—that co-occur in North American prairie grassland, and impacts on native prairie communities. Employing greenhouse microcosms, we observed invasive–invasive interactions by growing 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 pairs of invaders, or all three invaders together. For insight into mechanisms of species interactions, measurements of above- and below-ground resources were made. We predicted that the invasive legume would facilitate the other invasive species via nitrogen enrichment, promoting non-additive impacts on native communities. We observed that the legume promoted yields of the other invasive species and was associated with higher soil ammonium content. However, we also observed strongly asymmetric competition between co-occurring invasive species. Impacts of invaders on functionally diverse native communities were similar among invasion treatments and less than expected based on invasive impacts in monoculture. These results warrant future tests of functional group identity and diversity as predictors of co-occurring invasive species interactions and impacts.