Résumé
The viability of a metapopulation of the rare, cliff-dwelling Centaurea corymbosa depends critically on the persistence of extant local populations, as it has a very low colonising ability. The variation in components of seed production in space and time was investigated and compared with the variation of supposedly causal plant and population traits. Temporal variation in the number of capitula per plant and in the number of ovules per capitulum were related to resource limitation between years and flowering periods, respectively. The seed-to-ovule ratio also varied widely, because of variation in fertilization rate, and, to a lesser extent, in abortion rate; predation being negligible. Isolated individuals had lower fertilization rates than individuals within dense patches. But despite a high density, the smallest studied population showed the lowest fertilization rates and highest abortion rates. This Allee effect on seed production represents a possible threat for small and/or low-density populations. Reinforcing populations of low density and supplying the smallest populations with new genotypes, by seed deposition into clefts of rocks, are suggested as possible conservation management strategies for C. corymbosa.