Aims: Mobile, coastal dunefields around the world are under threat from invasive plants, which may out-compete native plant species. These aliens may also accelerate stabilisation of the dunes, to the complete exclusion of early successional native flora. In a mobile dunefield we examine the impact of the increasing abundance of alien species on substrate stability and successional trends of the native vegetation. Study area: Tawhirihoe Scientific Reserve, Manawatū, New Zealand. Methods: We recorded species’ covers and environmental factors in quadrats placed randomly over the mobile dunefield, and analysed the vegetation and its successional patterns via multivariate analysis. We explored the degrees of stabilisation and nativeness, referencing changes over the last three decades. Results: Our analysis reveals seven vegetation types, three with a high native component and following an established successional trend, and the others becoming dominated by alien grasses and herbs, and associated with increasing dune stability. Biodiversity is trending towards aliens, especially behind the foredunes, and aliens occupy nearly double the area of the mobile dunefield as do natives. Coverage of unvegetated or mobile sand has declined to 21 % and is projected to decline further. Conclusions: Only the foredunes and dune-slack wetlands are now in a mostly natural state, while native rear dune vegetation is becoming rare, and natural succession appears to be interrupted. Alien species over-stabilise the dunefield, facilitating further alien invasion, the longer-term implications of which are unknown. Intervention to destabilise the dunefield seems the most viable management option.<br> Taxonomic reference: Nga Tipu o Aotearoa (http://nzflora.landcareresearch.co.nz) [accessed 10 Jan 2023].
