The present study (1) describes the rates and patterns of Ammophila arenaria (marram grass) invasion in a large transgressive dune system (Mason Bay) and on a prograding foredune-ridge barrier (Doughboy Bay), Stewart Island, New Zealand; (2) examines the impact of Ammophila on dune morphology and indigenous dune biota; and (3) assesses the significance of geomorphic processes in accounting for the patterns observed. Processes of Ammophila invasion are interpreted from evidence of landform development and vegetation change; field observations and survey of dune landforms and dune vegetation; the aerial photographic record and historic accounts of the local botany.

The area dominated by Ammophila in the Mason Bay study area has increased from 1.4 ha in 1958, to 17.8 ha in 1978, to 74.9 ha in 1998; a 5,204 percent increase. Ammophila invasion of active dune systems in the study areas is clearly associated with dune forming processes—shadow dune development; migration of long-walled parabolic dunes; stoss face blowout development; and barrier progradation. The primary mechanism of native species displacement appears to be burial rather than competition for nutrients. Ammophila traps sand and builds dunes at rates that may exceed the threshold of tolerance of local native species. Desmoschoenus spiralis, the dominant indigenous foredune species, cannot co-exist with Ammophila in the active dune systems investigated.