Extending the observational record to provide new insights into invasive alien species in a coastal dune environment of New Zealand

Coastal habitats are regarded to be highly vulnerable to the impacts of invasive alien species. These impacts can be particularly visible in areas of national cultural and heritage significance, raising public awareness of a growing global trend and often requiring urgent changes to management practices. New Zealand has a relatively long history of invasive alien species with the introduction of non-native marram grass (Ammophila arenaria) for sand stabilisation and erosion control since the mid-nineteenth century. Of national importance, the sand dunes of the Hokianga Harbour are considered to be the spiritual birthplace of Māori culture in New Zealand and have experienced substantial vegetation change over the last century. Here we report a multi-disciplinary study combining palaeoecology with historic and contemporary observations to better characterise the changing distribution and mechanisms of spread of invasive alien species on the Hokianga headland. Our analysis indicates the vegetation established on the headland is primarily linked to late Pleistocene water-retaining, lignite deposits. We find, however, an abrupt increase in the area colonised by invasive alien species during the late twentieth century, most probably linked to reduced sediment supply in the Hokianga Harbour. Urgent management strategies may be required if the present dune headland is to be conserved, particularly against a backdrop of rising sea level which will most probably limit sediment resupply.