Introduction. This study investigated the ecology of Petalophyllum ralfsii in an oceanic dune system, including relationships with grazing livestock, hydrology and habitat restoration.

Methods. The study site was two large slacks at Newborough Warren, NW Wales, UK. In areas of one slack, there are 6-yr-old ‘restoration scrapes’, where vegetation was stripped to restore early successional habitat. Since 2001, both slacks have had continuous conservation grazing, primarily by ponies. Locations of P. ralfsii were recorded using high-precision GPS. Vegetation was recorded by relevés and hydrology investigated from dipwell data.

Results. Petalophyllum ralfsii was recorded at 1322 locations, mostly along pony trails, but also in sparsely vegetated depressions and restoration scrapes. The most frequent associates were Carex flacca, Didymodon tophaceus, Equisetum variegatum and Ptychostomum pseudotriquetrum. Wetter locations could be flooded for prolonged periods in some winters. Drier locations did not flood at all during the five winters of the study. Within occupied 5 × 5 m grid cells, vertical range of locations was no more than 16 cm. Locations in restoration scrapes are confined to areas where the water table is relatively high.

Conclusions. Livestock grazing had a major positive impact on P. ralfsii. Trampling along pony trails provided competitive release and, due to greater soil bulk density, prolonged water availability at the surface. Ponies likely provided directed dispersal into suitable microhabitats via transport of diaspores on their hooves. Restoration scrapes have been colonised by P. ralfsii, but the population is small, likely related, at least in part, to water availability.