Andrew O'Reilly-Nugent, Elizabeth Wandrag, Jane Catford, Bernd Gruber, Don Driscoll, Richard Duncan

1. Non-native species can dominate plant communities by competitively displacing native species, or because environmental change creates conditions favourable to non-native species but unfavourable to native species. We need to disentangle these alternative mechanisms so that management can target competitively dominant species and reduce their impacts.
2. Joint-species distribution models (JSDMs) can potentially quantify competitive impacts by examining how species respond to environmental variation and to changes in community composition. We describe a JSDM to model variation in plant cover, which detected declines in species abundance in the presence of a dominant competitor.
3. We applied our model to an experiment in an invaded grassy-woodland community in Australia where we manipulated biomass removal (through slashing and grazing by kangaroos) along a fertility gradient. Non-native species dominated plant cover at high fertility sites in the absence of biomass removal. Using a JSDM, we determined that three of the 72 non-native plant species (Bromus diandrus, Acetosella vulgaris and especially Avena fatua) were having a strong competitive impact on the community, driving changes in composition and reducing the cover of both native and non-native species, particularly in the absence of grazing. The dominant annual grasses (Bromus diandrus and Avena fatua) were two of the tallest species in the community and were good competitors for light under conditions of high fertility and low grazing. Consequently, their impacts were greatest on smaller statured species.
4. Synthesis. We demonstrate a method to measure competitive impact using a JSDM, identify species driving compositional change through competitive displacement, and identify where on the landscape competitive impacts are greatest. This information is central to managing plant invasions: by targeting dominant non-native species with large competitive impacts, management can reduce impacts where they are greatest. We provide details of the modelling procedure and reproducible code to encourage further application.