Community‐level direct and indirect impacts of an invasive plant favour exotic over native species

Abstract

Abstract Indirect interactions mediated by shared enemies or mutualists (i.e. apparent competition) can influence whether invasive plants harm or benefit co‐occurring species. However, studies to date have largely examined single pairwise interactions, limiting our understanding of the interplay among different types of interactions and whether indirect impacts systematically favour native or exotic species. Predicting indirect interaction strength has also proven challenging, and it remains unclear whether the strengths of different indirect interactions are correlated. We conducted a field experiment in a grassland invaded by Scotch broom Cytisus scoparius to compare the strength of its indirect impacts, via both soil fungi and herbivores, on 21 native and exotic legume species growing in pots buried in the ground. Direct interactions of plants with soil fungi were controlled using nylon mesh pot windows of differing porosity (1 or 38 µm) to prevent or allow soil fungi hyphal growth. Arthropod herbivores were controlled through spraying pyrethrum pesticide. To assess indirect impacts, interactions were compared between plants adjacent to or 50 m away from an extensive Scotch broom invasion. We measured plant performance (survival, height and biomass), arthropod and hare herbivory, and rhizobia nodulation. Despite increasing arthropod herbivory of both native and exotic plant species, Scotch broom had a net positive impact on their survival and growth, through sheltering them from abiotic stress, and indirectly via beneficial soil fungi and release from hare browsing. Soil fungi also increased arthropod herbivory, decreased rhizobia nodulation and disproportionately promoted the growth of exotic plants. Overall, exotic plants experienced stronger interactions, which favoured them with beneficial soil fungi and rhizobia but not hare browsing. Finally, indirect interaction strength was not correlated among indirect interactions mediated by different interaction partners. Synthesis. We demonstrate that invaders affect their competitors through multiple interacting indirect pathways that were stronger than direct ‘nurse plant’ effects, emphasizing the importance of a community‐level approach to studying biological invasions. Exotic species experienced stronger positive and negative impacts than natives, but were facilitated overall, potentially contributing to exotic dominance in communities.