The population ecology of the annual plantCakile edentulawas studied on a gravel bar in Halifax County, Nova Scotia, where it grows in a two phase mosaic consisting of (1) open shingle or gravel and (2) thick mats of deadZostera marinawrack. Cohorts of seedlings were marked and regularly counted in both habitats. Reproductive output and seed dispersal were also studied.There were marked differences inC. edentulaecology between the two habitats. In general, survivorship and reproductive output were both greater in shingle. However, caterpillar grazing in late summer was concentrated on shingle plants, with the eventual result that net reproductive output was greater in wrack. There was no evidence of seed movement between wrack and shingle.Seedlings ofC. edentulaoften grew in dense clusters around the remains of the previous year's parent(s). In both habitats,C. edentulaseedling density declined with distance from the centre of clusters. Reproductive output increased with distance in both habitats; survivorship increased with distance only in wrack. Distal fruit segments were dispersed further than proximal segments; thus seedlings derived from distal segments tended to grow further from the previous year's parent, and in turn had higher survivorship and reproductive output. Distal fruit segments are normally thought to function primarily for long-distance dispersal; on shingle beaches "long-distance dispersal" of less than a metre could significantly improve the reproductive success of a seedling.