We analyse the population and spatial structures of coastal annual-plant communities, across ten dunes and three years, to explore the role of seed mass in structuring these communities. One suggestion is that annual-plant communities are structured by competition-colonization trade-offs driven by difference among species in seed-allocation strategies, while another perspective is that seed mass influences the ways in which species respond to environmental variation. In support of the competition-colonization trade-off, the two largest-seeded species found on the dunes (Erodium cicutarium and Geranium molle) were negatively associated with the other guild members at the 10-mm scale in 1995, suggesting they locally excluded smaller-seeded species in that year (when population densities were high). In support of the environmental response hypothesis, populations of annual plants declined between 1995 and 1996 on eight of the ten dunes, underscoring the importance of year-to-year environmental fluctuations in determining population sizes. The species that became relatively uncommon also became more aggregated in space, and this effect was most pronounced among the small-seeded species. Thus, small-seeded species may be forced to retreat into refuges when conditions are unfavourable, where reduced frequencies of interspecific contacts may increase their chances of persistence. We also show that small-seeded species sometimes reach much higher population densities than larger-seeded species, consistent with earlier findings, but reason that this abundance/seed mass relationship could have resulted from either a competition-colonization trade-off or from different responses of small- and large-seeded species to environmental variation. We conclude that dune-annual species with contrasting seed masses respond differently to environmental variation, while the competition-colonization trade-off plays a lesser role in community dynamics than previously considered.