An analysis of the ecological characteristics (life form, seed-dispersal mode, pollination mode, and reproductive mode) of about 1800 species of the flora of northwest Spain indicates that the most frequent characteristics are hemicryptophyte or therophyte life form, semachorous or barochorous seed dispersal, entomophilous pollination, and hermaphroditism. Following binary classification of the above characteristics (herbaceous versus woody life form, animal-assisted versus abiotic seed dispersal, animal-assisted versus abiotic pollination, dioecous versus hermaphrodite reproductive mode), statistically significant associations were found for all pairs of characteristics except pollination mode and reproductive mode (in disagreement with the results of previous studies which have detected significant associations between dioecy and abiotic pollination) and pollination mode and life form. To investigate the evolutionary plasticity of the different characters, we used nested analysis of variance with (in the case of life form, for example) percentage of species in genus with herbaceous life form as response variable, and with genus, family, order and subclass as nested factors and class as crossed factor. The results of this analysis indicated that higher-level grouping (class or subclass) accounted for the majority of total variance only in the case of pollination mode, suggesting either that pollination mode was typically determined early in evolutionary history and that there is currently little plasticity in this characteristic (i.e., phylogenetic constraints), or that this characteristic has been subject to stabilizing selection (i.e., phylogenetic niche conservatism). The other ecological characters, by contrast, showed high variability at lower levels in the taxonomic hierarchy.