OIKOS 96: 421–432, 2002 Are differences in seed mass among species important in structuring plant communities? Evidence from analyses of spatial and temporal variation in dune-annual populations David A. Coomes, Mark Rees, Peter J. Grubb and Lindsay Turnbull Coomes, D. A., Rees, M., Grubb, P. J. and Turnbull, L. 2002. Are differences in seed mass among species important in structuring plant communities? Evidence from analyses of spatial and temporal variation in dune-annual populations. – Oikos 96: 421–432. We analyse the population and spatial structures of coastal annual-plant communi- ties, 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-al- location strategies, while another perspective is that seed mass influences the ways in which species respond to environmental variation. In support of the competition-col- onization 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 densi- ties than larger-seeded species, consistent with earlier findings, but reason that this abundance/seed mass relationship could have resulted from either a competition-col- onization 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-colo- nization trade-off plays a lesser role in community dynamics than previously consid- ered. D. A. Coomes and P. J. Grubb, Dept of Plant Sciences, Uni. of Cambridge, Downing Street, Cambridge, UK CB32EA (daid.coomes@plantsci.cam.ac.uk).– M. Rees and L. Turnbull, Dept of Biology, Imperial College at Silwood Park, Ascot, Berkshire, UK SL57PY. Over the last 30 years, population ecologists have made substantial advances in understanding the ways in which density-dependent and density-independent fac- tors determine the dynamics of annual plants (Harper 1977, Watkinson et al. 2000). Yet little is known about the ways in which these populations of annual plants interact with one another within communities (Law and Watkinson 1989, Rees et al. 1996). One perspective is that simple rules underpin community dynamics. For example, Rees (1995) has argued that seed mass is very important in structuring the dynamics of dune-annual communities, basing his arguments on the following Accepted 20 September 2001 Copyright © OIKOS 2002 ISSN 0030-1299 OIKOS 96:3 (2002) 421