Disentangling the impacts of plant co-invasions: additive, antagonistic and synergistic

Although the impacts of single-species invasions on resident communities have been widely studied, the ecological effects of co-invasion (simultaneous invasion by multiple alien species) remain largely unexplored. In this study, using an analytical framework, we investigate the individual and combined effects of two global plant invaders (Leucanthemum vulgare and Lupinus polyphyllus) on the aboveground vegetation (plant richness, diversity and biomass) and belowground soil variables (pH, salinity, electrical conductivity, total dissolved solids, organic carbon [OC], phosphorous, and potassium) of grassland communities in the Himalaya. We studied these ecological variables in comparable plots under the following four scenarios: both invaders absent (i.e. uninvaded), either of the two invaders present (i.e. single species invasion), and both invaders present (i.e., co-invasion). Our results show invasion effects vary with the invasion scenarios and the studied variables. The single species invasion plots had lower mean plant richness and diversity but higher mean soil OC than the uninvaded plots. The co-invasion plots had significantly increased mean plant biomass and soil OC, but had a non-significant effect on plant richness and diversity compared to the uninvaded plots. The overall combined effect of species’ co-invasion was additive in nature, but the effect varied significantly with choice of the response variable. We found synergistic effects of co-invasion on species richness and diversity, antagonistic effects on soil pH, and additive effects on the remaining response variables when compared with the predicted additive effect of the two invaders. Our findings suggest that further spread of L. polyphyllus will impact the resident plant community more severely when co-invading with L. vulgare than when invading alone, thereby lending support to the invasional meltdown hypothesis. Nevertheless, antagonistic and additive impacts for soil pH and other response variables respectively hint towards other alternate mechanisms at play. Overall, our study showcases a proof-of-concept to empirically estimate the additive, antagonistic and synergistic impacts of plant co-invasions, and also provides a model for designing co-invasion studies on other taxa/systems/scales elsewhere. Finally, based on the insights gained from this study, we highlight priority areas of future research in species’ co-invasions.