Life history differences between rare and common flowering plant species of California and the British Isles

Generalizations about rare and endangered species are important for the dev elopment of conservation management policy and for understanding of the nat ure of rarity. Therefore, we sought such generalizations by statistically c omparing eight life history and reproductive traits of rare and common plan t species in two better-studied but climatically and geographically distinc t Holarctic floras--those of California and the British Isles. Trends were often similar in both floras. Rare and common species differ significantly for a number of characters. In both floras, a woody habit was significantly more frequent for the rare species than the common species. Also, in both floras, monocarpy was less frequent in the rare species, significantly so i n California. The rare species of both floras were, on average, significant ly shorter in stature than the common species. This trend in stature remain ed significant when nonwoody species were compared but did not do so for wo ody species. Rare and common species differed with:regard to their frequenc y of inflorescence structure types, but these differences were not concorda nt for both floras. The rare species of the British Isles had significantly higher proportions of mixed and polymorphic flower colors; the trend was n ot significant in California. In both floras, rare species had a higher fre quency of many-seeded fruits, significantly so in California. California's rare species had a significantly higher frequency of dehiscent fruits than common species; the trend was reversed and not significant for the British Isles. We found no significant differences for characters associated with f ruit dispersal. Finally in California, the median altitude of the range of the common species was significantly greater than that of the rare species. At present, these correlates cannot be assigned as possible causes or cons equences of rarity. Nonetheless, they may represent "high-risk" traits that could be used to identify other species already at risk or potentially at risk of becoming rarer.