The pervasive impact of invasive species has motivated considerable research to understand how characteristics of invaded communities, such as native species diversity, affect the establishment of invasive species. Efforts to identify general mechanisms that limit invasion success, however, have been frustrated by disagreement between landscape-scale observations that generally find a positive relationship between native diversity and invasibility and smaller-scale experiments that consistently reveal competitive interactions that generate the opposite relationship. Here we experimentally elucidate the mechanism explaining the large-scale positive associations between invasion success and native intertidal diversity revealed in our landscape-scale surveys of New England shorelines. Experimental manipulations revealed this large-scale pattern is driven by a facilitation cascade where ecosystem-engineering species interact nonlinearly to enhance native diversity and invasion success by alleviating thermal stress and substrate instability. Our findings reveal that largescale diversity-invasion relationships can be explained by small-scale positive interactions that commonly occur across multiple trophic levels and functional groups. We argue that facilitation has played an important but unrecognized role in the invasion of other well studied systems, and will be of increasing importance with anticipated climate change.

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Though species interactions across local environmental gradients are well studied, the way in which species interactions shift between different habitats on a landscape scale has received less attention. We hypothesised that interactions among a suite of shoreline plant species shift across a hydrodynamic-exposure gradient, leading to generation of apparently distinct habitat types (e.g. bare cobble beaches, vegetated cobble beaches, fringing marshes and salt marshes). We examined hydrodynamic forcing and found that it was strongly correlated with shoreline habitat type. A transplant experiment revealed that all plants were rapidly crushed and abraded in bare cobble areas with high hydrodynamic energy and were out-competed by grasses in the low-energy salt marshes. Vegetated cobble beaches are inhabited by the largest amount of plant species under intermediate conditions, where hydrodynamic energy is sufficiently low to allow the establishment of the ecosystem engineer Spartina and sufficiently high to prevent the monopolisation of space by competitively dominant, but stress intolerant grasses. Experimentally reducing physical and biotic stresses (hydrodynamics and interspecific competition) on bare cobble beaches and salt marshes, respectively, enabled forbs to persist across the whole gradient. These results demonstrate that the outcome of interspecific interactions at landscape scales is driven by background physical conditions, and that this can result in the development of what are considered distinct habitats.@en