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Aim The smooth cordgrass Spartina alterniflora is an iconic ecological engineer that has inspired some of the more insightful concepts and perspectives in modern theoretical ecology and environmental management, from population to community and ecosystem scales. Although it is currently considered native over more than 100 degrees of latitude along the Atlantic coast of the Americas, we challenge the default assumption that this cordgrass is native to South America, and propose an alternative hypothesis that it was introduced in the 18th or early 19th century by human activity. Location World-wide. Methods We applied nine criteria that have been proposed for the recognition of non-native species. These criteria consider the historical, biological, geographic, dispersal, ecological and evolutionary evidence that classically underpin the distinctions between native and non-native species. Results Our results strongly support the hypothesis that S. alterniflora is not native to South America. Herbarium collections, historical floristic descriptions and extensive literature reports clearly indicate that S. alterniflora was absent prior to the early 1800s in South America. S. alterniflora shows a reduced morphological, ecological and physiological variability along the South American coast and demonstrably increased in both regional extent and abundance over the 20th century. Main conclusions We conclude that what are now extensive S. alterniflora marshes in this region were probably bare mudflats and that there have been vast unrecorded and thus overlooked shifts in bird, fish and invertebrate biodiversity, and immense shifts in algal vs. detritus production, with the concomitant trophic cascades that these changes imply. Our results change the way we perceive 'natural' coastal ecosystems on the Atlantic coast of South America as a whole. Indeed, most South American coastal marshes currently are what we term 'ecological mirages'- illusions that have seriously hampered our ability to recognize the nature of pre-existing native ecosystems. [ABSTRACT FROM AUTHOR]

Global biodiversity is both declining and being redistributed in response to multiple drivers characterizing the Anthropocene, including synergies between biological invasions and climate change. The Antarctic marine benthos may constitute the last biogeographic realm where barriers (oceanographic currents, climatic gradients) have not yet been broken. Here we report the successful settlement of a cohort of Mytilus cf. platensis in a shallow subtidal habitat of the South Shetland Islands in 2019, which demonstrates the ability of this species to complete its early life stages in this extreme environment. Genetic analyses and shipping records show that this observation is consistent with the dominant vectors and pathways linking southern Patagonia with the Antarctic Peninsula and demonstrates the potential for impending invasions of Antarctic ecosystems.