Invasion dynamics and potential future spread of sea spurge across Australia’s coastal dunes

Aim Invasive species provide an opportunity to study biogeography in action, allowing us to observe how species adapt and fill their environmental niche when introduced to new ecological settings. Here we use sea spurge-a foredune specialist plant species native to Europe which has recently spread across Australia's southern coasts-as a model system to explore species' environmental niches adaptations and potential for future spread following introduction outside their native range. Location Europe and Australia. Taxon Sea spurge, Euphorbia paralias, Euphorbiaceae. Methods We compiled presence-absence data of E. paralias from >190,000 vegetation surveys in the native and invaded range. We combined presence-absence data with information on climate, soil, coastal morphology and human pressure, to test whether E. paralias' environmental niche has shifted following invasion and used species distributions models (SDMs) to map its invasion potential under current and future climatic conditions. Results The environmental niche of E. paralias has shifted since reaching Australia, expanding into areas further away from people, closer to the shoreline and with higher temperatures. SDMs revealed that alongside broad-scale gradients in temperature and rainfall, the distribution of E. paralias is also constrained by soil substrate and dune morphology-highlighting the importance of these fine-scale drivers in shaping invasion dynamics in coastal environments. Moreover, SDMs suggest that future expansion in Australia will result from continued niche filling, not changes in climatic suitability. Main conclusions Despite its impressive dispersal ability, E. paralias has not yet reached equilibrium in its invaded range and is likely to continue to expand its distribution in Australia regardless of climate change. E. paralias' key to success has been its ability to suit novel environments. We provide one of the first examples of how to leverage distribution data and SDMs to test hypotheses about niche conservatism and expansion in coastal dune invasive plant species.